http://repo.m5sim.org/wiki/index.php?title=Register_Indexing&feed=atom&action=historyRegister Indexing - Revision history2024-03-29T09:03:48ZRevision history for this page on the wikiMediaWiki 1.29.2http://repo.m5sim.org/wiki/index.php?title=Register_Indexing&diff=7662&oldid=prevStever: /* Caveats */ - delete discussion of rename now that it's committed2013-10-18T14:17:39Z<p><span dir="auto"><span class="autocomment">Caveats: </span> - delete discussion of rename now that it's committed</span></p>
<table class="diff diff-contentalign-left" data-mw="interface">
<col class='diff-marker' />
<col class='diff-content' />
<col class='diff-marker' />
<col class='diff-content' />
<tr style='vertical-align: top;' lang='en'>
<td colspan='2' style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 14:17, 18 October 2013</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l52" >Line 52:</td>
<td colspan="2" class="diff-lineno">Line 52:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The description above is intended to illustrate the typical usage of these index types.  There may be exceptions that don't precisely follow this description, but I got tired of writing "typically" in every sentence.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The description above is intended to illustrate the typical usage of these index types.  There may be exceptions that don't precisely follow this description, but I got tired of writing "typically" in every sentence.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The terms 'relative' and 'unified' were invented for use in this documentation, so you are unlikely see them in the code until the code starts catching up with this page.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The terms 'relative' and 'unified' were invented for use in this documentation, so you are unlikely see them in the code until the code starts catching up with this page.</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del style="font-weight: bold; text-decoration: none;">* The illustration above uses FP_Reg_Base and Misc_Reg_Base to indicate the offsets for the respective register classes.  The actual constants in the code are called FP_Base_DepTag and Ctrl_Base_DepTag.  I don't like these names, and propose renaming the values in the code to match the illustration above if there are no objections.  Alternate proposals are welcome.</del></div></td><td colspan="2"> </td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>*  This discussion pertains only to the *architectural* registers.  An out-of-order CPU model such as O3 adds another layer of complexity by renaming these architectural registers (using the flattened register indices) to an underlying physical register file.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>*  This discussion pertains only to the *architectural* registers.  An out-of-order CPU model such as O3 adds another layer of complexity by renaming these architectural registers (using the flattened register indices) to an underlying physical register file.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
</table>Steverhttp://repo.m5sim.org/wiki/index.php?title=Register_Indexing&diff=7637&oldid=prevStever: /* Caveats */2013-07-31T06:12:21Z<p><span dir="auto"><span class="autocomment">Caveats</span></span></p>
<table class="diff diff-contentalign-left" data-mw="interface">
<col class='diff-marker' />
<col class='diff-content' />
<col class='diff-marker' />
<col class='diff-content' />
<tr style='vertical-align: top;' lang='en'>
<td colspan='2' style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 06:12, 31 July 2013</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l52" >Line 52:</td>
<td colspan="2" class="diff-lineno">Line 52:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The description above is intended to illustrate the typical usage of these index types.  There may be exceptions that don't precisely follow this description, but I got tired of writing "typically" in every sentence.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The description above is intended to illustrate the typical usage of these index types.  There may be exceptions that don't precisely follow this description, but I got tired of writing "typically" in every sentence.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The terms 'relative' and 'unified' were invented for use in this documentation, so you are unlikely see them in the code until the code starts catching up with this page.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The terms 'relative' and 'unified' were invented for use in this documentation, so you are unlikely see them in the code until the code starts catching up with this page.</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">* The illustration above uses FP_Reg_Base and Misc_Reg_Base to indicate the offsets for the respective register classes.  The actual constants in the code are called FP_Base_DepTag and Ctrl_Base_DepTag.  I don't like these names, and propose renaming the values in the code to match the illustration above if there are no objections.  Alternate proposals are welcome.</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>*  This discussion pertains only to the *architectural* registers.  An out-of-order CPU model such as O3 adds another layer of complexity by renaming these architectural registers (using the flattened register indices) to an underlying physical register file.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>*  This discussion pertains only to the *architectural* registers.  An out-of-order CPU model such as O3 adds another layer of complexity by renaming these architectural registers (using the flattened register indices) to an underlying physical register file.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
</table>Steverhttp://repo.m5sim.org/wiki/index.php?title=Register_Indexing&diff=7636&oldid=prevStever at 06:08, 31 July 20132013-07-31T06:08:33Z<p></p>
<table class="diff diff-contentalign-left" data-mw="interface">
<col class='diff-marker' />
<col class='diff-content' />
<col class='diff-marker' />
<col class='diff-content' />
<tr style='vertical-align: top;' lang='en'>
<td colspan='2' style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 06:08, 31 July 2013</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l1" >Line 1:</td>
<td colspan="2" class="diff-lineno">Line 1:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>CPU register indexing in gem5 is a complicated by the need to support multiple ISAs with sometimes very different register semantics (register windows, condition codes, mode-based alternate register sets, etc.).  In addition, this support has evolved gradually as new ISAs have been added, so older code may not take advantage of newer features or terminology.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>CPU register indexing in gem5 is a complicated by the need to support multiple ISAs with sometimes very different register semantics (register windows, condition codes, mode-based alternate register sets, etc.).  In addition, this support has evolved gradually as new ISAs have been added, so older code may not take advantage of newer features or terminology.</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">== Types of Register Indices ==</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>There are three types of register indices used internally in the CPU models: relative, unified, and flattened.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>There are three types of register indices used internally in the CPU models: relative, unified, and flattened.</div></td></tr>
<tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l5" >Line 5:</td>
<td colspan="2" class="diff-lineno">Line 7:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>=== Relative ===</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>=== Relative ===</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>A '''relative register index''' is the index that is encoded in a machine instruction.  There is a separate index space for each <del class="diffchange diffchange-inline">type </del>of register (integer, floating point, etc.), starting at 0.  The register <del class="diffchange diffchange-inline">type </del>is implied by the opcode.  Thus a value of "1" in a source register field may mean integer register 1 (e.g., "%r1") or floating point register 1 (e.g., "%f1") depending on the type of the instruction.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>A '''relative register index''' is the index that is encoded in a machine instruction.  There is a separate index space for each <ins class="diffchange diffchange-inline">class </ins>of register (integer, floating point, etc.), starting at 0.  The register <ins class="diffchange diffchange-inline">class </ins>is implied by the opcode.  Thus a value of "1" in a source register field may mean integer register 1 (e.g., "%r1") or floating point register 1 (e.g., "%f1") depending on the type of the instruction.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>=== Unified ===</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>=== Unified ===</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>While relative register indices are good for keeping instruction encodings compact, they are ambiguous, and thus not convenient for things like managing dependencies.  To avoid this ambiguity, the decoder maps the relative register indices into a '''unified register space''' by adding <del class="diffchange diffchange-inline">type</del>-specific offsets to relocate each relative index range into a unique position.  Integer registers are unmodified, and continue to start at zero.  Floating-point register indices are offset by (at least) the number of integer registers, so that the first FP register (e.g., "%f0") gets a unified index that is greater than that of the last integer register.  Similarly, miscellaneous (a.k.a. control) registers are mapped past the end of the FP register index space.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>While relative register indices are good for keeping instruction encodings compact, they are ambiguous, and thus not convenient for things like managing dependencies.  To avoid this ambiguity, the decoder maps the relative register indices into a '''unified register space''' by adding <ins class="diffchange diffchange-inline">class</ins>-specific offsets to relocate each relative index range into a unique position.  Integer registers are unmodified, and continue to start at zero.  Floating-point register indices are offset by (at least) the number of integer registers, so that the first FP register (e.g., "%f0") gets a unified index that is greater than that of the last integer register.  Similarly, miscellaneous (a.k.a. control) registers are mapped past the end of the FP register index space.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>=== Flattened ===</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>=== Flattened ===</div></td></tr>
<tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l19" >Line 19:</td>
<td colspan="2" class="diff-lineno">Line 21:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>A key distinction between the generation of unified and flattened register indices is that the former can always be done statically while the latter often depends on dynamic processor state.  That is, the translation from relative to unified indices depends only on the context provided by the instruction itself (which is convenient as the translation is done in the decoder).  In contrast, the mapping to a flattened register index may depend on processor state such as the interrupt level or the current window pointer on SPARC.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>A key distinction between the generation of unified and flattened register indices is that the former can always be done statically while the latter often depends on dynamic processor state.  That is, the translation from relative to unified indices depends only on the context provided by the instruction itself (which is convenient as the translation is done in the decoder).  In contrast, the mapping to a flattened register index may depend on processor state such as the interrupt level or the current window pointer on SPARC.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>=== <del class="diffchange diffchange-inline">Illustration </del>===</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>=== <ins class="diffchange diffchange-inline">Combining Register Index Types </ins>===</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> </div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">Although the typical progression for modifying register indices is relative -> unified -> flattened, it turns out that ''relative vs. unified'' and ''flattened vs. unflattened'' are orthogonal attributes.  Relative vs. unified indicates whether the index is relative to the base register for its register class (integer, FP, or misc) or has the base offset for its class added in.  Flattened vs. unflattened indicates whether the index has been adjusted to account for runtime context such as register window adjustments or alternate register file modes.  Thus a relative flattened register index is one in which the runtime context has been accounted for, but is still expressed relative to the base offset for its class.</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> </div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">A single set of class-specific offsets is used to generate unified indices from relative indices regardless of whether the indices are flattened or unflattened.  Thus the offsets must be large enough to separate the register classes even when flattened addresses are being used.  As a result, the unflattened unified register space is often discontiguous. </ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">[[File:graphics/Gem5-regs.png]]</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">== Illustration ==</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">=== Combinations ===</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">[[File:Gem5-regs.png|right|frame]]</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">Although the typical progression for modifying register indices is relative -> unified -> flattened</del>, <del class="diffchange diffchange-inline">it turns out that ''relative vs. unified'' and ''flattened vs. unflattened'' are orthogonal attributes.  Relative vs. unified indicates whether the index is relative to the base register for its register class </del>(<del class="diffchange diffchange-inline">integer</del>, FP, <del class="diffchange diffchange-inline">or </del>misc) <del class="diffchange diffchange-inline">or has the base offset for its class added in</del>.  <del class="diffchange diffchange-inline">Flattened vs. unflattened indicates whether </del>the <del class="diffchange diffchange-inline">index has been adjusted to account </del>for <del class="diffchange diffchange-inline">runtime </del>context <del class="diffchange diffchange-inline">such as register window adjustments or alternate register file modes.  Thus a relative flattened register index is one in which the runtime context has been accounted for, but is still expressed relative to the base offset for its class</del>.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">As an illustration</ins>, <ins class="diffchange diffchange-inline">consider a hypothetical architecture with four integer registers </ins>(<ins class="diffchange diffchange-inline">%r0-%r4)</ins>, <ins class="diffchange diffchange-inline">three </ins>FP <ins class="diffchange diffchange-inline">registers (%f0-%f2)</ins>, <ins class="diffchange diffchange-inline">and two </ins>misc<ins class="diffchange diffchange-inline">/control registers (%msr0-%msr1</ins>).  <ins class="diffchange diffchange-inline">In addition, </ins>the <ins class="diffchange diffchange-inline">architecture supports a complete set of alternate integer and FP registers </ins>for <ins class="diffchange diffchange-inline">fast </ins>context <ins class="diffchange diffchange-inline">switching</ins>.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">Thus in the example above, register %f1 in the alternate FP </del>register file <del class="diffchange diffchange-inline">could be referred to via the relative flattened index 4 as well as the relative unflattened index 1</del>, the unified <del class="diffchange diffchange-inline">unflattened index 9</del>, <del class="diffchange diffchange-inline">or the unified flattened index 12</del>.  <del class="diffchange diffchange-inline">Note that </del>the <del class="diffchange diffchange-inline">difference between </del>the <del class="diffchange diffchange-inline">relative and unified indices is always 8 (regardless of flattening)</del>, <del class="diffchange diffchange-inline">and </del>the <del class="diffchange diffchange-inline">difference between </del>the <del class="diffchange diffchange-inline">unflattened </del>and <del class="diffchange diffchange-inline">flattened indices is 3 </del>(<del class="diffchange diffchange-inline">regardless of </del>relative <del class="diffchange diffchange-inline">vs. unified status</del>)<del class="diffchange diffchange-inline">.</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">The resulting </ins>register file <ins class="diffchange diffchange-inline">layout</ins>, <ins class="diffchange diffchange-inline">along with </ins>the unified <ins class="diffchange diffchange-inline">flattened register file indices</ins>, <ins class="diffchange diffchange-inline">is shown at right</ins>.  <ins class="diffchange diffchange-inline">Although </ins>the <ins class="diffchange diffchange-inline">indices in </ins>the <ins class="diffchange diffchange-inline">picture range from 0 to 15</ins>, the <ins class="diffchange diffchange-inline">actual set of valid indices depends on </ins>the <ins class="diffchange diffchange-inline">type of index </ins>and (<ins class="diffchange diffchange-inline">for </ins>relative <ins class="diffchange diffchange-inline">indices</ins>) <ins class="diffchange diffchange-inline">the register class as well:</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">{| border="1 px" align="center" style="text-align: center; border-collapse: collapse"</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">|'''Relative unflattened'''||Int: 0-3; FP: 0-2; Misc: 0-1</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">|-</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">|'''Unified unflattened'''||0-3, 8-10, 14-15</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">|-</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">|'''Relative flattened'''||Int: 0-7; FP: 0-5; Misc: 0-1</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">|-</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">|'''Unified flattened'''||0-15</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">|}</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">Although </del>the <del class="diffchange diffchange-inline">gem5 code is unfortunately not always clear about which type of </del>register index <del class="diffchange diffchange-inline">is expected by a particular function</del>, <del class="diffchange diffchange-inline">functions whose name incorporates a register class (e</del>.<del class="diffchange diffchange-inline">g., readIntReg</del>()<del class="diffchange diffchange-inline">) expect a relative register index</del>, and <del class="diffchange diffchange-inline">functions that expect a </del>flattened <del class="diffchange diffchange-inline">index often have "flat" in the function name</del>.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">In this example, register %f1 in </ins>the <ins class="diffchange diffchange-inline">alternate FP </ins>register <ins class="diffchange diffchange-inline">file could be referred to via the relative flattened index 4 as well as the relative unflattened index 1, the unified unflattened </ins>index <ins class="diffchange diffchange-inline">9</ins>, <ins class="diffchange diffchange-inline">or the unified flattened index 12</ins>. <ins class="diffchange diffchange-inline"> Note that the difference between the relative and unified indices is always 8 </ins>(<ins class="diffchange diffchange-inline">regardless of flattening</ins>), and <ins class="diffchange diffchange-inline">the difference between the unflattened and </ins>flattened <ins class="diffchange diffchange-inline">indices is 3 (regardless of relative vs. unified status)</ins>.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">=</del>== Caveats <del class="diffchange diffchange-inline">=</del>==</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>== Caveats ==</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">* Although the gem5 code is unfortunately not always clear about which type of register index is expected by a particular function, functions whose name incorporates a register class (e.g., readIntReg()) expect a relative register index, and functions that expect a flattened index often have "flat" in the function name.</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">* Although the general case is complicated, the common case can be deceptively simple.  For example, because integer registers start at the beginning of the unified register space, relative and unified register indices are identical for integer registers.  Furthermore, in an architecture with no (or rarely-used) alternate integer registers, the unflattened and flattened indices are (almost always) the same as well, meaning that all four types of register indices are interchangeable in this case.  While this situation seems to be a simplification, it also tends to hide bugs where the wrong register index type is used.</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The description above is intended to illustrate the typical usage of these index types.  There may be exceptions that don't precisely follow this description, but I got tired of writing "typically" in every sentence.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The description above is intended to illustrate the typical usage of these index types.  There may be exceptions that don't precisely follow this description, but I got tired of writing "typically" in every sentence.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The terms 'relative' and 'unified' were invented for use in this documentation, so you are unlikely see them in the code until the code starts catching up with this page.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The terms 'relative' and 'unified' were invented for use in this documentation, so you are unlikely see them in the code until the code starts catching up with this page.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>*  This discussion pertains only to the *architectural* registers.  An out-of-order CPU model such as O3 adds another layer of complexity by renaming these architectural registers (using the flattened register indices) to an underlying physical register file.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>*  This discussion pertains only to the *architectural* registers.  An out-of-order CPU model such as O3 adds another layer of complexity by renaming these architectural registers (using the flattened register indices) to an underlying physical register file.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">=</del>== Outstanding Questions ==<del class="diffchange diffchange-inline">=</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>== Outstanding Questions ==</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div> </div></td><td colspan="2"> </td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">* The SPARC code looks broken, in that the FP_Base_DepTag value is much smaller than NumIntRegs (see src/arch/sparc/registers.hh).  Is it really broken, or is there some subtlety not captured here that allows it to work?</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>__NOTOC__</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>__NOTOC__</div></td></tr>
</table>Steverhttp://repo.m5sim.org/wiki/index.php?title=Register_Indexing&diff=7635&oldid=prevStever: /* Combinations */2013-07-31T05:31:19Z<p><span dir="auto"><span class="autocomment">Combinations</span></span></p>
<table class="diff diff-contentalign-left" data-mw="interface">
<col class='diff-marker' />
<col class='diff-content' />
<col class='diff-marker' />
<col class='diff-content' />
<tr style='vertical-align: top;' lang='en'>
<td colspan='2' style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 05:31, 31 July 2013</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l25" >Line 25:</td>
<td colspan="2" class="diff-lineno">Line 25:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>=== Combinations ===</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>=== Combinations ===</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">To complicate things a little further</del>, it turns out that relative vs. unified and flattened vs. unflattened are orthogonal attributes.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">Although the typical progression for modifying register indices is relative -> unified -> flattened</ins>, it turns out that <ins class="diffchange diffchange-inline">''</ins>relative vs. unified<ins class="diffchange diffchange-inline">'' </ins>and <ins class="diffchange diffchange-inline">''</ins>flattened vs. unflattened<ins class="diffchange diffchange-inline">'' </ins>are orthogonal attributes<ins class="diffchange diffchange-inline">.  Relative vs. unified indicates whether the index is relative to the base register for its register class (integer, FP, or misc) or has the base offset for its class added in.  Flattened vs. unflattened indicates whether the index has been adjusted to account for runtime context such as register window adjustments or alternate register file modes.  Thus a relative flattened register index is one in which the runtime context has been accounted for, but is still expressed relative to the base offset for its class.</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> </div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">Thus in the example above, register %f1 in the alternate FP register file could be referred to via the relative flattened index 4 as well as the relative unflattened index 1, the unified unflattened index 9, or the unified flattened index 12.  Note that the difference between the relative and unified indices is always 8 (regardless of flattening), and the difference between the unflattened and flattened indices is 3 (regardless of relative vs. unified status).</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> </div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">Although the gem5 code is unfortunately not always clear about which type of register index is expected by a particular function, functions whose name incorporates a register class (e.g., readIntReg()) expect a relative register index, and functions that expect a flattened index often have "flat" in the function name</ins>.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>=== Caveats ===</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>=== Caveats ===</div></td></tr>
</table>Steverhttp://repo.m5sim.org/wiki/index.php?title=Register_Indexing&diff=7632&oldid=prevStever at 04:07, 31 July 20132013-07-31T04:07:37Z<p></p>
<table class="diff diff-contentalign-left" data-mw="interface">
<col class='diff-marker' />
<col class='diff-content' />
<col class='diff-marker' />
<col class='diff-content' />
<tr style='vertical-align: top;' lang='en'>
<td colspan='2' style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 04:07, 31 July 2013</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l18" >Line 18:</td>
<td colspan="2" class="diff-lineno">Line 18:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>A key distinction between the generation of unified and flattened register indices is that the former can always be done statically while the latter often depends on dynamic processor state.  That is, the translation from relative to unified indices depends only on the context provided by the instruction itself (which is convenient as the translation is done in the decoder).  In contrast, the mapping to a flattened register index may depend on processor state such as the interrupt level or the current window pointer on SPARC.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>A key distinction between the generation of unified and flattened register indices is that the former can always be done statically while the latter often depends on dynamic processor state.  That is, the translation from relative to unified indices depends only on the context provided by the instruction itself (which is convenient as the translation is done in the decoder).  In contrast, the mapping to a flattened register index may depend on processor state such as the interrupt level or the current window pointer on SPARC.</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">=== Illustration ===</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">[[File:graphics/Gem5-regs.png]]</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">=== Combinations ===</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">To complicate things a little further, it turns out that relative vs. unified and flattened vs. unflattened are orthogonal attributes.</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>=== Caveats ===</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>=== Caveats ===</div></td></tr>
</table>Steverhttp://repo.m5sim.org/wiki/index.php?title=Register_Indexing&diff=7631&oldid=prevStever: /* Caveats */2013-07-31T01:13:12Z<p><span dir="auto"><span class="autocomment">Caveats</span></span></p>
<table class="diff diff-contentalign-left" data-mw="interface">
<col class='diff-marker' />
<col class='diff-content' />
<col class='diff-marker' />
<col class='diff-content' />
<tr style='vertical-align: top;' lang='en'>
<td colspan='2' style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 01:13, 31 July 2013</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l23" >Line 23:</td>
<td colspan="2" class="diff-lineno">Line 23:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The description above is intended to illustrate the typical usage of these index types.  There may be exceptions that don't precisely follow this description, but I got tired of writing "typically" in every sentence.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The description above is intended to illustrate the typical usage of these index types.  There may be exceptions that don't precisely follow this description, but I got tired of writing "typically" in every sentence.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The terms 'relative' and 'unified' were invented for use in this documentation, so you are unlikely see them in the code until the code starts catching up with this page.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The terms 'relative' and 'unified' were invented for use in this documentation, so you are unlikely see them in the code until the code starts catching up with this page.</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">*  This discussion pertains only to the *architectural* registers.  An out-of-order CPU model such as O3 adds another layer of complexity by renaming these architectural registers (using the flattened register indices) to an underlying physical register file.</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">=== Outstanding Questions ===</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>__NOTOC__</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>__NOTOC__</div></td></tr>
</table>Steverhttp://repo.m5sim.org/wiki/index.php?title=Register_Indexing&diff=7630&oldid=prevStever at 01:04, 31 July 20132013-07-31T01:04:24Z<p></p>
<table class="diff diff-contentalign-left" data-mw="interface">
<col class='diff-marker' />
<col class='diff-content' />
<col class='diff-marker' />
<col class='diff-content' />
<tr style='vertical-align: top;' lang='en'>
<td colspan='2' style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 01:04, 31 July 2013</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l23" >Line 23:</td>
<td colspan="2" class="diff-lineno">Line 23:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The description above is intended to illustrate the typical usage of these index types.  There may be exceptions that don't precisely follow this description, but I got tired of writing "typically" in every sentence.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The description above is intended to illustrate the typical usage of these index types.  There may be exceptions that don't precisely follow this description, but I got tired of writing "typically" in every sentence.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The terms 'relative' and 'unified' were invented for use in this documentation, so you are unlikely see them in the code until the code starts catching up with this page.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* The terms 'relative' and 'unified' were invented for use in this documentation, so you are unlikely see them in the code until the code starts catching up with this page.</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">__NOTOC__</ins></div></td></tr>
</table>Steverhttp://repo.m5sim.org/wiki/index.php?title=Register_Indexing&diff=7629&oldid=prevStever at 01:03, 31 July 20132013-07-31T01:03:43Z<p></p>
<table class="diff diff-contentalign-left" data-mw="interface">
<col class='diff-marker' />
<col class='diff-content' />
<col class='diff-marker' />
<col class='diff-content' />
<tr style='vertical-align: top;' lang='en'>
<td colspan='2' style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 01:03, 31 July 2013</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l1" >Line 1:</td>
<td colspan="2" class="diff-lineno">Line 1:</td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">A "nickle tour" of flattening and </del>register indexing in the <del class="diffchange diffchange-inline">CPU models</del>.  </div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">CPU </ins>register indexing in <ins class="diffchange diffchange-inline">gem5 is a complicated by </ins>the <ins class="diffchange diffchange-inline">need to support multiple ISAs with sometimes very different register semantics (register windows, condition codes, mode-based alternate register sets, etc.).  In addition, this support has evolved gradually as new ISAs have been added, so older code may not take advantage of newer features or terminology</ins>.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">First, an instruction has identified that it needs </del>register <del class="diffchange diffchange-inline">such and such as determined by its encoding (or </del>the <del class="diffchange diffchange-inline">fact that it always uses a certain register</del>, <del class="diffchange diffchange-inline">or ...). For the sake of argument, lets say we're talking about SPARC, the register is %g1</del>, and <del class="diffchange diffchange-inline">the second bank of globals is active. From the instructions point of view, the unflattened register is %g1, which, likely, is just represented by the index 1</del>.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">There are three types of </ins>register <ins class="diffchange diffchange-inline">indices used internally in </ins>the <ins class="diffchange diffchange-inline">CPU models: relative</ins>, <ins class="diffchange diffchange-inline">unified</ins>, and <ins class="diffchange diffchange-inline">flattened</ins>.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">Next, we need to map from the instruction's view of the register file(s) down to actual storage locations. Think of this like virtual memory. The instruction is working within an index space which is like a virtual address space, and it needs to be mapped down to the flattened space which is like physical memory. Here, the index 1 is likely mapped to, say, 9, where 0-7 is the first bank of globals and 8-15 is the second.</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">=== Relative ===</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">This </del>is the <del class="diffchange diffchange-inline">point where the CPU gets involved</del>. <del class="diffchange diffchange-inline">The </del>index <del class="diffchange diffchange-inline">9 refers to an actual </del>register <del class="diffchange diffchange-inline">the instruction expects to access</del>, <del class="diffchange diffchange-inline">and it's the CPU's job to make that happen</del>. <del class="diffchange diffchange-inline">Before this point</del>, <del class="diffchange diffchange-inline">all the work was done </del>by the <del class="diffchange diffchange-inline">ISA with no insight available to the CPU</del>, <del class="diffchange diffchange-inline">and beyond this </del>point <del class="diffchange diffchange-inline">all </del>the <del class="diffchange diffchange-inline">work is done by </del>the <del class="diffchange diffchange-inline">CPU with no insight available to the ISA</del>.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">A '''relative register index''' </ins>is the <ins class="diffchange diffchange-inline">index that is encoded in a machine instruction</ins>. <ins class="diffchange diffchange-inline"> There is a separate </ins>index <ins class="diffchange diffchange-inline">space for each type of </ins>register <ins class="diffchange diffchange-inline">(integer, floating point</ins>, <ins class="diffchange diffchange-inline">etc</ins>.<ins class="diffchange diffchange-inline">)</ins>, <ins class="diffchange diffchange-inline">starting at 0.  The register type is implied </ins>by the <ins class="diffchange diffchange-inline">opcode.  Thus a value of "1" in a source register field may mean integer register 1 (e.g.</ins>, <ins class="diffchange diffchange-inline">"%r1") or floating </ins>point <ins class="diffchange diffchange-inline">register 1 (e.g., "%f1") depending on </ins>the <ins class="diffchange diffchange-inline">type of </ins>the <ins class="diffchange diffchange-inline">instruction</ins>.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">The CPU is free to provide a register directly like the simple CPU by having an array and just reading and writing the 9th element on behalf of the instruction. The CPU could, alternatively, do something complicated like renaming and mapping the flattened index further into a physical register like O3.</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">=== Unified ===</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">While relative register indices are good for keeping instruction encodings compact, they are ambiguous, and thus not convenient for things like managing dependencies.  To avoid this ambiguity, the decoder maps the relative register indices into a '''unified register space''' by adding type-specific offsets to relocate each relative index range into a unique position.  Integer registers are unmodified, and continue to start at zero.  Floating-point register indices are offset by (at least) the number of integer registers, so that the first FP register (e.g., "%f0") gets a unified index that is greater than that of the last integer register.  Similarly, miscellaneous (a.k.a. control) registers are mapped past the end of the FP register index space.</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">One important property of all this, which makes sense if you think about the virtual memory analogy, is that the size of the index space before flattening has nothing to do with the size after. The virtual memory space could be very large (presumably with gaps) and map to a smaller physical space, or it could be small and map to a larger physical space where the extra is for, say, other virtual spaces used at other times. You need to make sure you're using the right size (post flattening) to size your tables because that's the space of possible options.</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">=== Flattened ===</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">One other tricky part comes from </del>the <del class="diffchange diffchange-inline">fact </del>that <del class="diffchange diffchange-inline">we add offsets into </del>the <del class="diffchange diffchange-inline">indices </del>to <del class="diffchange diffchange-inline">distinguish ints from floats from miscs</del>. <del class="diffchange diffchange-inline">Those offsets might be one thing </del>in <del class="diffchange diffchange-inline">the preflattening world</del>, <del class="diffchange diffchange-inline">but then need </del>to <del class="diffchange diffchange-inline">be something else </del>in <del class="diffchange diffchange-inline">the post flattening world to keep things from landing </del>on top of <del class="diffchange diffchange-inline">each other without leaving gaps</del>. <del class="diffchange diffchange-inline">It</del>'s <del class="diffchange diffchange-inline">easy to make a mistake here</del>, <del class="diffchange diffchange-inline">and it</del>'s <del class="diffchange diffchange-inline">one </del>of the <del class="diffchange diffchange-inline">reasons I don't like this offset idea </del>as a <del class="diffchange diffchange-inline">way </del>to <del class="diffchange diffchange-inline">keep </del>the <del class="diffchange diffchange-inline">different types separate</del>. <del class="diffchange diffchange-inline">I'd rather see a two dimensional index where </del>the <del class="diffchange diffchange-inline">second coordinate was a </del>register <del class="diffchange diffchange-inline">type</del>. <del class="diffchange diffchange-inline">But </del>in the <del class="diffchange diffchange-inline">world </del>as <del class="diffchange diffchange-inline">it exists today</del>, this <del class="diffchange diffchange-inline">is something </del>you <del class="diffchange diffchange-inline">have to keep track of</del>.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">Unified register indices provide an unambiguous description of all </ins>the <ins class="diffchange diffchange-inline">registers </ins>that <ins class="diffchange diffchange-inline">are accessible as instruction operands at a given point in </ins>the <ins class="diffchange diffchange-inline">execution.  Unfortunately, due </ins>to <ins class="diffchange diffchange-inline">the complex features of some ISAs, they do not always unambiguously identify the actual state that the instruction is referencing</ins>. <ins class="diffchange diffchange-inline"> For example, </ins>in <ins class="diffchange diffchange-inline">ISAs with register windows (notably SPARC)</ins>, <ins class="diffchange diffchange-inline">a particular register identifier such as "%o0" will refer </ins>to <ins class="diffchange diffchange-inline">a different register after a "save" or "restore" operation than it did previously.  Several ISAs have registers that are hidden </ins>in <ins class="diffchange diffchange-inline">normal operation, but get mapped </ins>on top of <ins class="diffchange diffchange-inline">ordinary registers when an interrupt occurs (e.g</ins>.<ins class="diffchange diffchange-inline">, ARM</ins>'s <ins class="diffchange diffchange-inline">mode-specific registers), or under explicit supervisor control (e.g.</ins>, <ins class="diffchange diffchange-inline">SPARC</ins>'s <ins class="diffchange diffchange-inline">"alternate globals").</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> </div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">We solve this problem by maintaining a '''flattened register space''' which provides a distinct index for every unique register storage location.  For example, the integer portion </ins>of the <ins class="diffchange diffchange-inline">SPARC flattened register space has distinct indices for the globals and the alternate globals, as well </ins>as <ins class="diffchange diffchange-inline">for each of the available register windows.  The "flattening" process of translating from a unified or relative register index to </ins>a <ins class="diffchange diffchange-inline">flattened register index varies by ISA.  On some ISAs, the mapping is trivial, while others use table lookups </ins>to <ins class="diffchange diffchange-inline">do </ins>the <ins class="diffchange diffchange-inline">translation</ins>.</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> </div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">A key distinction between </ins>the <ins class="diffchange diffchange-inline">generation of unified and flattened </ins>register <ins class="diffchange diffchange-inline">indices is that the former can always be done statically while the latter often depends on dynamic processor state</ins>. <ins class="diffchange diffchange-inline"> That is, the translation from relative to unified indices depends only on the context provided by the instruction itself (which is convenient as the translation is done </ins>in the <ins class="diffchange diffchange-inline">decoder).  In contrast, the mapping to a flattened register index may depend on processor state such </ins>as <ins class="diffchange diffchange-inline">the interrupt level or the current window pointer on SPARC.</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> </div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">=== Caveats ===</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> </div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">* The description above is intended to illustrate the typical usage of these index types.  There may be exceptions that don't precisely follow this description</ins>, <ins class="diffchange diffchange-inline">but I got tired of writing "typically" in every sentence.</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">* The terms 'relative' and 'unified' were invented for use in </ins>this <ins class="diffchange diffchange-inline">documentation, so </ins>you <ins class="diffchange diffchange-inline">are unlikely see them in the code until the code starts catching up with this page</ins>.</div></td></tr>
</table>Steverhttp://repo.m5sim.org/wiki/index.php?title=Register_Indexing&diff=2882&oldid=prevKsewell: Created page with 'A "nickle tour" of flattening and register indexing in the CPU models. First, an instruction has identified that it needs register such and such as determined by its encoding (…'2010-08-23T02:22:55Z<p>Created page with 'A "nickle tour" of flattening and register indexing in the CPU models. First, an instruction has identified that it needs register such and such as determined by its encoding (…'</p>
<p><b>New page</b></p><div>A "nickle tour" of flattening and register indexing in the CPU models. <br />
<br />
First, an instruction has identified that it needs register such and such as determined by its encoding (or the fact that it always uses a certain register, or ...). For the sake of argument, lets say we're talking about SPARC, the register is %g1, and the second bank of globals is active. From the instructions point of view, the unflattened register is %g1, which, likely, is just represented by the index 1.<br />
<br />
Next, we need to map from the instruction's view of the register file(s) down to actual storage locations. Think of this like virtual memory. The instruction is working within an index space which is like a virtual address space, and it needs to be mapped down to the flattened space which is like physical memory. Here, the index 1 is likely mapped to, say, 9, where 0-7 is the first bank of globals and 8-15 is the second.<br />
<br />
This is the point where the CPU gets involved. The index 9 refers to an actual register the instruction expects to access, and it's the CPU's job to make that happen. Before this point, all the work was done by the ISA with no insight available to the CPU, and beyond this point all the work is done by the CPU with no insight available to the ISA.<br />
<br />
The CPU is free to provide a register directly like the simple CPU by having an array and just reading and writing the 9th element on behalf of the instruction. The CPU could, alternatively, do something complicated like renaming and mapping the flattened index further into a physical register like O3.<br />
<br />
<br />
One important property of all this, which makes sense if you think about the virtual memory analogy, is that the size of the index space before flattening has nothing to do with the size after. The virtual memory space could be very large (presumably with gaps) and map to a smaller physical space, or it could be small and map to a larger physical space where the extra is for, say, other virtual spaces used at other times. You need to make sure you're using the right size (post flattening) to size your tables because that's the space of possible options.<br />
<br />
One other tricky part comes from the fact that we add offsets into the indices to distinguish ints from floats from miscs. Those offsets might be one thing in the preflattening world, but then need to be something else in the post flattening world to keep things from landing on top of each other without leaving gaps. It's easy to make a mistake here, and it's one of the reasons I don't like this offset idea as a way to keep the different types separate. I'd rather see a two dimensional index where the second coordinate was a register type. But in the world as it exists today, this is something you have to keep track of.</div>Ksewell