Difference between revisions of "X86 Instruction decoding"
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== Predecoder == | == Predecoder == | ||
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== Decoder generation == | == Decoder generation == | ||
Revision as of 22:43, 17 May 2009
Overview
X86 instruction encodings have several unique characteristics which make them harder to deal with than the encodings for the other ISAs M5 supports. Despite that, x86 is decoded using the same basic mechanisms as the other ISAs.
At the lowest level, instructions can take any number of bytes (up to a maximum) and can have any alignment. That means that when a CPU brings in bytes of memory to decode, it may contain several instructions, the end of one and the start of the next, the middle of an instruction, etc. It's the predecoder's job to take the incoming stream of bytes and to turn them into a stream of objects which represent the encoding of a single instruction from beginning to end in a unique and uniform way. The predecoder also makes the guarantee that if any two instruction representations are the same, the instruction object that implements it is also the same.
Next, the instruction representations need to be translated into an instruction object which the CPU can execute. This is the job of the x86 decoder which is implemented as ISA description files and automatically generated.
Finally, the instruction object is handed back to the CPU. If the object is a regular instruction it can be executed directly. If the object is a macroop, aka a microcoded instruction, the CPU pulls microops out of it one at a time and actually executes them.
Predecoder
