Determining microcode entry points and prefix bytes using a parallel logic technique

A method of accessing a microcode sequence that effectuates a function of a complex instruction comprising the steps of:

• generating a first address based on a first presumed prefix byte;
• generating a second address based on a second presumed prefix byte;
• selecting an entry point address from either said first address or said second address based on a state of a prefix byte of said complex instruction;
• and
• accessing a first microcode instruction of said microcode sequence using said entry point address.

A microprocessor comprising:

• an instruction cache;
• a first instruction unit coupled to said instruction cache;
• a microcode instruction unit coupled to said instruction cache including:
• a storage device configured to store a plurality of microcode instructions, wherein a subset of said plurality of microcode instructions effectuates a function of a complex instruction, and said complex instruction includes an opcode byte and a prefix byte;
• a plurality of address generators configured to generate an address of a first microcode instruction of said plurality of microcode instructions that effectuate said function of said complex instruction, wherein a first address generator generates a first address based on said opcode byte and a first presumed prefix byte, and a second address generator generates a second address based on said opcode byte and a second presumed prefix byte;
• and
• a selection circuit coupled to said plurality of address generators and to said storage device, wherein said selection circuit is configured to select said first address if said first presumed prefix byte corresponds to said prefix byte of said complex instruction, and said selection circuit is configured to select said second address if said second presumed prefix byte corresponds to said prefix byte of said complex instruction;
• a decode unit coupled to said first instruction unit and said microcode instruction unit;
• and
• a functional unit coupled to said decode unit.