One μop Doesn't Necessarily = One IA32 Instruction

Like the P6 processor, the Pentium® 4 processor family does not execute the variable-length IA32 instructions. Rather, each IA32 instruction is decoded into a series of one or more μops (primitive, fixed-length instructions) which, when executed by the processor core, have the same effect on the processor's state as would the IA32 instruction.

Just because a μop is retired doesn't necessarily mean that an IA32 instruction is being retired. While most IA32 instructions translate into a single μop, some decode into a number of μops (perhaps even hundreds or more). There are two implications related to this:

• The processor only recognizes interrupts and exceptions on IA32 instruction boundaries, not on μop boundaries.

• The processor's registers are only updated on IA32 instruction boundaries (when all of the μops associated with an IA32 instruction have completed execution).