The Unabridged Pentium 4 IA32 Processor Genealogy

Speculative Execution

As mentioned earlier, the Pentium® 4 processor can execute instructions speculatively (as could the P6 processor). The Pentium® 4 processor can speculatively execute instructions that reside downstream of a conditional branch instruction before the branch has been executed. Consider the following code fragment:

      ---
      ---
      Jcc  bypass
      ---
      mov  eax,[0100]    ;read could be executed before Jcc
      ---
      mov  ecx,es:[1234] ;this one as well
bypass

The processor could execute the four instructions beneath the conditional branch and then execute the branch. However, remember that the results of an μop's execution are not stored in the actual processor register set. Rather, they are stored in alias registers. If when the branch is finally executed, ...