Instruction Set Changes

Non-MMX Instructions

CMPXCHG8B

Compare and exchange eight bytes. Compares two 8-byte data objects:

  • Compares 64-bit value in EDX:EAX with a value in memory.

  • If equal, the value in ECX:EBX is stored into the specified memory operand.

  • If unequal, the contents of the memory operand is copied into EDX:EAX.

Can be used with a LOCK prefix to execute it as an atomic operation.

The processor never produces a locked read without producing a subsequent locked write. The destination operand always receives a write regardless of the compare result. The destination operand is written back if the compare fails; otherwise, the source operand is written into the destination.

RDTSC

Loads the current 64-bit TSC value into EDX:EAX (upper 32 ...

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