Intel x86 Assembly Language & Microarchitecture Optimization Zeroing a register
Example
The obvious way to zero a register is to MOV in a 0—for example:
B8 00 00 00 00 MOV eax, 0
Notice that this is a 5-byte instruction.
If you are willing to clobber the flags (MOV never affects the flags), you can use the XOR instruction to bitwise-XOR the register with itself:
33 C0 XOR eax, eax
This instruction requires only 2 bytes and executes faster on all processors.
