The obvious way to zero a register is to
MOV in a
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.