How many ways to set a register to zero?

assembly x86 x86-16 tasm
user173973 picture user173973 · Jan 28, 2011 · Viewed 43.2k times · Source

I'm curious how many ways are there to set a register to zero in x86 assembly. Using one instruction. Someone told me that he managed to find at least 10 ways to do it.

The ones I can think of are:

xor ax,ax
mov ax, 0
and ax, 0

Answer

GJ. picture GJ. · Jan 28, 2011

There are a lot of possibility how to mov 0 in to ax under IA32...

    lea eax, [0]
    mov eax, 0FFFF0000h         //All constants form 0..0FFFFh << 16
    shr eax, 16                 //All constants form 16..31
    shl eax, 16                 //All constants form 16..31

And perhaps the most strange... :)

@movzx:
    movzx eax, byte ptr[@movzx + 6]   //Because the last byte of this instruction is 0

and...

  @movzx:
    movzx ax, byte ptr[@movzx + 7]

Edit:

And for 16 bit x86 cpu mode, not tested...:

    lea  ax, [0]

and...

  @movzx:
    movzx ax, byte ptr cs:[@movzx + 7]   //Check if 7 is right offset

The cs: prefix is optional in case that the ds segment register is not equal to cs segment register.

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