Comment by vardump
> - mov ax, 0 - needs 4 bytes (66 b8 00 00) - xor ax,ax - needs 3 bytes (66 31 c0)
You don't need operand size prefix 0x66 when running 16 bit code in Real Mode. So "mov ax, 0" is 3 bytes and "xor ax, ax" is just 2 bytes.
> - mov ax, 0 - needs 4 bytes (66 b8 00 00) - xor ax,ax - needs 3 bytes (66 31 c0)
You don't need operand size prefix 0x66 when running 16 bit code in Real Mode. So "mov ax, 0" is 3 bytes and "xor ax, ax" is just 2 bytes.
My fault: I just compiled the instruction with an assembler instead of looking up the actual instruction from documentation.
It makes much more sense: resetting ax, and bc (xor ax,ax ; xor bx,bx) will be 4 octets, DWORD aligned, and a bit faster to fetch by the x86 than the 3-octet version I wrote before.