SIB byte question

Berhanie · 11-15-2004, 10:19 PM

Hello.

I'm going through the NASM x86 assembly. My question is this:
Is it true that the command MOV EAX,[ESP] would be longer than MOV EAX,[EBX] after each is converted to machine language? The reason I believe that it is the case is that the first would have to make use of the SIB byte (since there are restrictions on the ESP register in indirect addressing) while the second does not make use of SIB. I just want to make sure I understand.

CroMagnon · 11-16-2004, 10:30 PM

It has been a long time since I did this sort of thing, but I don't believe MOV EAX,[ESP] is supported by the x86 instruction set at all... at least, there's nothing listed in the Pentium manual for [ESP] that I can see.

If anything, this would have to be coded as POP EAX, PUSH EAX instead (2 bytes).

Berhanie · 11-17-2004, 11:32 AM

Thank you for your comment, CroMagnon. I was able to check the machine code for those statements. MOV EAX,[ESP] translates to 8B 04 24, while MOV EAX,[EBX] translates to the shorter 8B 03.

CroMagnon · 11-17-2004, 04:11 PM

Sorry, I see my mistake now... you are correct, it does work but needs the SIB byte because the [ESP] addressing isn't supported alone, only in the form [ESP+reg*index]. The 24 byte says that there is no register or index to add on.