movq in GAS

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pal

Joined: 26 Aug 2008
Posts: 227

pal 06 Jul 2009, 12:52

Wrong forum I know so move it if you want to.

Obviously I don't use GAS but I found something which made me think. In GAS the mov mnemonic have suffixes which determine what size operands are being used right e.g. movb $0x10,%al

When working with 64-bit registers the suffix is a q yeah, e.g. movq $0xDEADC0DEDEADC0DE,%rax or something right.

What happens when you want to use the MMX instruction mnemonic movq then? How does the assembler know whether you are using their suffixed version or the MMX version? Is it just to do with whether or not there is an XMM register involved?

Also what is up with GAS syntax? Why is it the opposite of all others Confused

Cheers.

06 Jul 2009, 12:52

drhowarddrfine

Joined: 10 Jul 2007
Posts: 533

drhowarddrfine 06 Jul 2009, 14:01

pal wrote:

Also what is up with GAS syntax? Why is it the opposite of all others

GAS was built upon ATT syntax which was around long before most of the others so the question should be why the others are the opposite. But GAS was designed to be used by language compilers and not humans.

06 Jul 2009, 14:01

Fanael

Joined: 03 Jul 2009
Posts: 168

Fanael 06 Jul 2009, 15:12

Maybe

Code:

movqq (%eax), %mm0 //;movq mm0, qword [eax]

? If it doesn't work, then try the following code:

Code:

.intel_syntax noprefix //;switch to Intel syntax
movq mm0, qword ptr [eax]
.att_syntax prefix //;switch back to AT&T syntax

06 Jul 2009, 15:12

pal

Joined: 26 Aug 2008
Posts: 227

pal 06 Jul 2009, 16:03

drhowarddrfine: Good point; I forgot it was based on the AT&T syntax.

Fanael: Good idea I guess; if I download GAS I'll try it.

Cheers.

06 Jul 2009, 16:03

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