What do the dollar ($) and percentage (%) signs represent in x86 assembly?

Question

I am trying to understand how the assembly language works for a micro-computer architecture class, and I keep facing different syntaxes in examples:

sub $48, %esp
mov %eax, 32(%esp)

What do these codes mean? What is the 32 operand an addition to the esp register?

Solution 1

Thats not Intel syntax, its AT&T syntax, also called GAS syntax.

the $ prefix is for immediates (constants), and the % prefix is for registers (they are required¹).

For more about AT&T syntax, see also the [att] tag wiki.

---

¹ Unless the noprefix option is specified, see here & here. But usually noprefix is only used with .intel_syntax noprefix, to get MASM-like syntax.

Solution 2

Compared to Intel syntax, AT&T syntax has many differences

• $ signifies a constant (integer literal). Without it the number is an absolute address
• % denotes a register
• The source/destination order is reversed
• () is used for memory reference, like [] in Intel syntax

So the above snippet is equivalent to

sub esp, 48         ; esp -= 48
mov [esp+32], eax   ; store eax to the value at the address `esp + 32`

Solution 3

Yes, "32(%esp)" indicates an offset of 32 from %esp.

Solution 4

As @Necrolis said, that's written in AT&T syntax. It means:

subtract 48 from the register esp (the stack pointer).
store the contents of eax to the four bytes starting at (esp + 32).

View more solutions

Comments

[juliensaad]

Thanks a lot, I did not know where to look for this

[Taryn East]

Hi - I'm not sure this actually answers the question... what are the $ and % ?

[phuclv]

@TarynEast they are prefixes for immidiates and registers like Necrolis has said

[phuclv]

the % prefix is not required if using noprefix directive stackoverflow.com/questions/549347/…

[Taryn East]

Your post showed up in the "low quality" review queue... for not having enough explanation in it. My recommendation was based on that... Feel free to edit it to say what you have said above ;)

[Necrolis]

@LưuVĩnhPhúc: Interesting, I've see that directive before, probably cause its extremely poorly documented :( sourceware.org/binutils/docs/as/i386_002dVariations.html

[Incerteza]

that's funny but I've never seen percent mark % being used.

[Ciro Santilli OurBigBook.com]

GAS calls it "Memory References", documented at: sourceware.org/binutils/docs-2.18/as/i386_002dMemory.html x86 concept summarized brilliantly at: en.wikipedia.org/wiki/X86#Addressing_modes