x86-64 AT&T versus Intel syntax
x86 (both 32- and 64-bit) has two alternative syntaxes available for it. Some assemblers can only work with one or the other, while a few can work with both. We will primarily use the AT&T syntax in this course, but you should be aware of the Intel syntax.
This table summarizes the main differences between AT&T and Intel syntax:
| AT&T | Intel | |
|---|---|---|
| Comments | // |
; |
| Instructions | Tagged with operand sizes: addq |
Untagged add |
| Registers | %eax,%ebx, etc. |
eax, ebx, etc. |
| Immediates | $0x100 | 0x100 |
| Indirect | (%eax) |
[eax] |
| General indirect | displacement(reg, reg, scale) |
[base + reg + reg * scale + displacement] |
Note that in the generalized indirect format, the Intel syntax will computer base + displacement for you, as the opcodes really only allow a single immediate displacement. In the AT&T syntax, you have to do this yourself, using the result of the sum as the third value in the parenthesis.
The main difference between Intel and AT&T syntax is that AT&T makes the sizes of the instruction operands explicit by appending suffixes to the instruction name, whereas Intel leaves the size implicit.