Introduce the Intel and AT&T Syntax about assembly language

Intel and AT&T Syntax

Intel and AT&T syntax Assembly language are very different from each other in appearance,
and this will lead to confusion when one first comes across AT&T syntax after having learnt Intel syntax first, or vice versa.
So lets start with the basics. Linux use AT&T syntax default.

1.1 Direction of Operands

The direction of the operands in Intel syntax is opposite from that of AT&T syntax.
In Intel syntax the first operand is the destination, and the second operand is the source whereas in AT&T syntax the first operand is the source and the second operand is the destination.
The advantage of AT&T syntax in this situation is obvious.
We read from left to right, we write from left to right, so this way is only natural.

Example:

Intel Syntax AT&T Syntax
instr dest,source instr source,dest
mov eax,[ecx] movl (%ecx),%eax

1.2 Prefixes

In Intel syntax there are no register prefixes or immed prefixes.
In AT&T however registers are prefixed with a ‘%’ and immed’s are prefixed with a ‘$’.
Intel syntax hexadecimal or binary immed data are suffixed with ‘h’ and ‘b’ respectively.
Also if the first hexadecimal digit is a letter then the value is prefixed by a ‘0’.

Example:

Intel Syntax AT&T Syntax
mov eax, 1 movl $1, %eax
mov ebx, 0ffh movl $0xff, %ebx
int 80h int $0x80

1.3 Memory Operands

Memory operands as seen above are different also. In Intel syntax the base register is enclosed in ‘[’ and ‘]’ whereas in AT&T syntax it is enclosed in ‘(’ and ‘)’.

Example:

Intel Syntax AT&T Syntax
mov eax, [ebx] movl (%ebx), %eax
mov eax, [ebx+3] movl 3(%ebx), %eax

The AT&T form for instructions involving complex operations is very obscure compared to Intel syntax. The Intel syntax form of these is segreg:[base+index*scale+disp]. The AT&T syntax form is %segreg: disp(base,index,scale).

Index/scale/disp/segreg are all optional and can simply be left out. Scale, if not specified and index is specified, defaults to 1. Segreg depends on the instruction and whether the app is being run in real mode or pmode. In real mode it depends on the instruction whereas in pmode its unnecessary. Immediate data used should not ‘$’ prefixed in AT&T when used for scale/disp.

Example:

Intel Syntax AT&T Syntax
instr foo,segreg:[base+index*scale+disp] instr %segreg:disp(base,index,scale), foo
mov eax, [ebx+20h] movl 0x20(%ebx), %eax
add eax, [ebx+ecx*2h] addl (%ebx,%ecx,0x2), %eax
lea eax, [ebx+ecx] leal (%ebx,%ecx), %eax
sub eax, [ebx+ecx*4h-20h] subl -0x20(%ebx,%ecx,0x4), %eax

As you can see, AT&T is very obscure. [base+index*scale+disp] makes more sense at a glance than disp(base,index,scale).

1.4 Suffixes

As you may have noticed, the AT&T syntax mnemonics have a suffix. The significance of this suffix is that of operand size. ’l’ is for long, ‘w’ is for word, and ‘b’ is for byte. Intel syntax has similar directives for use with memory operands, i.e. byte ptr, word ptr, dword ptr. “dword” of course corresponding to “long”. This is similar to type casting in C but it doesnt seem to be necessary since the size of registers used is the assumed datatype.

Example:

Intel Syntax AT&T Syntax
mov al,bl movb %bl,%al
mov ax,bx movw %bx,%ax
mov eax,ebx movl %ebx,%eax
mov eax, dword ptr [ebx] movl (%ebx),%eax