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.
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:
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Intex Syntax mov eax,1 mov ebx,0ffh int 80h |
AT&T Syntax movl $1,%eax movl $0xff,%ebx int $0x80 |
Direction of Operands.
The direction of the operands in Intel syntax is opposite from that of AT&T syntax. In Intel syntax the first operand is the destination, and the second operand is the source whereas in AT&T syntax the first operand is the source and the second operand is the destination. The advantage of AT&T syntax in this situation is obvious. We read from left to right, we write from left to right, so this way is only natural.
Example:
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Intex Syntax instr dest,source mov eax,[ecx] |
AT&T Syntax instr source,dest movl (%ecx),%eax |
Memory Operands.
Memory operands as seen above are different also. In Intel syntax the base register is enclosed in '[' and ']' whereas in AT&T syntax it is enclosed in '(' and ')'.
Example:
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Intex Syntax mov eax,[ebx] mov eax,[ebx+3] |
AT&T Syntax movl (%ebx),%eax movl 3(%ebx),%eax |
The AT&T form for instructions involving complex operations is very obscure compared to Intel syntax. The Intel syntax form of these is segreg:[base+index*scale+disp]. The AT&T syntax form is %segreg:disp(base,index,scale).
Index/scale/disp/segreg are all optional and can simply be left out. Scale, if not specified and index is specified, defaults to 1. Segreg depends on the instruction and whether the app is being run in real mode or pmode. In real mode it depends on the instruction whereas in pmode its unnecessary. Immediate data used should not '$' prefixed in AT&T when used for scale/disp.
Example:
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Intel Syntax instr foo,segreg:[base+index*scale+disp] mov eax,[ebx+20h] add eax,[ebx+ecx*2h lea eax,[ebx+ecx] sub eax,[ebx+ecx*4h-20h] |
AT&T Syntax instr %segreg:disp(base,index,scale),foo movl 0x20(%ebx),%eax addl (%ebx,%ecx,0x2),%eax leal (%ebx,%ecx),%eax 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).
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:
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Intel Syntax mov al,bl
mov ax,bx
mov eax,ebx
mov eax, dword ptr [ebx]
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AT&T Syntax movb %bl,%al
movw %bx,%ax
movl %ebx,%eax
movl (%ebx),%eax
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