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Opcode syntax: Difference between revisions
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== Important Details ==
There are some substantial differences between the AT&T syntax and the Intel
Here are a few key things to look for, when moving to AT&T syntax:
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*'''Escapes:''' Special characters are written as C-style escapes (\n, \", \#, \\, ...).
*'''Comments:''' Either C-style ( /* ... */ ) or shell style (# ...).
*'''Directive syntax:''' Directives begin with a period (".align 4" to align on a
*'''Strings:''' Defined using special directives, .ascii (or .asciz for a zero-terminated string). Example: msg: .ascii "Hello, World!\n"
*'''Current location address:''' Indicated by a period (".", equivalent to Intel syntax "$").
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The AT&T syntax format for macros:
<syntaxhighlight lang="asm">
.macro <name> <args>
<operations>
.endm
</syntaxhighlight>
Example:
<syntaxhighlight lang="asm">
.macro write string
movw string, %si
call printstr
.endm
</syntaxhighlight>
This would be equivalent to the NASM macro:
<syntaxhighlight lang="asm">
%macro write 1
mov si, %1
call printstr
%endmacro
</syntaxhighlight>
Additionally, the cpp and
== Converting small snippets of code from Intel syntax to AT&T ==
You can use the following script to convert short snippets of code (one liners) from Intel syntax to AT&T syntax:
== Sources ==▼
<syntaxhighlight lang="bash">#!/bin/bash
set -e
# Usage:
[http://www.delorie.com/djgpp/v2faq/faq17_1.html DJGPP AT&T Assembly Tutorial]<br>▼
#
[http://asm.sourceforge.net//howto/Assembly-HOWTO.html Linux Assembly HOWTO]<br>▼
# ./inteltoatt [16|32|64] "mov eax, eax \n xor ecx, edx"
[https://savannah.nongnu.org/projects/gas-user/ GAS/AS End User Help Project]<br>▼
#
[http://savannah.nongnu.org/projects/pgubook/ Programming from the Ground Up]<br>▼
case "$1" in
16|32|64)
bits="$1"
shift ;;
*)
bits="32" ;;
esac
code="$1"
nasm="$(mktemp)"
obj="$(mktemp)"
objdump="$(mktemp)"
case "$bits" in
16) m="i8086" ;;
32) m="i386" ;;
64) m="i386:x86-64" ;;
esac
echo -e "BITS $bits\n$code" > "$nasm"
nasm "$nasm" -o "$obj"
objdump -D -b binary -m $m -Maddr${bits},data${bits} "$obj" > "$objdump"
lineno="$(egrep -m 1 -n '<\.data>\:$' "$objdump" | cut -d':' -f1)"
lineno=$((lineno+1))
tail -n +$lineno "$objdump"
</syntaxhighlight>
=== External Links ===
[[Category:Assembly]]
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