I have been looking for an practical tool that would print the opcodes of any Intel 64-bit or 32-bit instruction in Linux, eg. something like Hiew's assembler in DOS. A web-based service would be one option too.
As I wasn't able to find any, I made my own bash script, that creates an assembly source file from command line parameters (instruction[s] and <32/64>), compiles, links and disassembles it and shows the correct rows of disassembly. But is there already some program that would show all the possible encodings for any given instruction, eg. for mov eax,ebx? My approach using nasm, ld and ndisasm obviously only gives one possible encoding for each instruction.
With this script I can get the encodings used by nasm for 64 and 32-bit code, eg:
/home/user/code/asm$ showop 'nop;add eax,ebx;cpuid' 64
00000000 90 nop
00000001 01D8 add eax,ebx
00000003 0FA2 cpuid
But how could I get easily all the possible opcode encodings? Is there already some program available for that?
Here's the code:
#!/bin/bash
# usage: showop instructions bits
asminstr=$1
bits=$2
# asminstr="nop;nop;nop;nop;add eax,ebx;nop;nop;nop"
# bits=64
numberofinstr=`echo $asminstr | grep -o ";" | wc -l`
((numberofinstr++))
if [ -f tempasmfile.asm ]
then
rm tempasmfile.asm
fi
if [ -f tempobjfile.o ]
then
rm tempobjfile.o
fi
if [ -f tempexefile ]
then
rm tempexefile
fi
printf "[bits $bits]\nsection .text\nglobal _start\n\n_start:\n`echo $asminstr | sed 's/;/\\n/g'`\n" >tempasmfile.asm
nasm -f elf$bits tempasmfile.asm -o tempobjfile.o
ld tempobjfile.o -o tempexefile
if [ $bits -eq 32 ]
then
ndisasm -b $bits -e 0x60 tempexefile | head -n $numberofinstr
elif [ $bits -eq 64 ]
then
ndisasm -b $bits -e 0x80 tempexefile | head -n $numberofinstr
fi
rm tempasmfile.asm
rm tempobjfile.o
rm tempexefile