Many of you may recall the old DOS program--debug. Though outdated in many respects, one of the nice things about it was that one could easily find the byte-sequence for a given instruction without having to go through the steps of writing a program, compiling, disassembling, examining the file contents, .... Enter the instruction, then dump the instruction address. 'debug' regrettably does not do 32 bit instructions.
Does anyone know of a tool that does something similar for 32-bit x86 instructions? I don't want to go through the whole compile process; I just need to be able to enter a couple of instructions and have it spew out the length of the instruction and its byte sequence.
DOS debug was an interactive assembler as well as a debugger, entering assembly code resulted in that line being converted immediately to machine code - which is what you dumped out.
So all you need is to automate your favourite assembler with a script or batch-file.
Here's a bash function I came up with in a minute or two using the popular nasm assembler:
opcode() {
echo $* > tmp.S && nasm tmp.S -o tmp.o && od -x tmp.o
rm -f tmp.o tmp.S
}
Takes less than a second. Invocation looks like this:
$ opcode mov eax, [ebx]
0000000 6667 038b
0000004
$ opcode fadd st0,st1
0000000 c1d8
0000002
Not brilliant, but you can tweak od command-line for better output. This idea should work with any command-line assembler as long as you tell it to use a simple binary output format.