Metasm is a cross-architecture assembler, disassembler, linker, and debugger. It is written in such a way that it is easy to add support for new architectures. For now, the following architectures are in:

• Intel Ia32.txt (16 and 32bits)

• Intel X86_64.txt (aka Ia32 64bits, X64, AMD64)

• MIPS

• PowerPC

• Sh4

Supports low and high-level debugging support (Ia32 only for now) under Windows, Linux and remote (via a gdbserver). Metasm is included in Metasploit by default.

• Install Metasm gem

  gem install metasm

  More about installation here.

Converting Assembly to Op-code - metasm-shell.rb

You can find metasm-shell in ruby gems default path after installation. In my case, it's located in /var/lib/gems/2.1.0/gems/metasm-1.0.2/samples

• Run it

ruby metasm-shell.rb
type "exit" or "quit" to quit
use ";" for newline
​
asm>

as you can see you are now in the shell's prompt

• Find assembly op-code

asm> nop nop
"\x90\x90"
asm> call [eax]
"\xff\x10"
asm> push esp
"\x54"
asm> pop eax
"\x58"

Note: it is possible to do exactly the same thing with a metasploit embedded tool: nasm_shell.

$ /opt/metasploit/tools/exploit/nasm_shell.rb
nasm > jmp esp
00000000  FFE4              jmp esp