gistfile1.rb

    asm.block do
      # low nybble of nth byte
      movzx(EBX, AL)
      and_(BL, 0x0f)        # isolate low nybble
      movzx(EDX, [:byte, ESI+EBX])
      mov([EDI], DL)
      dec(EDI)
      # high nybble of nth byte
      movzx(EBX, AL)
      and_(BL, 0xf0)        # isolate high nybble
      shr(BL, 4)
      mov(DL, [ESI+EBX])
      mov([EDI], DL)
      dec(EDI)
      shr(EAX, 8)
      loop_(loop_label)
      # write(int fd, char *s, int n)
      mov(EAX, 4)               # SYS_write
      lea(ECX, [var(h)])        # ecx = &s
      args = [1,                # fd = 1 (STDOUT)
              ECX,              # s = &s
              11]               # n = 11 (excluding term, max # of chars to print)
      case platform
      when 'darwin'             # on the stack, right to left (right @ highest addr)
        ####
        # setup bogus stack frame
        push(EBP)
        mov(EBP, ESP)
        sub(ESP, 36)
        ####
        args.reverse.each { |a| push(a) }
        push(EAX)
        int(0x80)
        ####
        # teardown bogus stack frame
        xor(EAX, EAX)
        add(ESP, 36)
        pop(EBX)
        leave
        ####
      when 'linux'
        mov(EBX, args[0])
        mov(ECX, args[1])
        mov(EDX, args[2])
        int(0x80)
      end
    end