#!/bin/sh
# -*- mode: org -*-

# I was thinking folding-mode might provide a neat way to make a
# non-subroutine-factored assembly program readable in Emacs, but
# C-c @ C-q just is not a a usable keybinding.  Org-mode is more
# reasonable but its asterisks are not valid as assembly comments.

# But packaging this as a shell script that assembles the rest of
#  the file, minus the org-mode headers, does work:

sed 's/^\*.*//' <<'EOF' | as -o folded.o && ld folded.o -o folded && ls -l folded

* Preliminary setup for gas and whatnot
# These persuade gas to produce error messages pointing to the correct line number,
# a real time saver.  Note that the line number must be edited if you add any code
#  above this line, and without .file the .line directive fails to work:

.file "folded.s"
.line 20

* System calls
** System call numbers
.equiv __NR_exit, 1

** System call macros
*** sys1
.macro sys1 call_no, a
        be \a, %ebx
        sys0 \call_no
.endm

*** sys0
.macro sys0 call_no
        be \call_no, %eax
        ## There's a new, faster instruction for system calls, but I
        ## don't know how to use it yet.
        int $0x80
.endm

** `be`: Peephole-optimization of setting a register

### Set dest = src.  Usually just `mov src, dest`, but sometimes
### there's a shorter way.
.macro be src, dest
        .ifnc \src,\dest
	.ifc \src,$0
        xor \dest,\dest
        .else
        .ifc \src,$1
        xor \dest,\dest
        inc \dest
        .else
        .ifc \src,$2
        xor \dest,\dest
        inc \dest
        inc \dest
        .else
	mov \src, \dest
        .endif
        .endif
        .endif
        .endif
.endm

* Main program

.globl _start
_start:
    sys1 $__NR_exit, $42   # exit code 42