AS86 is a 80x86 assembler, both 16-bit and 32-bit, part of Bruce Evans' C Compiler (BCC). It has mostly Intel-syntax, though it differs slightly as for addressing modes.
A completely outdated version of AS86 is distributed by HJLu just to compile the Linux kernel, in a package named bin86 (current version 0.4), available in any Linux GCC repository. But I advise no one to use it for anything else but compiling Linux. This version supports only a hacked minix object file format, which is not supported by the GNU binutils or anything, and it has a few bugs in 32-bit mode, so you really should better keep it only for compiling Linux.
The most recent versions by Bruce Evans are published together with the FreeBSD distribution. Well, they were: I could not find the sources from distribution 2.1 on :( Hence, I put the sources at my place:
The Linux/8086 (aka ELKS) project is somehow maintaining bcc (though I don't think they included the 32-bit patches). See around (or ) and . I haven't followed these developments, and would appreciate a reader contributing on this topic.
Among other things, these more recent versions, unlike HJLu's, support Linux a.out format, so you can link you code to Linux programs, and/or use the usual tools from the GNU binutils package to manipulate your data. This version can co-exist without any harm with the previous one (see according question below).
BCC from 12 march 1995 and earlier version has a misfeature that makes all segment pushing/popping 16-bit, which is quite annoying when programming in 32-bit mode. I wrote a patch at a time when the TUNES Project used as86: . Bruce Evans accepted this patch, but since as far as I know he hasn't published a new release of bcc, the ones to ask about integrating it (if not done yet) are the ELKS developers.
Here's the GNU Makefile entry for using bcc to transform .s asm into both a.out .o object and .l listing:
%.o %.l: %.s bcc -3 -G -c -A-d -A-l -A$*.l -o $*.o $< |
Remove the %.l, -A-l, and -A$*.l, if you don't want any listing. If you want something else than a.out, you can see the docs of bcc about the other supported formats, and/or use the objcopy utility from the GNU binutils package.
The docs are what is included in the bcc package. I salvaged the man pages that used to be available from the FreeBSD site at . Maybe ELKS developers know better. When in doubt, the sources themselves are often a good docs: it's not very well commented, but the programming style is straightforward. You might try to see how as86 is used in ELKS or Tunes 0.0.0.25...
AS86 has some simple macro support, but I couldn't find docs. Now the sources are very straightforward, so if you're interested, you should understand them easily. If you need more than the basics, you should use an external filter (see below).
Linus is buried alive in mail, and since HJLu (official bin86 maintainer) chose to write hacks around an obsolete version of as86 instead of building clean code around the latest version, I don't think my patch for compiling Linux with a modern as86 has any chance to be accepted if resubmitted. Now, this shouldn't matter: just keep your as86 from the bin86 package in /usr/bin/, and let bcc install the good as86 as /usr/local/libexec/i386/bcc/as where it should be. You never need explicitly call this "good" as86, because bcc does everything right, including conversion to Linux a.out, when invoked with the right options; so assemble files exclusively with bcc as a frontend, not directly with as86.
Since GAS now supports 16-bit code, and NASM looks promising, maybe Linux will get rid of AS86, anyway? Who knows!