[jan-jaap]

Aside from that:
MIPS Risc/os: the thing that IRIX was based on in its early stages.

This. RISCos 4.52 media (binaries and source code) are on bitsavers.org. That's right, the AT&T System V R3 kernel, userland, standalone programs and firmware source. MAME can emulate the MIPS RC2030/RS2030 systems.

The very first MIPS IRIS was the 4D/60 "G". The compute part is the MIPS R2300 board set. I believe it's identical to what MIPS (the company) used in their MIPS M/500 system, which was the first MIPS UNIX workstation (and still supported by RISCos 4.52). The R2300 board in the 4D/60 had firmware which was of course the MIPS firmware with support for the "G" (CLOVER1) graphics. Later that year SGI released their first home made CPU board (IP4), a.k.a. the 4D/70. The IP4 firmware was of course mostly identical to the R2300 firmware.

I have RISCos running inside MAME. It can compile the kernel and most of the standalone code and firmware. This is RISCos 4.52 from 1991, roughly IRIX 4 days. So you're not looking at an ancestor of IRIX here, but something that evolved from a common source. Less developed, I'd say, RISCos doesn't even have shared libraries which IRIX 3 already had. The compiler is also largely the same if not identical.

Some other bits from the (very) early days can be found in the file sgidiag.tar.Z on bitsavers again. This is mostly for the original 68K IRIS, but inside are some (very) early MIPS binaries also:

* fx.old : fx version 1.1, @(#)Mips Computer Systems SAIO Version GL5-W5.0 MIPS OPT Tue Feb 3 13:51:32 PST 1987 sgi
That's from before the IRIX 1.0 release, presumably for the R2300 board set.

* fx.new: fx version 1.2, @(#)Mips Computer Systems SAIO Version 4D1-1.0 MIPS OPT Thu May 14 04:20:59 PDT 1987 sgi
* sash.new: sash v1.0, @(#)Mips Computer Systems Version 1.0 MIPS OPT Thu May 14 04:33:38 PDT 1987 SGI
4D1-1.0, a.k.a. IRIX 1.0 ... new

* sash.ip4: Version 2.0 IP4 OPT Fri Aug 28 03:59:26 PDT 1987 SGI
It seems that IRIX 2.0 already supported the IP4, but this predates the GT graphics (introduced 4/88) so this is for the 4D/70 "G"

There are also some kernels for R2300 and IP4 but they're newer I believe (4D1-3.2 maybe). Without userland it won't go very far.

When I get back to work on my Professional IRIS I'll try to boot this stuff, but it probably won't work because my GT graphics are too new... Interestingly, I believe the IP4 and R2300 boards are mostly identical as far as the OS is concerned so it may even be possible to boot (or port) RISCos on the Professional IRIS even if it wont support the graphics.

NB: since the SGI PROM firmware obviously derives from the AT&T codebase, it is pretty clear why it was never released.