alt.hn

2/8/2026 at 10:25:25 PM

Reverse Engineering the Prom for the SGI O2

 https://mattst88.com/blog/2026/02/08/Reverse_Engineering_the_PROM_for_the_SGI_O2/

by mattst88

2/8/2026 at 10:56:09 PM

In the PC world this would be known as "BIOS modding".

The first two instructions looked legitimate, but the third looked unlikely to be a real instruction.

Given that the first appears to be a branch, that's not surprising. When disassembling, not following the flow will likely not give you anything meaningful. If the author is reading this: have you tried Ghidra?

That said, this seems a lot simpler than PC BIOSes in structure, as the latter are usually written in a combination of C and Asm (I can see why no one wanted to write MIPS Asm) and are self-extracting compressed archives.

by userbinator

2/9/2026 at 8:50:27 PM

> I can see why no one wanted to write MIPS Asm

At least in comparison to x86 assembly, MIPS assembly seemed very elegant and rich to me at the time. I wanna say that MIP R4K had 32 integer registers and 16/32 double- or single-precision float registers, but don't quote me on that. Either way, it was an embarrassment of riches :)

by nathan_douglas

2/8/2026 at 10:52:35 PM

I often see superbly restored SGI equipment at VCF and also own a few SGI equipment that I hope to get to some point in my life but I have never seen any interesting new software or usage of these machines other than the stock "cool" demo programs(The file manager, the gears demo and others running at the same time). Is there any actual cool homebrew occuring on these platforms?

by nebula8804

2/8/2026 at 11:59:19 PM

I think the lack of a real usable emulator for SGIs is holding back any kind of homebrew. I say this as one of the developer's that got SGI Indy emulation working in MAME. Yes, it works, but it's too slow and too old to be usable. I spent some time after the MAME effort working on a custom high performance emulator for Crimson/Onyx/Reality Engine, but I've kind of burned out again. Maybe some day if I'm really driven again, and had help. I've done most of the reverse engineering already, it's just a lot of code.

I think that if a high performance, usable emulator for some of the big systems existed I think some of the old software might be rediscovered and show up on the internet.

by tyfighter

2/9/2026 at 1:05:28 AM

I think the problem was that the machines were always very expensive, even used.

My Fuel has an SSD and Id use it daily except:

- It's loud

- It's single core

- It's a furnace

- It's very very loud

It has a fairly modern Emacs, ssh and a non distracting UX. The browser is the only real thing that is too old to be useful, feature and performance wise, but that's just bonus points productivity wise (besides, rdesktop into a modern machine and you can watch youtube)

If I had a 900 MHz O2 loaded with RAM, and an SSD (SCSI SSD, ha!) it'd probably be my daily driver.

by rngfnby

2/9/2026 at 6:14:55 AM

I have a 600MHz RM7k O2, and my 700MHz R16k Fuel blows it out of the water. The O2 isn't that quiet or that quick even with upgrades!

What SSD are you running? I'm still on 10k SCSI drives selected for the quietness of their bearings.

by theodric

2/9/2026 at 12:58:13 PM

Right now I cant get to the machine (off, in the basement) but it is some run-of-the-mill SATA drives using a SATA expansion card.

It works great but I just use it for /opt since I ran out time to move more of the machine into it.

You cant boot off the SSD, so I still use a SCSI but you can replace that too if you boot the SGI off the network.

Silent SGI:

Having gotten rid of the SCSI drives completely w/ the network boot, you can put a modern, more silent, PSU [1] (but hurry, ones w/ enough current on 5V (?) are rare), and then replace the GPU and CPU fans and turn of environmental monitoring.

[1] i had to replace mine; my 500 MHz Fuel is notorious for bad psu

by rngfnby

2/9/2026 at 5:35:07 PM

Do not turn off environmental monitoring. That's for debugging only. That's how people are cooking the video cards. Please get your fuel /properly/ repaired by say weblacky on irixnet. The reason why? With env monitoring off, the system won't respond to overheating on the graphics card and it'll cook it alive. The fuel has notoriously bad airflow (air doesn't move right angles)

by ThatGuyRaion

2/9/2026 at 6:21:40 PM

[dead]

by rngfnby

2/9/2026 at 3:52:45 PM

You could use the NVME driver to load the filesystem and boot the kernel diskless...

by ThatGuyRaion

2/9/2026 at 7:58:28 AM

Yes, I have a restored Indigo 2. I fixed at lot of things in the PSU.

And yes:

- It's loud

- It's single core

- It's a furnace

- It's very very loud

:-D

by Aldipower

2/9/2026 at 3:48:52 PM

I keep my Challenge S running 24/7 :)

by ThatGuyRaion

2/9/2026 at 3:48:27 PM

Your contributions to the Indy alongside Ryan's contributions were neat, truly. You plowed the road so others can navigate it. There's a rumor about a faster Indy emulator... but don't hold your breath yet. (Not a project I'm part of, but I've been told snippets)

The OS/hardware though, has serious limitations that while no problem for me, definitely pisses off people. Examples:

No atomics/Thread local support. Doesn't matter that someone ported GCC 15 -- you can't make use of many useful newer language features.

Immediate Mode OpenGL only. There's no direct hardware access. Not a problem for me, but every SGI out there is fixed function only. I've had people bitch to high hell we don't have shaders.

and in general, some people just think the OS is janky. I love it, but not everyone is me.

by ThatGuyRaion

2/9/2026 at 4:11:25 PM

> Not a problem for me, but every SGI out there is fixed function only.

Is that true? I remember sgi had a shader library for modeling light aimed at the automotive market. All the demos and examples were showing off car paint colours in different environments.

by Grosvenor

2/9/2026 at 3:03:05 AM

They got the N64 running on the MiSTer. So an indy should be possible, they're closely related systems.

I'd love an Onyx/RE on an FPGA someday. Next to my FPGA cray.

by Grosvenor

2/9/2026 at 5:20:03 AM

The CPUs are close, but the Indy is otherwise pretty different from the N64. Totally different graphics architecture, and - relevant to getting it on MiSTer - it’s a workstation rather than a video game console, necessitating quite a bit more complexity. I’d be really surprised if it could be squeezed on.

(Though, full disclosure, I said the same thing about the N64 before the core for it came out - the folks working on MiSTer are incredible.)

by wk_end

2/9/2026 at 5:37:45 AM

Huh. I had thought the n64 was basically an Indy xz graphics. What was the rcp closest to?

I was always confused why sgi didn’t throw the rcp on a pci card and dominate the pc graphics market.

by Grosvenor

2/9/2026 at 7:31:52 AM

To my knowledge - and I'm not an expert here - the N64 hardware is pretty unique and doesn't really resemble any of SGI's other chipsets. Not in precise capabilities - the XZ, for instance, didn't even support hardware texture mapping - and not in overall technical design.

It does seem a little bit like an ultra-simplified, integrated version of the RealityEngine [0]. The RealityEngine had "6, 8, or 12 Geometry Engines" split out across three to six boards, each powered by an Intel i860XP, that then passed their work along to Fragment Generators. This roughly corresponds to the RSP, which was just another MIPS core (with vector/matrix extensions), passing its work along to the RDP on the N64. I'm not sure how programmable the RealityEngine's pipeline was compared to the surprisingly flexible RSP.

Remember, the constraints for a graphics workstation are really different than for a game console - especially on the low-end, totally different corners are going to be cut. An Indy still needed to be able to generate a high resolution display and allow modelling complex scenes for film and TV; but while some degree of real-time 3D was important, it was expected that artists could be modelling using wireframe or simplified displays. A game console was displaying low-resolution and relatively low-detail scenes, but they still wanted them to look aesthetically "complete" - shading, textures, fog, lighting, particles - while running at real-time speeds. SGI used their expertise and built something custom-fit for the job at hand, rather than just reusing an existing solution.

[0] https://cseweb.ucsd.edu/~ravir/274/15/papers/p109-akeley.pdf

by wk_end

2/10/2026 at 1:21:03 AM

I would have loved to have that paper when I was learning 3D and OpenGL.

by Grosvenor

2/9/2026 at 3:50:30 PM

Nay, the N64 is pretty unique hardware-wise. Conceptually it's vaguely similar to the O2, the RCP is an R4000 fixed function CPU with some extra graphics instructions IIRC.

by ThatGuyRaion

2/8/2026 at 11:11:35 PM

I'm not aware of any cool homebrew, but there is a certain level of cool being able to compile the code for some N64 games using the original IDO compiler on original hardware. You can even compile one of the many decompiled games like Super Mario 64, Banjo Kazooie and more that all will produce the exact binary shipped on the cartridge byte for byte, all from reverse engineered work to create byte matching equivalent C code.

by foldor

2/9/2026 at 3:49:21 PM

It runs like arse though.

I'm working on some OpenGL stuff at least.

by ThatGuyRaion

2/8/2026 at 11:17:52 PM

Yes! I don't use my O2 a lot (I think the PSU is flaky, and I'm not super interested in IRIX), but I'm aware of at least https://forums.sgi.sh/index.php, among other similar sites, full of people porting/developing software for IRIX. It's a pretty active community for a 90s workstation platform, the most active one I'm aware of!

by spijdar

2/9/2026 at 12:47:57 AM

The main watering hole for the hobbyist community around these machines vanished from the internet a while back, taking the forums and info around porting software with it.. I guess some of it is available via wayback etc.

by jrmcauliffe

2/9/2026 at 1:34:09 AM

Awesome work! Enriching the disassembly with known constants and labels is great, great stuff.

As somebody else suggested, try Ghidra's decompiler. It produces very sloppy C code, but still reads faster than assembly most of the time.

Now enriching Ghidra's decompiler output to clean up the C code, that would be a neat trick, and one that Ghidra isn't doing.

by the_biot

2/9/2026 at 2:09:45 PM

I've loaded the PROM in Ghidra. The ability to decompile to C is great.

I looked at some particular parts of the PROM that took a while to understand to see if I could have understood them quicker with Ghidra. In particular, the part of `sloader` that searches for the `post1` and `firmware` sections and then calls `post1(&firmware)`. Given that I already understand how this works, I can see that this is happening from the decompiled C, but the lack of labels, comments, etc really hampers my ability to understand from the decompiled C alone.

This might all be down to inexperience with the tool.

The ability to iteratively add a label, rerun the decompiler, reread the decompiled assembly, make more inferences was really the key to building an understanding for me.

Another aspect I'm unsure how to handle in Ghidra is that the base address differs between sections of the firmware. E.g. the `firmware` section is copied to RAM and executed from `0x81000000`. It's not clear to me how to configure this in a granular way, rather than a single base address for the whole PROM image.

by mattst88

2/9/2026 at 4:31:34 PM

Annotating the decompiled code is very much part of the workflow in using Ghidra, but the UI is not really intuitive. If you right-click on variables, function names etc there are a bunch of options for annotation. You can also edit the function signature, which can cause the decompilation to improve.

I know you can do various base address manipulations, but your case seems like it would be hard -- the base address is in the file, or has to be calculated.

by the_biot

2/9/2026 at 1:58:14 PM

>Enriching the disassembly with known constants and labels is great

speaking of PC BIOSes there is/was a great disassembler called Sourcer https://corexor.wordpress.com/2015/12/09/sourcer-and-windows... with its Bios Preprocessor producing very readable sources full of comments and enumerated hardware IO.

Author of Sourcer Frank van Gilluwe also wrote a somewhat companion book "The Undocumented PC, A Programmer's Guide to I/O, CPUs, and Fixed Memory Areas".

by rasz

2/9/2026 at 2:36:51 AM

Thanks!

I'll give Ghidra a try!

by mattst88

2/9/2026 at 7:40:25 AM

The successor to SGI, after several acquisitions and bankruptcies, is Hewlett Packard Enterprise. There's a forum for abandoned HP products.[1] The SGI O2 has been mentioned, but not in recent years.

[1] https://community.hpe.com

by Animats

2/9/2026 at 12:44:37 AM

Oh, the PROM, not the prom.

by hinkley

2/9/2026 at 1:10:54 AM

Yes, I submitted it as "PROM". I have no idea why it has been changed.

by mattst88

2/9/2026 at 5:31:17 AM

I think it’s to prevent SHOUTY TITLES. I also think there are some hardcoded acronyms that are safe.

by MBCook

2/9/2026 at 3:40:53 PM

Nice work Matt.

by ThatGuyRaion

2/9/2026 at 2:37:27 AM

I for one is awaiting for the world to completely decompile and or reverse engineering the IBM mainframe microcodes for all their machines.

Number 1 because Mainframes without the microcode is sent to the junkyard.

by unixhero

2/9/2026 at 3:55:20 PM

"We weep for the blood of a bird, but not for the blood of a fish. Blessed are those with a voice."

Applies here. SGI hardware holds interest because "ooh pretty animations/GL". IBM is great stuff, but it's all workhorses.

by ThatGuyRaion