z386: An Open-Source 80386 Built Around Original Microcode

https://nand2mario.github.io/posts/2026/z386/

by wicket • 2 months ago

135 37 2 months ago

nand2mario.github.io

mmastrac• 2 months ago

Did the microcode disassembly find any useful backdoors to read microcode without decapping?

nand2mario3• 2 months ago

Not really. The 386 does not have an interface to read the ROM direclty. Instead, it uses the Built-In Self-Test (BIST) to verify the ROM's contents. It's basically a checksum-like mechanism that verify the integrity of the CROM.

NooneAtAll3• 2 months ago

there are several iterations of 386 and only one was disassembled

so there's still a chance

cbdevidal2• 2 months ago

Of course they tested Doom :-D

They might also run Linux kernel 3.7, that supported i386. Gray386linux is still maintained, and runs a patched 3.7 kernel.

https://github.com/marmolak/gray386linux

KellyCriterion2• 2 months ago

haha, this was my first thought when I read the headline, because this "classic test" always comes up here :-))

nand2mario3• 2 months ago

Doom (DOS extender games) is an easier protected mode application as it does not use complex page tables. Linux is much more complex.

cbdevidal2• 2 months ago

I have faith in you!!

RiverCrochet• 2 months ago

If modern x86 microcode ever gets sufficiently understood and reverse engineered I bet someone gets a PLAYDOOM instruction working. (I know it's been at least partially implemented on FPGAs)

KellyCriterion2• 2 months ago

Question: Are there today any 386 instances running somewhere in the basement to do some productive stuff, maybe processing only some controller data once a day?

I remember the link some month ago where that one small shop ran completely on an old Amiga (?IIRC, not sure, was linked here)

Around 98/99 I was involved in a small IT-management company serving SME around the region, we had a client producing distinct metal objects with a big press; this got feeded once a day with a 5.25 floppy from another machine with production data - and it was still in use while we had already ethernet/USB/3.5 floppies etc. :-D

bitwize3• 2 months ago

A few years ago there was a story where the single Amiga that ran an entire US school district's HVAC was replaced with a system costing like 1.5 million dollars, after 30 years of dutiful service.

I can't think of examples offhand but you bet your ass there are donut shops and auto body repair services running 386s to do POS, inventory, and the like. Some of them may be driving terminals off Xenix.

xp842• 2 months ago

Funny thing about this is that the character-based systems of that era, whether PC-type or host+terminal type, were most of the time so much faster and more responsive than the laggy, over-animated, touchscreen trash they always replace them with in order to get big screens and prettier graphics.

asdefghyk3• 2 months ago

RE "....a system costing like 1.5 million dollars, after 30 years of dutiful service....."

I immediately wondered ... how long the new system would last or be used .... and how long it would be problem free ?

AviationAtom• 2 months ago

You missed a fun part of that story! The person who programmed it was a kid in the district at the time. They continued to hire him to come back and maintain it any time they had issues, which apparently was fairly rare.

asdefghyk3• 2 months ago

RE "...A few years ago there was a story where the single Amiga that ran an entire US school district's HVAC....."

https://web.archive.org/web/20150616052358/http://www.geek.c...

From cira ..2018 ...

userbinator• 2 months ago

There's likely plenty of them still in use in industrial/embedded applications.

geon• 2 months ago

I think there are industrial pcs with a 486-compatible soc.

A ton of industrial equipment are still using win 3.1.

bzzzt• 2 months ago

Consequence of the 'if it works, why mess with it?' mindset. It will become a problem when those boards give out and spares are gone.

BirAdam• 2 months ago

No. That’s why companies like Vortex are still in business. There’s even a company producing new Win98 machines for this reason.

UncleOxidant2• 2 months ago

Kind of surprised this only takes 18K LUTs. That's a fairly small FPGA these days.

mycall• 2 months ago

How does it compare to something like VexRisc [0] with respect to its capabilities? I know having x86 support is a metric all by itself.

[0] https://github.com/spinalhdl/vexriscv

phire• 2 months ago

No contest really: RISCV is a much better ISA, VexRISC is a hyper-optimised implementation of it (for FPGAs), and it's not hindered by trying to be microcode compatible.

The roughly equivalent VexRISC configuration (full with MMU) is only 2736 LUTs, running at 124 Mhz (on Cyclone V, which I'm pretty sure is the same arch)

UncleOxidant2• 2 months ago

A lot of RISCV implementations are going to be better and take a lot less resources in an FPGA due to the much more-easily-decoded ISA. What I was saying above was that I was surprised that they could get an x86 into that few LUTs as the x86 ISA is not well suited to efficient implementations.

ChrisArchitect• 2 months ago

80386 Microcode Disassembled

https://news.ycombinator.com/item?id=48247004

dang• 2 months ago

Related ongoing thread:

80386 Microcode Disassembled - https://news.ycombinator.com/item?id=48247004 - May 2026 (42 comments)

spankibalt3• 2 months ago

Sadly, without an accompanying FPGA-implementation of an FPU it's much less useful for productivity work/research, i.e. outside of 08/15 gaming and application fare. Same with Ao486, which only implements a 486SX.

nand2mario3• 2 months ago

I think there are two reasons we haven’t seen an FPGA FPU yet, especially for MiSTer.

First, FPUs are complex and FPGA support for floating point is limited. There's DSP blocks for integer additions and multiplications. But very little FP support.

Second, the CPU itself may not be fast enough for an FPU to matter much. Quake wants at least a 75 MHz Pentium, while ao486-MiSTer is closer to a 486-66. So we probably need both a faster CPU design and a faster FPGA. Maybe Altera's new Agilex 5 will be useful here.

spankibalt3• 2 months ago

Aye. Well, hopefully something comes to pass as a lot of industry-specific applications demand the (emulated) physical presence of an FPU. Keeping that in mind, it's obviously high-level bitching (as we say in Germany); I, as a fan of IBM PCs and compatibles, am very fascinated by your and others' awesome work in this domain.

z386: An Open-Source 80386 Built Around Original Microcode

Related Stories