80386 microcode disassembled

mettamage

For me, this is peak Hacker News. I am happy I took the hard courses at uni to understand a post like this. I’m also happy that HN was there to stimulate this thinking at the time (2015). Even if I now don’t really do anything with my humble knowledge of low level programming, every time it feels consciousnesses enriching. And it’s an awesome feeling. For people that don’t have access to a uni, I recommend nand2tetris.org

bmenrigh

The black box analysis needed to decode this is incredibly hard but also incredibly fun and rewarding to pull off. Very impressive work.

liendolucas

> ...they mentioned that it would be interesting to get high resolution images of the 80386 die and try to extract the microcode from it. Can someone explain how is that from a high resolution image of the die the microcode can be reconstructed? I'm really curious, what's the process? Is the output some sort of Verilog? Does the process involve recognizing each and every transistor and model a circuit from that? I'm fascinated that something like this is possible at all...

trollbridge

I checked reenigne's blog a few days ago. "Hmm, nothing posted since 2020. Oh well." It's especially fun seeing his blog going back 33 years .

p1esk

Here’s a great book explaining microprogramming from ground up: https://www.amazon.com/Computation-Structures-Optical-Electr... Easy to find a free pdf

yukIttEft

If you put this into an emulator, would it boot linux?

Dwedit

Meanwhile the original ARM didn't use any microcode at all.

danborn26

This is an incredible piece of reverse engineering. Seeing the actual microcode implementation helps demystify how these older processors handled complex operations.

Levitating

I wonder if an OpenFletcher[1] would be able to get such images [1]: https://openflexure.org/projects/microscope/

themafia

Wow. Virtual86 modes, the floating point unit, and memory paging really created an explosion of complexity within the microcode. There's sort of a wild west nostalgia that came with the 8086 and 8088 chips and a sense of approachable individual adventure that came along with it. Staring into the 386 is like staring into the cold and dispassionate industrial machine future that Fritz Lang was trying to portray in Metropolis. Still fun to look at though. Great post.

ChrisArchitect

Related: z386: An Open-Source 80386 Built Around Original Microcode https://news.ycombinator.com/item?id=48248014

dang

Related ongoing thread: z386: An Open-Source 80386 Built Around Original Microcode - https://news.ycombinator.com/item?id=48248014 - May 2026 (22 comments)

cobbzilla

beautiful work! any plans for the 80387 coprocessor?

userbinator

I agree with the first comment there, that it's important to know which revision of the 386 this came from, since the 386 did receive many small changes over its 22-year production run.