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80386 microcode disassembled

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Re: 80386 microcode disassembled

#21

Post by mettamage »

drivers99 wrote: post #17
deskamess wrote: post #7

Earlier quoted context omitted.

Do you know if nand2tetris covers/uses microcode?

It doesn’t. I posted a reply to the same comment before I saw your question. Even the books I mentioned didn’t really get into it. I tried a search for some that did and ran across Constructing a Microprogrammed Computer by O.J. Mengali which looks interesting. It says it has you implement the microcode for 4 different architectures. I’m going to check it out.

Ah that's a shame. I had a computer systems course at uni where we were playing around with the microcode from the MIC-1 created by Tanenbaum. I sort of figured that Nand2Tetris just had that in it.

Re: 80386 microcode disassembled

#23

Post by photochemsyn »

bri3d wrote: post #14
liendolucas wrote: post #12

Earlier quoted context omitted.

So what you actually need is a program that navigates through the huge image of the die and detects if the structure that is looking at is a 1 or a 0? This at the fundamental level is a cross between machine learning and image processing?

Yes, exactly. Historically you would make some simple image processing software that will align the grid and then look for properties at each specific bit position. Usually die shots are highly imperfect (the delayering usually leaves some artifacts or damage) so frequently merging multiple scans is important as well. Travis Goodspeed has a neat tool for this workflow at https://github.com/travisgoodspeed/maskromtool…

The full workflow seems to look something like this, with the added complications relative to the 8086 microcode being that the 80386 microcode acts as an orchestration layer on top of hardwired engines, programmable logic arrays, and fault/protection redirection. The 8086 microcode does all that algorithmically, reusing the same hardware instead of having dedicated transistors.

1. Extract the ROM bits. 2. Determine physical-to-logical bit ordering. 3. Identify microinstruction boundaries. 4. Infer field boundaries. 5. Associate fields with hardware destinations (check with die tracing). 6. Decode instruction-dispatch programmable logic arrays. 7. Associate x86 instructions with microcode entry points. 8. Infer repeated idioms: moves, ALU ops, termination, calls, tests, redirects. 9. Decode accelerator protocols. 10. Validate against known architectural behavior.

Re: 80386 microcode disassembled

#25

Post by electroly »

liendolucas wrote: post #12
jdblair wrote: post #9

Earlier quoted context omitted.

Yes, literally this. No verilog decode, just looking for signals in the image of a 1 vs. a 0. For example, a 1 may be the existence of a transistor at a particular intersection of wiring.

So what you actually need is a program that navigates through the huge image of the die and detects if the structure that is looking at is a 1 or a 0? This at the fundamental level is a cross between machine learning and image processing?

I helped out on this image-to-bits transcription, doing manual verification of the automated work. I did the whole thing by hand: I sliced the ROM images into strips that excluded parts of the image that don't encode bits, used my tablet and stylus to manually place a black dot on every 1 bit, then wrote a trivial program that detected the presence or absence of the black dot in each cell. From my perspective, the ROM is organized like a series of "ladders" where the 1 bits are missing legs of the ladder, and I was placing dots on the missing legs. I compared my results with the ML output and manually re-checked each bit where we disagreed.

http://brianluft.com/images/2026/05/386_microcode_bits.jpg -- my fully annotated result. I was working from a higher-quality PNG; this is highly compressed because it's a big image.

Re: 80386 microcode disassembled

#29

Post by Levitating »

liendolucas wrote: post #5

> ...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…

Just look at the images[1].

> The photo above shows part of the microcode ROM. Under a microscope, the contents of the microcode ROM are visible, and the bits can be read out, based on the presence or absence of transistors in each position.

[1]: https://www.righto.com/2020/06/a-look-at-die-of-8086-process...

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