80386 Microcode Disassembled
reenigne.org84 points by nand2mario 2 hours ago
84 points by nand2mario 2 hours ago
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.
> ...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...
Here's a video of some guys de layering the chips for the Nintendo 64 lockout mechanism. It's pretty in-depth and it goes over a lot of different ways they do this.
The microcode is in a ROM. It's a regular structure where a 1 looks different to a 0.
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.
Right. And the best way to think about microcode is as code for a wacky, custom VLIW processor that implements the programmer-level x86 (in this case) instruction set. Various fields in the microcode send signals to different parts of the processor to activate them, routing values along internal busses and between registers, functional units and memory to cause the processor to execute the x86 instructions.
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 and the blog mentions John McMaster’s bitract: https://github.com/SiliconAnalysis/bitract although I think most people working on these projects usually just one-off it as the mentioned Discord users in the blog post eventually did.
More modern devices are of course more difficult due to layers, feature size, and less visually obvious ROM bit designs.
Anyway, the impressive part of this project was really understanding the undocumented microcode assembly language through inference and trace following; the 1s and 0s look like they were the easy part!
The black box analysis needed to decode this is incredibly hard but also incredibly fun and rewarding to pull off. Very impressive work.
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
Just building your own microprocessor from gates is an easier way to learn about designing microcode and understanding how processors work(ed). But it can't hurt to study a few simple old designs like RISC or Transputer. The 80386 is on the other side of that spectrum, needlessly complicated because they wanted to be backwards compatible with an old bad design.
There certainly is no need to go to university to learn chip design. Watching a few Alan Kay talks [3] or browsing Bitsavers computer designs [4] are good starting points.
We made an easier way (than FPGA) to simulate and convert your gate level design into transistors on a chip (for less than $200 in 2026). We call it Morphle Logic [1].
Eventually you grow into making the largest fastest and cheapest supercomputer wafer scale integration [2].
[1] https://github.com/fiberhood/MorphleLogic/blob/main/README_M...
[2]https://www.youtube.com/watch?v=vbqKClBwFwI
[3] https://www.youtube.com/watch?v=f1605Zmwek8
[4] http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/...
> needlessly complicated because they wanted to be backwards compatible with an old bad design.
It's not really needless complication of there is a reason for the complication. Obvioudsly in this case the need to be backward compatible with an old design made the implemtation more complicated than if they didn't need to do that. There were very, very strong business reasons why backward compatibility was a design requirment.
I did nand2tetris a couple times, but it emphasizes simplicity in every level of abstraction. That in itself is an amazing lesson and has been an inspiration, but that also means it skips things like microcode. In college (in the 1990s) I took a EE class as part of my CS degree that went through how an 8086-like[0] CPU is made, a lot like nand2tetris but without necessarily making each part an assignment. It did cover how microcode worked where there was an internal program counter that stepped through a table of control words whose bits directly orchestrated each controllable piece of the CPU. We each got an instruction to implement on a simulator that the teacher had made previously. (I got DEC, decrement.)
In a way I guess the instructions in nand2tetris are the microcode. The bits of the instructions directly control the hardware with the first bit choosing 2 instruction types, so there’s only 1 step of code per instruction, unlike with microcode where an instruction can have any number of microcode steps.
In Ben Eater’s series of videos building an 8-bit CPU on breadboards he has ROMs that are indexed by the opcode (4 bits of the instruction) + a step counter to determine the control word. The ROM stands in for what could be done with sufficiently complicated logic gates. I like it as a next step on the hardware side as you get hands on experience with electronics and having to troubleshoot it.
It’s disappointing how it only has 16 bytes of RAM so you can’t really build higher levels of abstraction like you can with nand2tetris. But at that point you could (I should) either redo it with a better design (and put it on PCBs) or move on to the 6502 project, and then since that puts together a timer, CPU, ROM, RAM, I/O, UART, etc. mentally group those together and move on to microcontrollers that already have them together.
Anyone interested in reading about how a CPU could be made out of logic gates could also read Code by Charles Petzold (moves slower, recently updated) and/or Pattern on the Stone by Danny Hillis (moves faster).
Edit: I just checked Code (2nd edition) and that uses a 4 bit cycle counter and hard logic gates to determine what to do each cycle. But then it uses an array of diodes for part of the logic. Would that be considered microcode?
[0] there were classes that covered more advanced (pipelined) CPUs in another CS class but not at quite a low level where you felt like you could make one yourself