The 8085's register file reverse engineered

Note that in a wired-AND-capable current-mode logic, like open-collector TTL, you can get a similar RS latch out of a couple of inverters wired into a Henkin-sentence circuit like this, with the latch inputs being the latch outputs; but the behavior is slightly different than what Ken documents here. Here he says that, essentially, the 8085 tolerates bus-fight, but the bus drivers in the ALU are stronger than the bus drivers in the registers, and so after a brief bus fight, the register bit flips.

But with the wired-AND approach, bus fight doesn't exist; you just don't hook up any external zeroes to the Q and /Q lines if you want to read the latch, or to write it, you assert an external zero on one of them.

The trouble with open-collector logic, as I understand it (I've never built or debugged an open-collector circuit in my life) is that it's kind of slow. If I understand Ken's post, the circuit described here is a kind of optimization of the wired-AND latch I described above: there's a weak positive driver stage to speed up positive-going edges on the output. Is that right?

It's always interesting to see how real-world circuits depart from the purely digital, RTL ideal in order to save some resource. Jeri Ellsworth's work reverse-engineering old chips (which I hope to God is not secret!) found another example: dynamic stages in pipelines, where rather than using a flip-flop or even an entire latch, you just rely on the capacitance of an input to hold the previous value for long enough that the next stage in the pipeline can do its job. Works as long as you clock it fast enough, fails when you try to single-step the chip.

(Also, thank God, an actual hacking-related story on Hacker News, as opposed to "I lost 3lbs and a percent of body fat in a week" or "Every night when my dad would come home from work he would be filthy from working at the junkyard all day.")

http://zeptobars.ru/en/read/open-microchip-asic-what-inside-...

Really cool to see the different designs used in various components, or even compared to other CPUs/MCUs. The pictures above were taken by dissolving off the inline package casing with sulfuric acid. No 8085, but there's a picture of the Z80A available, and you can make out the similar areas to the 8085 (register file, control logic, ALU, etc... the quality is insane, the Z80A picture is 6200x6000 px). There's also some earlier photos at http://zeptobars.ru/en/read/how-to-open-microchip-asic-what-... if you want some more high-res silicon imagery.