Z80 Explorer – a Zilog Z80 netlist-level simulator | Hacker News Reader

Z80 Explorer – a Zilog Z80 netlist-level simulator

baltazarstudios.com

140 pointssegfaultbuserr13 comments

This is nuts. I learned Z80 as a kid and wrote tens of thousands of lines of code in it.

This isn’t just emulating the ISA, but the actual transistors. I could basically load the schematics of the Z80 on my computer and run it. That blows my mind. If Moore’s law keeps going, the simulation will be running at 2MHz in a few years and I guess I could play Meteor Mission II on it.

For some reason this really makes me feel very strongly that this is a simulator... within a simulator.

See also: Full simulation of MOS 6502 [0] by the Visual6502 team. The same team also did works on the Z80 [1], and it's actually the basis of the Z80 simulator presented in this article.

[0] http://visual6502.org/JSSim/index.html

[1] http://visual6502.org/JSSim/expert-z80.html

Shameless plug: I did a WASM "visual6502 remix" a little while ago with an integrated assembler and "execution history" (mostly for fun, but also because I found the original UI a bit hard to use for my emulator experiments, so I guess it was the same motivation as the Z80 Explorer project):

https://floooh.github.io/visual6502remix/

Eventually I want to do a version using the Z80 netlist of the visual6502 project as well (the actual transistor simulation code should remain the same, I used a C port of the visual6502 Javascript code called 'perfect6502'):

https://github.com/mist64/perfect6502

...I heard though that not all 'trap transistors' which should protect the Z80 from reverse engineering had been found and 'neutralized' in that netlist, so even though the ZEX test is running I'm not entirely sure whether the simulation still contains hidden problems (neither ZEXDOC nor ZEXALL have complete "coverage" of the Z80).

Small factoid. Microsoft's first foray into hardware was the Z80 Softcard for Apple ][, which was a plug in card that introduced Z80 and 6502 functionality. This was in 1980 and cost $365. A premium version also added 16k of RAM to the Apple ][ line.

https://www.versionmuseum.com/history-of/discontinued-micros...

I've used that, a few years later, and really enjoyed it. At the time I've used it Turbo Pascal for CP/M was also available and it was pure magic, practically instantaneous compared to anything else. Even then, that environment allowed me to write the program in the editor, compile it and run all in RAM, even if the RAM was smaller than 64 KB. I'd write something new, press just a few keys, and it either runs and writes the output or it brings me back to the editor and the line with an error, without any additional steps. The last time I've checked, not even the latest IDEs did the last step so smoothly -- I'd have to click around in the output window before.

This is getting a bit off-track now, but I did use Turbo Pascal in the mid eighties (on an Amstrad CPC464) too. That sure was an impressive tool, but it seems to get more impressive as more time passes (and progress in IDEs trails, perhaps inflated, expectations).

Thanks to the author for this OCD-level work. I always find it interesting that netlists out of die images are precise enough to run hardcore test routines.

Z80 has a special place in my heart because of Speccy.

To give proper credit (I think it's also mentioned in the blog post), the Z80 netlist was actually extracted by the visual6502 project. The data files are available in the visual6502 github repository:

https://github.com/trebonian/visual6502

I love this project, having done a fair bit of Z80 programming myself. For all it's baroqueness, the Z80 is still in common use in high schools and universities in the form of TI-84 calculator series. Looks like this project has a ZX Spectrum display emulator, it would be interesting to see if other peripherals would be possible as well.

Why not make a Verilog extractor? Then we can simulate it with standard tools. I'm pretty sure Verilog can simulate NMOS since it has pullups.

Another option is to extract a Spice netlist.

Generally speaking, Verilog simulation tools don't support the transistor-level primitives. You have to move up to at least the gate level to get something that'll simulate at all; even then, certain special combinations of gates (like registers) have to be specifically recognized to break combinational loops.

This was done for the 6502 from the Visual 6502 netlist, I suspect it could be done from the Z80:

http://www.aholme.co.uk/6502/Main.htm

But it does look like a lot of work, and it's not like you get especially readable Verilog from it.

Given other people have already built cycle accurate cores for these processors that are readable and maintained, it may not be worth it.

The 6502 conversion was especially difficult, as the processor made use of some wacky analog tricks (like capacitive storage) which can't be represented in Verilog at all. The author of that conversion had to do a great deal of manual work to detect these features and convert them to digital logic.

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