this post was submitted on 31 Aug 2024
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not necessary FPGA but can be re-writable: see
https://electronics.stackexchange.com/questions/243712/eeprom-is-a-read-only-memory-so-why-can-i-write-to-it
I am not good in that hardware emu branch but my guess is that they pick something that can drive and matches original clock speed as the old programmable rom was no longer produced. (the antique people would buy old broken ones and rip parts off them or try to restore them.)
ROMs back then got erased by UV light, EPROM. EEPROMs are a bit newer (though still ancient) and can be erased electronically, nowadays it's a very sane idea to just throw flash storage at the problem. I think you can get modern replacement for pretty much any ancient form factor.
The way those things are used are basically big logic tables: Instead of using a bunch of logic gates, you store the output that's expected given a certain input. Completely ancient technique, the limiting factor is storage space and sensibility -- storing all addition results of two 32 bit numbers uses a lot more transistors than a 32-bit adder, but if what you want to put in there isn't a thing that can be implemented few standard TTL components throwing storage at the problem makes sense even if you never plan to reprogram it because burning a custom set of transistors onto silicon is expensive.
Thanks for letting me know about this logic table thing, that explains my question when younger why some old computers had massive array of same components put together.
ps. my first computer was a 80286 knock off. By the time I get to high school(basically 80386 era) that have a computer tech club where member bring their old computer parts to share, they are mostly no longer functional. I basically donated my old 80286's 20MB hard drive for tear down and that's first time me and other member see what it looks like inside a hard drive.