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Using a foreign CPU architecture for a co-processor was a very common practice in the 90s, even early 2000s; e.g. the GameBoy Advance has a regular ARM CPU as it's brain, but it also hosts an entire GBC on board (including a custom Z80/8080 clone/hybrid) - mostly for backward compatibility, but it performs other regular duties (such as sound) even in "GBA mode".

GBA is a fascinating system, the main CPU is powerful enough to do some serious work - some games even had pretty decent 3D (check out Colin McRae Rally 2.0); all while remaining simple enough architecturally that you could mostly keep the whole thing mapped out in your head. You get to learn about many modern concepts such as instruction pipelining, caches, etc but it's not as complex as to overwhelm you, and you still get to play with funky quirky stuff like the sound chip, that throws you back into the 1980s. You can also still get them pretty cheap, I paid ~€120 for a GBA SP (AGS-101) with case, dongles & a flash cart, so I can trivially run homebrew stuff on real hardware.

https://en.wikipedia.org/wiki/Game_Boy_Advance; https://duckduckgo.com/?t=ffab&q=colin+mcrae+rally+gba&iax=i...; https://gbadev.org ; https://docs.rs/gba/latest/gba/

Nintendo DS did something similar, with the GBA’s ARM7 running in conjunction with the DS’s ARM9 and acting largely as an I/O coprocessor (IIRC) and handling backwards compatibility.

The Wii and Wii U go a little further; in addition to the main PPC CPU, there’s an ARM core handling auxiliary tasks (mostly security and I/O related, but it’s also involved in early boot IIRC). That one’s not there for backwards compatibility; it’s purely there as a coprocessor.

The 6502-based BBC Micro had this in the 80s as the Tube. This could run a Z80 and other processors.

The interface was initially used in the development of the Arm processor: https://en.wikipedia.org/wiki/Tube_(BBC_Micro)

> E.g. Commodore designs "borrowed" this from bigger computer systems when they put fully programmable 6502 based computers in their external floppy drives for their 8-bit line (you can load code into the memory of the 1541 floppy drive and execute it)

Yup Gwern mentions it in TFA. I had a Commodore 128 (well I still have it and it's still working!): Z80 chip in addition to the MOS ones (not just the CPU but the soundchip too).

I'd say though that back then none of these chips were added due to committees being infiltrated by three letters agencies or their chinese equivalent with the goal of introducing backdoors.

Bil Herd mentions in a few re-tellings that the C128 had power problems when they tried to use the CP/M cartridge originally intended for the C64, so it got "absorbed" into the design instead.

You should not forget Agnus, Gary, Paula, and Denise [1] one the main innovation with Commodore Amiga was the inclusion of dedicated processors for video and audio.

[1] https://ancientelectronics.wordpress.com/tag/amiga-2000/

Of those only the Copper is programmable though, and only marginally so.

I knew someone who even in the early 90s was supporting a stock control database he'd written on the Commodore PET that constructed queries in 6502 assembler from user input, and then loaded them onto the CPU in the drive which could then chunter away assembling output while the user-facing app continued to respond normally.

When I had an A2k as a teenager, I was amazed that the 6502* running my friends' C64s was just the keyboard controller of my very powerful machine :)

* Yes, technically the C64 had a 6510, but that didn't matter in this context

Don't fret about the 6510 - chip die shots show that the internal structure is essentially unchanged from the 6502 - they just added 6 GPIO pins, tristate address bus and a halt that works better.

My favorite multi-CPU system was the Apple ][ card for the Macintosh LC line, which is essentially an entire computer on an expansion card to allow schools to get newer computers and keep running their older educational software. Still looking for one for my LC II, someday.

Honorable mention to the MacCharlie, which was a PC in a box that stuck on the side of an original Macintosh and allowed running PC software over a serial connection. (I've heard there was a similar Amiga sidecar for the Commodore, but that's outside of my scope of experience.)

Some later Macintosh systems also had a PC Card -- an expansion card with an x86 CPU and chipset on it. The x86 system could (somehow!) use the RAM and storage of the host computer, and took over the system's video output with a weird adapter cable.

That's similar to the Bridgeboards for the Amiga 2000[1], with the added bit that the A2000 had two buses so you can see the board has connectors for both the Zorro bus on the Amiga and an ISA bus for the "PC side". For the Amiga 1000 there was also the "Sidecar"[2]

[1] https://bigbookofamigahardware.com/bboah/product.aspx?id=328

[2] https://en.wikipedia.org/wiki/Amiga_Sidecar

Sidecar was for Amiga 1000 [1]. For Amiga 2000 there was the somewhat simpler Bridgeboard / Janus, which plugged into both the Amiga Zorro bus and a separate ISA bus in the A2000, and kept the whole thing internal, and let it access hardware on both buses (mostly used for IO, and to show the video output from the PC side in a window on the Amiga.

The Sidecar plugged into the expansion port which was basically mostly the internal bus just exposed as-is.

[1] https://en.wikipedia.org/wiki/Amiga_Sidecar

My uncle had the super PET a 6502 system that could shift into a 6809 mode with a whole suite of amazing dev software from the University of Waterloo.