The MOS 7601 was the first CPU I learned how to write code for, circa late 1977 to early 1978. I would scour jumble sales for broken pong games and the other, more sophisticated console variants to cannibalize for parts, with one conversation about haggling with another buyer over who got what parts leading to a job.
There were several machines of the time that had pluggable cartridges, from Binatone, Telarude?/Telalude? (French or Italian company I recall, I forget), Telstra/Telstar, again, I forget, it was 45 years ago, along with many others, where the display circuitry, power and joytsticks were all on the main board, but each cartridge carried its own CPU and ROM, which was the fashion back in the day.
They were an absolute bugger to write code for, no complex ALU to speak of, no register storage, no RAM, you sort of just chased the CPU with a stream of instructions, stored stuff in decay buffers (you want a countdown from 10? charge up that cap for a brief period of time then let it leak) or on the display output circuitry that you could read back if you needed. If your code went wrong, you never knew it.
You were mostly bit-banging analogue circuits to get anything to display, same for the audio, though it could also talk to some AY audio synthesizer.
I recall the 7601 had a reasonably short lifespan in the marketplace for a variety of reasons.
There were two datasheets for it that were the bible, there might have been more but I only ever had access to bad photocopies of those two. One was the usual electrical characteristics and the other was the instruction set with the usual timings, delays and addressing modes.
The addressing modes were stupidly simple by the standards of today. There were EPROM variants (gold pin variant IIRC) available that you wiped with a strong ultraviolet light, but generally you had an off-board EEPROM and you tied two of the pins high to let it know it should use the external EEPROM.
When I tell people my first game I wrote fit in less than 500 (8-bit) bytes, they wonder how that is. The game was never released but Race Electronics and their desire to cash in on this new fad and exploit the younger, more gullible version of me, did give me my start in video game development so I am thankful for that, in a way. The next game I wrote fit in 2KBytes and I had the luxury of using a 6502 CPU and access to actual RAM on a CBM PET.