Booting Linux on the intel 4004 from 1971
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In 2012, I ran real Linux on an 8-bit microcontroller (AVR), setting a new world record for lowest-end-machine to ever run Linux. A natural extension of that project was into something faster and more practical, and I did that. Others also did follow-up work based on the original project. Some exciting work also happened based on my LinuxCard followup, my favourite being this gem. Nobody really tackled the actual record for about eleven years. In 2023, there was this advancement. It is still an AVR, so it is not much lower-end, but it does use an AVR with less RAM, so it counts. This is especially true since the author was clearly aiming to beat my record (as per the README). An even more impressive effort was seen, also in 2023, here. That one boots Linux (in emulation) on a MOS 6510. This is a much older-style 8-bit CPU and thus definitely counts as lower-end than an AVR. So, it seems that after 11 years on top (or…bottom), my record had been beaten. This would not do!
What would be lower-end than an AVR or a MOS 6510? AVR is a very modern pipelined architecture, delivering nearly 1.0 MIPS/MHz. The 6510 is also rather performant per-cycle. This was not always the case with CPUs. Squeezing even half that performance per MHz out of, say, an 8086 would be quite hard. But 8086 is a 16-bit chip, so it would not necessarily count as lower-end than an 8-bitter. Intel 8080 exists, and it is an 8-bit chip from 1974. Its instructions take 4-11 cycles, so it is more typical of the original 8-bitters. however, the 8080 is just an upgraded version of the Intel 8008 from 1972, so logically the 8008 would be a more tantalizing target anyways, being older and thus cooler. At this point, though, we’re approaching the start of microprocessor history, 1972 being only a year after the Intel 4004 came out. The 4004 is considered to be the first commercially-produced microprocessor. So, as long as I am going to go back in history, why not go all the way back? Plus, it being a 4-bit chip, this unambiguously sets a new low bar! Thus this project was born…
The result? It works.
