Linux on an 8bit Microcontroller (2012) (opens in new tab)

It's a pretty interesting question, even if a bit tangential to the article!

Their main advantage is the comparatively low transistor count. The chips can be made cheaply and don't need cutting-edge fabs. This also makes them exceptionally power-efficient and tolerant of a wide range of supply voltages, so they're easier to integrate into many designs.

Just as important, they are far easier to master. Single-cycle instruction execution, single chip-wide clock, simple interfaces to all peripherals. If you don't have an OS to abstract it all away, most of the contemporary 32-bit platforms are a serious pain. Adjusting clock speed on a typical Cortex-M chip requires following a carefully-choreographed sequence of steps, possibly fiddling with flash wait states, various bus clocks, PLLs, and so on. On an 8-bit MCU, you just flip a bit or two.

It's a weird misconception that "8-bit" means antiquated or slow. Modern 8-bit MCUs run at high speeds, have plenty of fast memory, and integrate all kinds of cool on-die peripherals - DACs, ADCs, op-amps, DMA controllers, USB controllers, and so on. And in a good majority of embedded applications, they are enough.

Of course, there's nothing sacred about 8-bit ALUs, so I wouldn't be surprised if 32-bit ALUs eventually take hold in this market segment. But the replacements would need to preserve that "I can learn this in a weekend" property that isn't preserved in most of the high-performance 32-bit chips available today.

It’s worth mentioning that the linked article was written over a decade ago.

- Even today, 8-bit MCUs can be significantly cheaper than 32-bit MCUs. A lot of microcontrollers are used for very simple tasks that simply don’t need anything more than what an 8-bit MCU can do. Although, these days it looks like 16-bit MCUs are comparable or sometimes cheaper when looking at Mouser and DigiKey? RL78-S1 may or may not count as 16-bit. I’m not familiar with it, and seeing some conflicting info.

- Plenty of legacy designs certainly continue to get manufactured, since updating the design and software to support a more advanced/cheaper processor would both require requalification of the product and require engineering time.

- If you need the absolute lowest power consumption, a simpler, smaller, slower core can be helpful.

- Existing expertise/familiarity

They're smaller (as in transistor count) and draw much less current than more powerful ones, which in some context (embedded/industrial, etc) is quite important. For simple tasks such as "read a sensor and if value isn' between values X and Y send this message over serial port and turn on relay on GPIO4" anything more than a smallish 8bit (sometimes even less) uC would be a huge waste of resources and potential source of problems since every piece of hardware or line of code added could hide a potential bug, therefore one adds only what is absolutely necessary, and that includes an OS and the beefier hardware required to run it.

Old processes have much lower static current(which is often dominant in mcu designs). building 32-bit mcu's using old processes is expensive.

In addition to low power and unparalleled reliability, the microcontroller make it possible to build a realtime system.

It's this slow not because of the MCU, though, but because the author is emulating another architecture in software. It would be fairly sluggish on a PC too.

This is both very impressive in terms of the sheer amount of work that went into that, but also a tiny bit disappointing because it's not really Linux running natively on an 8-bit chip.

I suspect that native Linux is technically doable and would be a lot faster, but would require a lot of fiddling with the kernel, to the point where you'd be rewriting as much as you're keeping. A lot of assembly shims to port, gotta get rid of all the MMU code, etc. AVR chips in particular also also a bit weird because they follow the Harvard architecture - separate addressing for code and data, which is not something the Linux kernel is made for.

FWIW, modern 8-bit MCUs are actually pretty darn fast. They have clocks all the way to 50 MHz or so, single-cycle execution for most opcodes, some have DMA and other cool gizmos on the die. They are orders of magnitude faster than the 8-bit tech we had back in the 1980s.

First decide what you're looking for:

Some kind of Raspberri Pi style desktop replacement (or capable of that). GPU, GB's of RAM, plug in USB peripherals, fast external storage, ...

Microcontroller: on the lower extreme of CPU power, no graphics, small flash/RAM, grab soldering iron to hook up peripherals. There's ~$0.10 parts there.

Boards like the Milk-V Duo fall in between. uC like, faster cpu, larger embedded RAM, may produce video output, not suitable to run modern desktop OS.

Very different beasts for very different applications.

VisionFive 2 is "the one" as far as I'm concerned. Very cheap and crucially it has a slot on the back for an NVMe drive.