See also the toplevel.

I would like to use a non-x86 architecture. This is mostly due to the lower power usages of a lot of other architectures, as well as compatible board sizes. To summarize my research, I've decided that my requirements are

• minimum of 32M of code storage for a boot loader
• support for an SD card, though having SATA support later would be nice
• minimum of 64M of memory (this is debatable, more is better)
• onboard serial UART

I've considered a bunch of candidate system boards:

• RISC-V: there's really two options for system boards, both by SiFive. The HiFive1 just doesn't have enough RAM (at 16K), but the HiFive Unleashed is prohibitively expensive. There's also not much information on the power usage of this board, but the open nature of the architecture is compelling.
• MIPS: the Onion2 is interesting, but it needs a dock or carrier board of some kind that increases the bulk. I'm not sure about the power usage, though.
• ARM: the Raspberry Pi is compelling, and there's lot of power usage information for these boards. Really, this comes down to the model 3 B+, with ARMv8, and the Zero WH, with ARMv6. The 3 B+ is nice for its builtin peripherals, but it has a high power usage with 2.5A recommended current draw. The Zero WH is smaller but its peripherals aren't great; it does have a recommended power supply rating of 1.2A, which is a lot better. I'm not sold on this given the Mali GPU.

Right now, the Pi is the most tenable option for early explorations, so I'm proceeding with that. I'd love for a suitable RISC-V board, though. It'd be nice to not get too far before having to rewrite a bunch of low-level code.