This is an old post, preserved for reference.
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ZXBaremulator is a bare metal ZX Spectrum emulator for the Raspberry Pi by José Luis Sánchez , and something I have been meaning to look at for a while.
RISC OS Pico I used with the Acorn Electron). Without any operating system in the way, it boots within a few seconds of powering on, and you are greeted with the familiar copyright notice.