[Sharlin]

Need a couple of instructions for accessing memory (and possibly loading immediates) but otherwise seems like a perfectly adequate general-purpose instruction set. Might be fun (for some values of "fun") to write a compiler backend for it.

[sparkie]

They're one byte opcodes, but not one byte ops. Most of them have operands which are encoded in a ModRM byte which follows the opcode. The ModRM may be followed by a SIB byte, and that may be followed by a a variable size immediate|displacement. There are also optional prefixes to the opcode.

[jeffbee]

Tons of these have immediate operands. The question becomes is ADD with an implicit register destination and an immediate value in the next byte a "1-byte opcode"?

[Sharlin]

Yes, indeed. I'd allow only mov to have a memory or immediate parameter as the only exception to one-byte encoding.

[benlivengood]

Push, pop, inc, and dec with a 16-bit register argument are one byte, so is ret. That technically gives you enough to do anything, but you can include jz/jnz (which do take immediate bytes, maybe cheating?), stosw, lodsw, clc, and stc to implement Brainfuck (a little harder to perform input/output with single byte instructions, but maybe pretend the OS uses int1 or int3 for calls).

[themafia]

You've always got the stack segment (SS) to play with and there's also:

https://www.felixcloutier.com/x86/xlat:xlatb