Cool idea! The REPL needs to handle whitespace after an instruction with no operands:

  > ld c,20h
  0E 20   (7 cycles)
  > ld c,20h [<-- trailing space here]
  0E 20   (7 cycles)
  > nop
  00      (4 cycles)
  > nop [<-- trailing space here]
  unknown instruction: nop

I also wish I could select from the completion list with Tab without typing more of the instruction, fish-style. I.e., you press Tab to go through completion 1, completion 2, ..., completion N, completion 1 again with Enter to confirm your pick.