If you're going to do something like that, you might as well steal from the register numbering bits instead. (many architectures have done this: Intel has lots

If you're going to do something like that, you might as well steal from the register numbering bits instead. (many architectures have done this: Intel has lots of short-form encodings for "op {al, ax, eax, rax} #imm", and ARM has Thumb, which basically chops the register count in half for all instructions to shorten their encoding)