I want 5-bit identifiers so that I can have more than ample devices on the backplane, but the reality is coding will be much simpler with 4-bit chunks... each part of an instruction can be indicated with a single hexadecimal character.

The first digit  of any actual instruction will be zero and can be ignored. The next 6-bits are the actual "instruction" in most cases, and is blocked in purple. The orange bit indicates a mode or status, and is not always required. The light blue block indicates a source device and the darker blue a destination or target device. A red block indicates the 4-bit ALU signal. The 4-bit black block indicates the source for conditionals... 0000 meaning the instruction is not conditional, 0001 meaning the instruction occurs when the carry flag is set, etc.