This weekend I simulated the microcode in Logisim (V2.7.1). A few things didn't work, so schematic changed a little. The files in the file section are updated.

Some things were added in microcode:

• Micro-instruction to clear Y register (overlaps with bit set/clr). For easy access to a fixed region (at address zero), for constants or for variables for the video interrupt.
• Micro-instruction to clear upper byte of B register (overlap with other bit set/clr). Now we can move an immediate byte to a word register (Rn) with the upper half of the word set to zero, in a single 16 bit instruction.

The microinstructions are simple, but a sequence of them will be quite capable. It will be possible to add the 'long' instruction type to our PDP-11 lookalike instructions, so 32 bit MOV, ADD (and more) can be handled !