No, not feature creep, just an ECO...

I want to have diagnostic code in the microcode and need some way to report an error condition.  What I've decided on is to reserve the last 15 instructions in microcode memory for error conditions.  These locations will contain instructions that jump to themselves.  Then I can check if the high 8 bits of the address are FF (hex) and display the contents of the lowest 4 bits on some board-mounted LEDs if so.  I looked at using a nifty hexadecimal display (TIL311/DIS1417/HDSP0762) but they are rather expensive (especially for something that is never supposed to happen).  So settled on a Kingbright WP914CK/$IDT display.

Updated schematics: