Reading http://www.hp9825.com/html/osborne_s_story.html ... and a familiar thought resurfaced !

I forgot the breakpoint(s).

At least one trap on a given instruction address would be nice, right ?

This happened before with the #Discrete YASEP  at 6. Dear SN74HC688  and other handy traps would be triggered on writing to a given register or on detection of a given value on the result bus. These trap signals would really help, not just with debugging the software, but also the hardware !

Another thought : add a cycle counter. However the electromechanical panel versions I have found are limited to 8 to 10 pulses per second only, while I expect more than twice that speed...

Examples found on eBay:

https://www.ebay.com/itm/W16-Mini-Pulse-Counter/162201671333

Maybe a predivider would help but... the missing least significant digit would be more than welcome.

Another eBay image from https://www.ebay.com/itm/253201849478:

And this one is limited to 10 I/S as well https://www.ebay.com/itm/Sodeco-TCEZ5E-5-digit-counter-with-manual-reset-Uses-a-28-Vdc-Pulse-TESTED/162734694869 :

So I doubt the claim that this chinese model works at 60Hz https://www.ebay.com/itm/New-DC-24V-50-60Hz-5-Digits-2-Wire-Lockable-Electronmagnetic-Counter/371715609836... The line "Input Power : DC 24V 50/60Hz" makes me believe it's an editing error.

But mostly, these counters don't provide an output signal when a given count is reached so their usefulness is "only informative". A nice decorative gadget.

Anyway, the bitwise comparison of two numbers is pretty easy with relays. First, wire the relays in series to make a large AND gate. Then complement the relays with other relays to make XORs. This amounts to 2 relays per bit. There is also a version of XOR with 1 relay per bit but it relies on both operands to use rail-to-rail signals (not just on/off).

For ease of design and economy, the values to compare could be entered in binary with individual SPDT switches. But you know, binary is SOOOOO 50s. Hexadecimal knobs please !

Unfortunately, my knobs are SPST. This means that one relay per bit is still required to transform the on/off signal into rail-to-rail signals. So we're back to 2 relays per bit. Convenience has won again...