Working with hysteresis logic is weird !
When you turn the machine on, you don't know the state of the outputs because they all default to off, which can be anything depending on the wiring, which is crazy because of all the interleaving.
For each logic level, the relays must be cycled to on and off to ensure that the state is reliable. For example, it means writing "all 1s" and "all 0s" to all 8 registers, which will also have the side effect of sending the addresses "all 1s" and "all 0s" to the data address decoders. The register read relays must also be exercised.
The "program" will certainly start with a few dozens of instructions that clear/set every bit, including status flags. To this end, going from "all 0s" to "all 1s" and vice versa can also exercise the adder, just add or subtract 1 to a register.
It will be interesting to find the shortest program that achieves that...
The power-on routine also requires all the parity bits to be in a legal state. This means scanning the whole memory and read/write data, first without halting the machine if a parity error occurs (which will occur because all the legal states have been lost), then a second run to check the machine has no fault.
The scanning can be done directly with the refresh circuitry (a simple LFSR and arbitration logic). We just need to count how many times the LFSR has wrapped around:
- cycle of 256/512 access, write all 1s while disabling parity checks (but write parity
- cycle, read the 1s, write all 0s and check parity
- cycle, read the 0s
The LFSR can also generate addresses for the register set and the ALU control lines. When the cycle counter reaches 4, the machine can start fetching instructions.
Considering a cycle rate of 25IPS, a 256-words machine will take around 10 second per cycle, or 30s to fully scan the memory 3 times. With 512 words, the power-on routine takes one minute...