It took a whole weekend but finally the register file is complete. I took a delivery of new boards last week and the first to be built is the register control board. This is used to drive the 16 individual registers. Once finished it looks quite impressive...

The control board takes three 4-bit values. The register to output on port A, the register to output on port B and the register to be written to. Our control board has three output headers that are 16 lines wide. So register zero takes bit 0 from each of the three headers as its input. Register one takes bit 1 from all three headers and so forth. This results in the monster wiring loom shown above. I had to make it all by hand using DuPont connectors, a crimper and some heat shrink tubing.

It would have gone much faster except for some errors that took a long time to notice. Each of the two register stacks had misconnected pins in the middle of the stacks. When connecting them together it can be tricky getting all the 100 long headers to match up. Well I hadn't noticed a couple that had missed on insertion and were not connected. Compounding this I found that one of the ribbon cables I was using to make a test connection had two of the wires transposed. Luckily it was a rainbow coloured cable so I eventually noticed that red and blue lines were in the opposite order at the other end of the cable. Otherwise I would still be scratching my head!