Off by one error

A project log for RISC Relay CPU

Scientific calculator with a brain built out of relays.

roelhroelh 07/04/2018 at 20:470 Comments

The Risc Relay CPU is in the Homebuilt CPUs WebRing now !!  Celebrate !!

In the last weeks I did build most remaining boards and did more testing. All registers are available now, and the second ALU board is ready, so the full 16 bit data handling is in place.

The new ALU and register cards were first tested with the test device, and the soldering problems that were found were corrected.

Then everything was put together. (Only the second Program Counter board, and the keyboard are still missing.) This was too much for my 2A benchtop power supply, it showed the LED's slightly dimming during operations. So I changed to a beefy 24V 10A supply.

This was tested:

All tests only worked after one or a few soldering issues were fixed. In the shift unit, one of the diodes (D609) was not connected on the schematic, so a short wire was needed. On the boards with 3 registers, 8 short wires were placed (this was planned), to enable register DH to receive input over the LINK bus (return address). The register pcb uses almost all 96 connector pins. Therefore, the link bus was connected to the output of register C (register C is not placed on these pcbs), and on the pcb there are wires from the register C outputs to the relay coils of register DH.

The subroutine call placed the return address in register DH, fine. But when, in the return instruction (LD PC,DH), the DH register is moved to the PC, the PC is incremented before the next instruction is fetched. That means that the instruction after the call is skipped ! A classic off-by-one error. I have not yet decided what to do about this, there are several possibilities.