Since I've been away from the Spikeputor Manufacturing Center in Texas, it's been awhile since I've been able to make any progress on the project. I finally got there and spent some time completing the last two items referenced in the Memory Module. These are two special 16-bit registers for storing the current instruction and current constant, if required. You'll recall that the instruction is read from memory in CPU Phase 0. During that phase, ISEL is set high so the INSTRUCTION register is updated. Meanwhile, the CONSTANT register is updated on every clock cycle, meaning that if a constant is needed for the current instruction, it will be loaded in CONSTANT during CPU phase 1. INSTRUCTION still holds the current instruction, since ISEL is only high during CPU Phase 0. Then, during CPU Phase 2, the operation is executed, using CONSTANT as an operand if needed.
Both registers are positive edge triggered D-Registers. The difference between the two is that INSTRUCTION has a Write Enable (EN) input so it can be selective about which memory outputs get stored.
Here's a photo of the finished registers. CONSTANT is on top. Notice the blue LED in the INSTRUCTION register to indicate the state of the EN input.
The last CPU module is the Program Counter circuitry. Once that's done, I can start integrating everything and get to an actual working stand-alone CPU! Everything else (and there's a bunch!) will be bells and whistles.