It's been a while since I posted an update, and that's mostly because it's been a while since I've done any work on this project. But over this weekend I got back on track.
I was in the midst of a setback the last time I posted -- the program counter unit was not going well. First I found that somewhere in the myriad of connections, was a bad connection or a short, and thus the program counter quit...counting. Then, as I tried to cram the 7-segment display onto the board, I realized that I was trying to do too much with my limited resources and skills.
So this weekend I started over and got the program counter (almost) finished. Here are a few pics:
Program counter attached to the clock/reset unit
Close-up of the program counter. The four chips in the middle are synchronous 4-bit counters that provide a 16-bit address to the three Atmel EEPROMs. The EEPROMs store 8-bit words using a 15-bit address, allowing for 32k 24-bit instructions. The instructions will appear on the 24-pin header to the right.
Rear of the program counter. As you can see, there are still a number of things missing. One, I still need to wire up the 24 instruction header pins to the EEPROMs. And second, the 40-pin socket that is currently unwired? That's for implementing jump instructions. So I still need to wire up the 16 "next instruction" pins to the 4-bit counters, but not until I get finished with the jump/shift unit.
What this boils down to is that I'm very close to having the ability to actually start executing some code very soon. I need to get an EEPROM programmer (I think I'll get a Willem programmer). Once I have that, I should be able to do a full system test with real code, and at least verify that I'm able to write code, compile it, burn it into the EEPROMs, and have it read out properly on the board.
After that, it's "just" a matter of getting the remaining modules done. Next up:
- Repeater board for the instructions. The instruction data needs to be fed to lots of different places over some very long cables. So I'll build a simple repeater board with a bunch of 8-channel buffers and 40-pin sockets and then wire it up to the program counter. The instructions will be distributed to each board using standard IDE cables.
- Jump/Shift unit (phase 1: basic jumps): I'll build out the basic components of the jump/shift unit, only implementing the JMP instruction (which contains the target address right in the instruction). This way I can test the jump functionality without needing the register file (which all the other jump instructions need).
- Register file: given the complexity, likely the first module that I'll try printing a circuit board for.
- Load/Store unit: needed to actually populate the register file with anything useful (and provides the system RAM)
- Finish the jump/shift unit
- I/O unit