After several weeks of digging through the details of other designs, re-reading my college texts, sketching out data paths, and writing test microcode for my various design ideas, I arrived at what I hope is a mostly-complete block diagram for the computer. I wrote out the microcode for a dozen or so instructions that I expect to have in the machine’s instruction set, to prove to myself that they were all computable in a reasonably efficient number of clock cycles. Most instructions look like they’ll require 3 or 4 clocks, with the most complicated read-modify-write instructions using indexed addressing modes taking about 10 clocks.

One of the possible ALU inputs is a hard-wired zero. When writing the sample microcode, I found that a zero input would help me shave a clock or two off many instructions. X[7] is another special ALU input. It’s the sign bit (bit 7) of X, replicated 8 times. It makes it easy to do sign extension when adding a signed 8-bit number to a 16-bit address, such as when doing a relative jump or using an indexed addressing mode. It feels a bit ugly, but it cuts two clock cycles off every branch instruction.

Note that the ALU data bus and memory data bus are separated by a bi-directional bus driver. This enables them to do work in parallel. For many instructions, the next opcode can be fetched from memory and stored in the OP register simultaneously with the last step of the computation by the ALU.

I haven’t yet created a detailed circuit schematic, but I know roughly what parts I’ll need. I made an approximate tally of 40 chips used in the design, plus a few more I’ll undoubtedly need for clock generation, signal buffering, and glue logic. The total should be under 50 chips, which I hope is few enough to fit onto a couple of circuit boards. Physical construction is still a ways off, but I’m always trying to think ahead.

The next step is to get feedback on the current design, and make sure there aren’t any major problems I overlooked. After that, I plan to move on to creating a Verilog model of the design, so I can simulate it and work out the flaws before I break out the soldering iron.