What's this thing all about??

Computer LEDs GIF

I few months back I stumbled upon some videos on YouTube by Ben Eater. Ben has an incredible video series on building an 8-bit computer with simple logic gates all on a huge(!) breadboard. After watching this I got inspired to dust off my old Verilog knowledge and try to "build" my own 8-bit computer. It's not the first and certainly not the best computer but it is a fun project. You can find the GitHub repo for the project here. I'm current developing this all on the Lattice LCMX02-1200HC on the TinyFPGA AX2 board. These are really cool little FPGA boards that get you up and running very quickly, their programmer is super simple.

The computer is a Von Neumann architecture machine with 256 bytes of memory/code space. Instructions are currently 8 bits wide and are either 1 or 2 bytes long. Addressing is absolute, with the small memory space that's currently in use adding offsets doesn't seem too important at the moment. Registers are 8 bits wids as well, there are the general purpose X, Y, and Z registers as well as the ALU register for performing math functions. There are currently 2 useable flags, the zero flag and the carry flag. These can be used in conjunction with comparisons and logical jumps to do some interesting stuff. The program for the computer to run is currently compiled into the JED file, it is expected to be a hex file with the name program.mem. Each line is a new address in memory.

Verilog showing expected file name.

In the future I'll be writing some additional code to retrieve the memory from an external erasable memory. In the meantime here's a sweet example program. It will count up by 1 until the ALU is equal to 0x55, once it matches that value it will reset to 0 and count up again.

Example program.

There are of course plenty of better ways to do this but I made the example purposefully extra convoluted to make it look more interesting and to be a better test 😄.

TODO List:

  • Proper division logic.
  • Add more Bitwise logic operations.
  • Fix the "stack" that I poorly coded.
  • Read program from external memory?
  • Use fewer slices...
  • Inputs! ⚡
  • Interrupts 🙏
  • Finish the compiler...it's no fun hand compiling 😬