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Progress on the ALU FPGA Design

A project log for Project Ember

Homebrew Retro-Inspired 32-bit CPU And Video Game System

TomTom 01/28/2022 at 02:170 Comments

I have made some great progress on the ALU in SystemVerilog in Vivado for the Spartan-7. I can now step through a sequence of ALU instructions, have them read from registers and immediate values, then save out the result to another register. The next step will be to add some additional instruction types to the decoder logic, probably memory load/store, and a few others like load immediate to have some data to operate on. Then I need to try stepping it on the FPGA hardware. For now, it is looking good in the simulator.

Vivado Simulator View of Ember ALU

I decided to go with SystemVerilog ultimately. Originally, I wanted to do the whole implementation in Verilog, but after finding roadblocks and bottlenecks (a lot of it was due to being a programmer for decades, Verilog was just too limiting), it was becoming apparent it would be much harder to do.

Anyway, as you can see from the simulator image above, there are benefits to using SystemVerilog and going all in. I was able to define typedef enums for all my types and values. Then I defined a number of structures for each encoded instruction word, defining the bits in each instruction and assigning them enums. This way, I can just load the 32-bit instruction and set it to a typedef logic union, then use the structure members just like it was C code! It looks like it's just as optimal, way easier to read, and even more convenient to simulate.

If you look closely at the sequence in the image, you can see all the values with names like the pipeline state at the top, names of instruction opcodes, registers by name, etc. I can then change their colors as well. People complain all the time about the "closed" tools like Vivado, Quantus, and the like, but so far it has been working for me...we'll see as I get farther into things. I have to say it is slow when building for hardware though...you do need a fast machine or builds take forever...

I plan to write up something on the SV code at some point, but right now I'm having so much fun just coding it! :)

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