Zylin ZPU: The worlds smallest 32 bit CPU with GCC toolchain

• zpu

  2 230 0.0 VHDL

  The Zylin ZPU

  

• mini-rv32ima

  16 1,479 6.6 C

  A tiny C header-only risc-v emulator.

Not GP, but this might scratch your itch https://github.com/cnlohr/mini-rv32ima

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• pinwheel

  1 2 9.0 Verilog

  A tiny RISC-V processor for hard-real-time FPGA-based applications. (by aappleby)

It will eventually be in https://github.com/aappleby/Pinwheel but that repo is currently a mess.

• zpugcc

  1 3 10.0 C

Yes, exactly, the gcc port is a very old version, and the build process frequently breaks on more modern systems. I did fork it [0] and apply a couple of fixes a year or so back, but I seem the be the only one who cares now, and even I don't care enough to pour large amounts of time into it!

[0] https://github.com/robinsonb5/zpugcc

• ZPUFlex

  1 34 10.0 VHDL

  A highly-configurable and compact variant of the ZPU processor core

For me the most interesting thing about ZPU is that its instructions are only a single byte each, which means that while the CPU is much smaller than most RISC-V implementations, the code density is significantly better than RV32 code. (Compressed RISC-V code can beat ZPU, but adding the compressed instruction set to a RISC-V core increases its size quite a bit.)

I did play with the ZPU quite a bit 8 or 9 years ago, and forked the ZPU Small variant to create ZPUFlex [0]. I borrowed a number of ideas from ZPU when I created my own CPU Project, EightThirtyTwo [1], some years later.

[0] https://github.com/robinsonb5/ZPUFlex

• EightThirtyTwo

  1 15 5.1 C

  An experimental CPU core with 8-bit instruction words and 32-bit registers

• minimax

  13 194 2.9 Verilog

  Minimax: a Compressed-First, Microcoded RISC-V CPU

Note that you can't compare LUT4 results (ZPU @ 440 LUTs) against LUT6 results (PicoRV32 @ 750 LUTs). The ZPU is remarkably small, and it's a bigger gap than a direct comparison shows.

SERV is a fair comparison, since it's architected for 4LUTs and I suspect the synthesis results come from iCE40 tools.

I have a contender in the "very small" space, too [1], although I don't claim it's as mature or complete as SERV. (If Minimax was excluded from your post on the basis of insanity, I'm OK with that.)

[1] https://github.com/gsmecher/minimax

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