serv VS minimax

In short: it works, though the implementation lacks the crystal clarity of FemtoRV32 and PicoRV32. The core is larger than SERV but has higher IPC and (very arguably) a more conventional implementation. The compressed instruction set is easier to expand into regular RV32I instructions than it is to execute directly.

https://github.com/olofk/serv

As for the size advantage: this mattered more when LUTs were precious and when PicoBlaze's competition was either similarly unorthodox (J1 Forth CPU) or several times larger (MicroBlaze). Nowadays, there are very small RISC-V cores like FemtoRV32 Quark or SERV. RISC-V benefits from mainstream open-source tooling and has momentum that's hard to beat.

The RISC-V spec does allow non-trapping behavior and SeRV in particular has non-trapping behavior, which is an important part of how it can fit into 200 4-input LUTs.

https://github.com/olofk/serv#good-to-know

It will be interesting to see if a Serv will fit with some usable gates left over.

On FPGAs, a register file probably fits better into distributed RAM than block RAM.

On Xilinx, for example: a 64-bit register file doesn't map efficiently to Xilinx's RAMB36 primitives. You'd need 2 RAMB36 primitives to provide a 64-bit wide memory with 1 write port and 2 read ports, each addressed separately. Only 6% (32 of 512) entries in each RAMB36 are ever addressable. It's this inefficient because ports, not memory cells, are the contented resource and BRAMs geometries aren't that elastic.

A 64-bit register file in distributed RAM, conversely, is a something like an array of DPRAM32 primitives (see, for example, UG474). Each register would still be stored multiple times to provide additional ports, but depending on the fabric, there's less (or no) unaddressed storage cells.

The Minimax RISC-V CPU (https://github.com/gsmecher/minimax; advertisement warning: my project) is what you get if you chase efficient mapping of FPGA memory primitives (both register-file and RAM) to a logical conclusion. Whether this is actually worth hyper-optimizing really depends on the application. Usually, it's not.

I haven't used it on a huge design (I'm usually a VHDL person), but it was a hassle-free replacement for iverilog when regression testing Minimax. Performance is substantially better; compilation times are worse.

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

I develop Minimax (https://github.com/gsmecher/minimax), an open-source RISC-V implementation. It's currently written in both VHDL and Verilog (the two implementations are equivalent, though I am likely to drop the VHDL implementation if it's too much work to keep them both.)

The best point-of-entry for "tiny" MCUs these days is FemtoRV32-Quark or SERV. I also maintain my own small RISC-V core (Minimax), though it's early on in graduating from "experiment" to "real design".