Jonesforth Port to RISC-V

The original Jonesforth is very cool, and I'm happy it's being ported to RISC-V. However, the in-code documentation[0] clearly has some errors due to blind find-replace:

  > On the right hand side I've written the actual RISC-V machine code.

... followed by a bunch of 5-byte x86 instructions (E8 followed by 4-byte addresses), and '"E8" is the x86 machine code to "CALL" a function'.

  > On an RISC-V machine it turns out that we can write this interpreter rather easily, in just two assembly instructions which turn into just 3 bytes of machine code.

Uncompressed RISC-V instructions are 4 bytes. Compressed instructions are 2 bytes. Without some completely unanticipated extension that goes against the current design, there will be no way to make a 3-byte instruction sequence for RISC-V. From what I can tell, the actual RISC-V implementation is 4 instructions,

  .macro NEXT
    ld a0, 0(s1)
    addi s1, s1, 8
    ld t0, 0(a0)
    jalr t0
 .endm

taking 16 bytes on machines that don't support the compressed instructions extension. If all 4 can be compressed, that's still 8 bytes.

[0]https://github.com/jjyr/jonesforth_riscv/blob/master/jonesfo...

What a coincidence, I was trying to compile it yesterday but for instance, the `fp` register is not recognized. I can rename it to `s0` (a synonym) but I'm wondering what should I do to avoid this error:

    jonesforth.S:540: Error: illegal operands `addi fp,fp,-8'

Any idea?

For those interested, I'm using the nixpkgs cross-toolchain. Some documentation is here[0]. Here is a possible `shell.nix` file content:

    { pkgs ? import <nixpkgs> {} }:
    let
      riscv64-pkgs = import <nixpkgs> {
        crossSystem = (import <nixpkgs/lib>).systems.examples.riscv64;
      };
    in
    {
      riscv64 = riscv64-pkgs.mkShell {
        buildInputs = [
        ];
        nativeBuildInputs = [
          riscv64-pkgs.buildPackages.gdb
        ];
      };
    }

This works well enough to compile my hello-world example (available here[1]) but I'd like to use that `fp` register naming...

[0]: https://nixos.wiki/wiki/Cross_Compiling

[1]: https://github.com/noteed/riscv-hello-asm

Looks amazing, but the fact that `make qemu` uses docker to run qemu and the readme almost (there is a warning about image size) fails to mention it is somewhat off-putting.

Where there are 3 forthers, there will be 4 FORTHs.