Soil

A low-level VM

When implementing my programming language Martinaise, I was faced with a difficult challenge. I wanted to achieve both of these goals simultaneously:

• Performance: Martinaise programs should be (nearly) as efficient as writing the corresponding machine code. Until now, this was achieved by allowing the developer to write functions in platform-specific assembly.

• Platform Independence: All Martinaise code should be platform independent. So, just writing assembly doesn't work.

The solution is pretty obvious:

Any problem in computer science can be solved with another layer of indirection. [...]

So, I invented Soil.

Similar to fantasy consoles such as PICO-8, Soil is a specification for a VM. It is on a similar abstraction level as existing CPU instruction sets: There are registers, memory, and instructions.

• Registers have sane names (a, b, c, d, e, f, stack pointer sp, status register st).

• Instructions are low-level and operate on registers. Soil follows the tradition of RISC instruction sets – there are only a few simple instructions that compose instead of many specialized instructions. For example, jumping based on whether a value is negative is encoded using these instructions:

  cmp a 0
  isless
  cjump label
  

• Memory has a fixed size. Code can just access any address within that range.

Because Soil is low-level, it's easy to write interpreters. There already exist a few implementations written in C, Zig, Dart, Rust, and x86_64 assembly.

Now, I can write Martinaise functions in platform-independent Soil assembly:

fun memcopy(from: Address, to: Address, amount: Int) asm {
  moveib a 8  add a sp load a a | from
  moveib b 16 add b sp load b b | to
  moveib c 24 add c sp load c c | amount
  moveib e 1
  cmp a b isless cjump .right_to_left
  .left_to_right: ..loop:
  move st c isequal cjump .done
  loadb d a storeb b d
  add a e add b e sub c e
  jump ..loop
  .right_to_left:
  add a c add b c sub a e sub b e | make a and b point to the last byte
  ..loop:
  move st c isequal cjump .done
  loadb d a storeb b d
  sub a e sub b e sub c e
  jump ..loop
  .done: ret
}