Bytecode VMs

// from src/vm.ts: an operand stack, a call stack of frames, and a loop
for (;;) {
  if (++steps > max) throw new Error(`step limit exceeded (${max}): infinite loop?`);
  const op = c.code[pc++];
  const frame = frames[frames.length - 1];
  switch (op[0]) {
    case 'PUSH': stack.push(op[1]); break;
    case 'LOAD': stack.push(frame.locals[op[1]]); break;
    case 'ADD': { const b = stack.pop()!, a = stack.pop()!; stack.push((a + b) | 0); break; }   // 32-bit ints
    case 'JZ': if (stack.pop() === 0) pc = op[1]; break;
    case 'CALL': { const f = c.fns.get(op[1])!; const locals = new Array(f.locals).fill(0);
      for (let k = op[2] - 1; k >= 0; k--) locals[k] = stack.pop()!;
      frames.push({ ret: pc, locals }); pc = f.entry; break; }
    case 'RET': { const f = frames.pop()!; pc = f.ret; break; }   // the return value stays on the stack
    case 'HALT': return { output, steps };
  }
}