stack pointer

void stack_in_bounds(struct machine *machine)
{
    // make sure that 0 <= SP <= 15
    // SP is signed, so no need to check for negative
    assert(machine->registers.stack_pointer < 16);
}

void stack_push(struct machine *machine, uint16_t value)
{
    stack_in_bounds(machine);
    machine->stack[machine->registers.stack_pointer] = value;
    machine->registers.stack_pointer++;
}

uint16_t stack_pop(struct machine* machine)
{
    machine->registers.stack_pointer--;
    stack_in_bounds(machine);
    /* intuitively, this seems to pop the _2nd_ value, not the top */
    return machine->stack[machine->registers.stack_pointer];
}