a simple-to-stupid stack vm

StackVM.cpp

#include <iostream>
#include <stack>

//a stupid stack machine

typedef enum : int {
    ADD, //0
    SUB,
    MUL,
    DIV,
    PUSH, //1
    POP, //0
    PRINT,//0
    JMP, //1
    CALL, //1, save returning ip for RET
    RET //0, if ret_ip == -1, exit. else jump to ret_ip
} InstType;

struct Inst {
    InstType typ;
    int val = 0;
};

struct VM {
    std::stack<int> s;
    Inst* insts;
    
    int get_pop() {
        int a = s.top();
        s.pop();
        return a;
    }
    
    int run() {
        int ip = 0;
        
        int a,b,ret_ip = -1; //"registers"
        while (1) {
            Inst inst = insts[ip];
            
            switch (inst.typ) {
                case ADD:
                    a = get_pop();
                    b = get_pop();
                    s.push(a + b);
                    ++ip;
                    break;
                case SUB:
                    a = get_pop();
                    b = get_pop();
                    s.push(a - b);
                    ++ip;
                    break;
                case MUL:
                    a = get_pop();
                    b = get_pop();
                    s.push(a * b);
                    ++ip;
                    break;
                case DIV:
                    a = get_pop();
                    b = get_pop();
                    s.push(a / b);
                    ++ip;
                    break;
                case PUSH:
                    a = inst.val;
                    s.push(a);
                    ++ip;
                    break;
                case POP:
                    s.pop();
                    ++ip;
                    break;
                case PRINT:
                    a = s.top();
                    std::cout << a << std::endl;
                    ++ip;
                    break;
                case JMP:
                    a = inst.val;
                    ip = a;
                    break;
                case CALL:
                    ret_ip = ip + 1;
                    a = inst.val;
                    ip = a;
                    break;
                case RET:
                    if (ret_ip < 0) {
                        return s.top();
                    } else {
                        ip = ret_ip;
                        ret_ip = -1;
                        break;
                    }
                
            }
        }
    }
};

void play() {
    Inst inst[] = {
        {JMP, 4},
        
        //func increment(int i) -> int
        {PUSH, 1},
        {ADD},
        {RET},
        
        {PUSH, 1},
        {PUSH, 2},
        {CALL, 1},
        {PRINT},
        {RET}
    };
    
    VM vm;
    vm.insts = inst;
    std::cout << vm.run() << std::endl;
    
}

int main(int argc, const char * argv[]) {
    play();
    return 0;
}