Simple stack virtual machine in Python

simple-vm.py

# -*- coding: utf-8 -*-
"""
A very simple stack-based virtual machine in Python

Example:
PSH 1 # push 1 into the stack
PSH 2 # push 2 into the stack
ADD # add the topest 2 values and pop them
POP # pop the top of the stack and print 3
HLT # end the program
"""

ins_set = ('PSH', 'ADD', 'POP', 'HLT')  # the instruction set

running = True  # the program is running?

ip = 0  # the instruction pointer
sp = -1  # the stack pointer

stack = [0] * 256  # size = 256

program = [
    "PSH", 1,
    "PSH", 2,
    "ADD",
    "POP",
    "HLT"
]


def fetch():
    global ip
    return program[ip]


def execute(instr):
    global ip
    global sp
    global running

    if instr not in ins_set:
        error("Illegal instruction")
        running = False
        return
    if instr == "HLT":
        running = False
        print("Program ends")
    elif instr == "PSH":
        sp += 1
        ip += 1
        stack[sp] = program[ip]  # push next value into the stack
    elif instr == "POP":
        top = stack[sp]
        sp -= 1
        print(f"You popped {top}")
    elif instr == "ADD":
        val1 = stack[sp]
        sp -= 1
        val2 = stack[sp]
        sp -= 1
        result = val1 + val2
        sp += 1
        stack[sp] = result
    else:
        pass


def error(msg):
    print(f"Error: {msg}")


def run():
    global ip
    while running:
        execute(fetch())
        ip += 1


if __name__ == '__main__':
    run()