even_fib_sum.st

            PSH 0
            PSH 1
LOOP:       SUM 2
            OUT
            DUP
            JMP EO_CHECK
EO_CONT:    OUT
            POP
            SSZ
            PSH 20
            BEQ END
            POP 2
            JMP LOOP

EO_CHECK:   PSH 1
            BEQ EO_RET
            POP
            PSH 0
            BEQ EO_RET
            POP
            ADD -2
            JMP EO_CHECK
EO_RET:     POP
            JMP EO_CONT

END:        NOP

fib.st

        PSH 0
        PSH 1
A:      SUM 2
        OUT
        SSZ
        PSH 20
        BEQ END
        POP 2
        JMP A
END:    NOP

stack_lang.py

import re
from time import sleep
from sys import argv, stderr

def runtime_error(*args):
    exit()

def nop(stack, args):
    return ("", stack)

def push(stack, args):
    if len(args) < 1:
        runtime_error()

    stack.append(int(args[0]))
    return ("", stack)

def pop(stack, args):
    n = -int(args[0]) if len(args) > 0 else -1
    return ("", stack[:n])

def jmp(stack, args):
    if len(args) > 0:
        return (args[0], stack)
    else:
        runtime_error()

def beq(stack, args):
    if len(args) > 0 and stack[-1] == stack[-2]:
        return (args[0], stack)
    else:
        return ("", stack)

def sum_stack(stack, args):
    start = -int(args[0]) if len(args) > 0 else 0
    stack.append(sum(stack[start:]))

    return ("", stack)

def add(stack, args):
    if len(args) < 1:
        runtime_error()
    else:
        stack[-1] += int(args[0])
    return ("", stack)

def duplicate(stack, args):
    if len(stack) < 1: runtime_error()
    stack.append(stack[-1])
    return ("", stack)

def print_last_stack(stack, args):
    print "==> %d" % stack[-1]
    return ("", stack)

def stack_size(stack, args):
    stack.append(len(stack))
    return ("", stack)

funcs = {
    "NOP" : nop,
    "PSH" : push,
    "POP" : pop,
    "JMP" : jmp,
    "BEQ" : beq,
    "SUM" : sum_stack,
    "ADD" : add,
    "DUP" : duplicate,
    "OUT" : print_last_stack,
    "SSZ" : stack_size
}

def run(program, debug=False):
    source_lines = program.split("\n")

    lines = []
    label_table = dict()

    for line in source_lines:
        line = line.strip()

        if len(line) == 0:
            continue

        label, instr, args_blob = re.search("^(?:(\w+):\s+)?(\w+)(?:\s+(.+))?$", line).groups()
        args = re.split("\s+", args_blob) if args_blob else []

        if label:
            label_table[label] = len(lines)

        lines.append((label, instr.upper(), args))

    stack = []
    PP, SP = (0, -1)
    LABEL, INSTR, ARGS = (0, 1, 2)

    while PP < len(lines):
        line = lines[PP]

        f = funcs.get(line[INSTR], runtime_error)
        jmp_label, stack = f(stack, line[ARGS])

        if jmp_label:
            # ignore unknown labels and just carry on in the program
            pos = label_table.get(jmp_label, PP)
            PP = pos
        else:
            PP += 1

        if debug:
            print >> stderr, "%-10s%-3s %s" % (line[LABEL] or "", line[INSTR], ", ".join(line[ARGS]))
            print >> stderr, stack[-10:]
            raw_input()

    print stack




def main():
    run(open(argv[1]).read(), argv[-1] == "d")

if __name__ == '__main__':
    main()