Simple stack on static memory

MAX_SIZE = 100
stack = [ 0 ] * MAX_SIZE
stack_size = 0

def push(x):
    global stack_size
    print('ok')
    stack[stack_size] = x
    stack_size += 1

def back():
    print( stack[stack_size-1] )

def pop():
    global stack_size
    back()
    stack_size -= 1

def size():
    print( stack_size )

def clear():
    global stack_size
    print('ok')
    stack_size = 0

while True:
    command = input()
    if command == 'exit':
        print('bye')
        break
    elif command[:4] == 'push':
        push(command.split()[1])
    else:
        exec( command + "()" )