A lot of popular virtual machines are stack based. The instruction sets take their inputs from a stack and then place their outputs on the same stack. It's pretty simple at face value but the virtual machines end up doing a lot of fancy things behind the scenes to make the code run fast.

Today I implement a REALLY REALLY BASIC stack based virtual machine that does basic math.

s = []

def e(cmd):
    global s
    f={'add' : 's.append(s.pop()+s.pop())',
       'dup' : 's.append(s[-1])',
       'sub' : 's.append(s.pop()-s.pop())',
       'mul' : 's.append(s.pop()*s.pop())',
       'div' : 's.append(s.pop()/s.pop())',
       'peak' : 'print s[-1]',
       'out' : 'print s.pop()'
       }[cmd]
    exec(f)

s.append(5)
s.append(1)
e('add')
e('dup')
s.append(12)
e('mul')
e('sub')
e('peak')
e('out')

The key of the dictionary is the instruction and the value defines what the instruction does.