See how it works (sample input 2): These are memory cells: 0:CALL 3 1:CALL 3 2:CALL 3 3:BELL&RET Before operatons stack is empty First the first command is executed. Stack becomes (1),IP-3 BELL&RET is executed. Then IP pop-ups to 1 (from stack). The second command is executed. Stack becomes (2) and so on. The result is 4 executed commands 'BELL&RET'