The article deals with testing SP in the main loop. At the call level of the main loop (0), the SP's value should always equal its initialized value. If SP is less than its initialized value, then more POPs than PUSHs have occurred, or more RETs than CALLs have occurred, or other some other SP manipulation has caused underflow. If SP is greater than its initialized value, the opposite has occurred.


Underflow is when more data has been removed from the stack than was put on the stack. Overflow is the opposite.