The ANSI M Standard describes certain M operations in terms of how a stack-based virtual machine would operate. A stack is a repository for tracking temporary information on a "last-in/first-out" (LIFO) basis. M program behavior can be understood using a stack-based model. However, the standard is not explicit in defining how an implementation must maintain a stack or even whether it must use one at all.

The stack model provides a trail of routines currently in progress that shows the location of all the M operations that performed the invocations leading to the current point.

The ZSHOW command makes this stack information available within GT.M. For more information, refer to the section "Using the Invocation Stack for Debugging" later in this chapter, and the ZSHOW section in the "Commands" chapter.