Library /sys$common/syshlp/dbg$help.hlb DEBUG, SHOW, CALLS, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The SHOW CALLS command shows a traceback that lists the sequence
    of active routine calls that lead to the routine in which
    execution is currently suspended. Any recursive routine calls
    are shown in the display, so you can use the SHOW CALLS command
    to examine the chain of recursion.

    SHOW CALLS displays one line of information for each call frame
    on the call stack, starting with the most recent call. The top
    line identifies the currently executing routine, the next line
    identifies its caller, the following line identifies the caller
    of the caller, and so on.

    Even if your program contains no routine calls, the SHOW CALLS
    command displays an active call. The reason for this is that
    your program has a stack frame built for it when it is first
    activated. Thus, if the SHOW CALLS display shows no active calls,
    either your program has terminated or the call stack has been
    corrupted.

    On VAX processors, the sequence of routine calls corresponds to
    the sequence of call frames on the call stack. Whenever a call is
    made to a routine as your program executes, the operating system
    creates a separate call frame on the call stack. Each call frame
    stores information about the calling routine, for example, the PC
    value that enables the SHOW CALLS command to symbolize module and
    routine information.

    On Alpha processors, a routine invocation can result in a stack
    frame procedure (with a call frame on the stack), a register
    frame procedure (with a call frame stored in the register set),
    or a null frame procedure (without a call frame). SHOW CALLS
    provides one line of information on all three: stack frame
    procedures, register frame procedures, and null frame procedures.
    (See the Alpha example below.)