The SHOW CALLS command shows a traceback that lists the sequence
    of active routine calls that lead to the routine in which
    execution appears suspended. Each recursive routine call is shown
    in the display, that is, 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 because your program has at least
    one stack frame built for it when it is first activated.

    On Alpha and Integrity server processors, you also usually
    see a system and sometimes a DCL base frame. Note that if the
    SHOW CALLS display shows no active calls, either your program
    has terminated or the call stack has been corrupted. As your
    program executes, whenever a call is made to a routine a new
    call frame is built on the stack(s) or in the register set.
    Each call frame stores information about the calling or current
    routine. For example, the frame PC value enables the SHOW CALLS
    command to symbolize to module and routine information. On Alpha
    processors, a routine invocation results in either a stack frame
    procedure (with a call frame on the memory stack), a register
    frame procedure (with a call frame stored in the register set),
    or a null frame procedure (without a call frame).

    On Integrity server processors, a routine invocation can result
    in a memory stack frame and/or a register stack frame. That is,
    there two stacks on Integrity servers, register and memory. An
    Integrity server routine invocation could result in call frames
    on one or the other or both of those stacks. Also, an Integrity
    server leaf routine invocation (that does not itself make calls)
    can result in a null frame procedure, without a call frame on
    either stack. SHOW CALLS provides one line of information,
    regardless of the which stack or register results. (See the
    examples below.)