TEST—AT subcommand

Use the AT subcommand to establish breakpoints where processing is to be temporarily halted so that you can examine the results of execution up to the point of interruption. Processing is halted before the instruction at the breakpoint is executed.

If you enter the TEST command with any operands, a pseudo or automatic breakpoint is established at +0 for the problem program being invoked under TEST. Therefore, do not specify AT +0.

If you set a breakpoint following a fullscreen TPUT macro and preceding a TGET macro, the fullscreen message is overlaid by the TEST line mode message (IKJ57024I). For more information, see z/OS TSO/E Programming Guide.

You cannot establish a breakpoint at:

The target of an execute instruction or the execute instruction itself.
An instruction that is to be modified by the execution of other in-line code before the execution of the breakpoint.
A user-written SVC exit.
An instruction that other code depends upon to be the same. See Example 7.

For some instructions, the AT subcommand establishes a single pass breakpoint that is removed automatically after the breakpoint hits. These instructions include:

all branch relative instructions (such as, BRC, BRAS, or BRCT).
all space-switching instructions (such as, PC, SAC, SACF, SSAR, PT, or PR).
any instructions that save the current PSW address except BAL, BALR, BAS, BASR, BSM, or BASSM.

A single pass breakpoint is removed automatically during execution of the GO subcommand. After resuming execution at the breakpoint, the breakpoint is no longer in effect until you establish it again by using the AT subcommand . If you want to establish the breakpoint, you must use the AT subcommand after you issue the GO subcommand, not before.