April 1, 1981 COMMANDS THAT ALTER THE CONTENTS OF STORAGE You can use the STORE, STCP, and ZAP commands to alter the contents of storage: Use the STORE command to alter the contents of specified registers and
locations in virtual machine storage. The contents of the following can
be altered: e Virtual machine storaqe locations (first-level virtual storage only) • General registers • Floating-point registers • Control registers (if available) • Program Status Word The STORE STATUS command can save certain information contained in low
storaqe. When debugging, you may find it advantageous to alter storage,
registers, or the PSW and then continue execution. This is a good
procedure for testing a proposed change. Also, you can make a temporary
correction and then continue to ensure that the rest of execution is
trouble-free. A procedure for using the STORE STATUS command when
debugging is as follows: • Issue the STORE STATUS command before entering a routine you wish to
debug. • When execution stops (because an address stop was reached or because
of failure), display the extended logout area. This area contains
the status that was stored before entering the routine. • Issue STORE STATUS again and display the extended logout area again. You now have the status information before and after the failure.
This information should help you solve the problemc Use the STCP command to alter the contents of real storage.
command cannot alter the real PSW or real registers.
The STCP the ZAP command to modify or dump MODULE, LOADLIB, or TITLIB files. ZA-Pcan be used to modify either fixed-or variable-length r.ODULE files.
It is for use bV system support personnel only. Part 1. Debugging with VM/370 41

page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829 ZAP makes use of control records to control processing. These
records can be submitted either from the terminal or from a disk file. Using the VER and REP control records, you can verify and replace
records or instructions in a control section (CSECT). Using the DUMP control record, you can dump all or part of a CSECT, an entire member of
a LOADLIB or TXTLIB file, or an entire MODULE file.
Debugging CP on a Virtual Machine Many CP problems can be isolated without standalone machine testing. It
is possible to debug CP by running it in a virtual machine. In most
instances, the virtual machine system is an exact replica of the system
running on the real machine. To set up a CP system on a virtual
machine, use the same procedure that is used to generate a CP system on
a real machine. However, remember that the entire procedure of running
service programs is now done on a virtual machine. Also, the virtual
machine must be described in the VM/370 directory. See VML11Q in g Virtual for directions on how to set up
the virtual machine.
CP Internal Trace Table CP has an internal trace table that records events that occur in the
real machine. The events that are traced are: • External interruptions • SVC interruptions • Program interruptions • Machine check interruptions • I/O interruptions • Free storage requests • Release of free storage • Entry into scheduler • Qlleue drop • Run user requests • Start IIO • Unstack I/O interruptions • Storing a virtual CSW • Test I/O • Halt Device • Unstack IOBLOK or TRQBLOK • NCP BTU (Network Control Program Basic Transmission Unit) • Spinning on a lock (attached processor environment) • SIGP (X'13') • Clear Channel instruction
An installation may optionally specify the size of
table. To do so, use the SYSCOR macro instruction in
Information on using this macro instruction is in the ang QgngratiQ!! the CP trace
module DMKSYS. !UJIQ-Planning· If an installation does not specify the trace table size or the size
specified is smaller than the default size, CP assigns the default size.
For each 256K bytes (or part thereof) of real storage available at IPL time, one page (4096 bytes) is allocated to the CP trace table.
Each entry in the CP trace table is 16 bytes long. There are trace
table entries for each type of event recorded. The first byte of each
42 IBM VM/370 System programmer's Guide