Page of GC20-1807-7 As Updated Aug 1, 1979 by TNL GN25-0492
The abnormal termination of an operating system (such as as or DOS) running under VM/370 appears the same as termination of the operating
system on a real machine. Refer to publications for that operating system for debugging information. However, all of the CP debugging
facilities may be used to help you gather the information you need.
Because certain operating systems (OS/VS1, OS/VS2, and DOS/VS) manage
their virtual storage themselves, CP commands that examine or alter
virtual storage locations should be used only in virtual=real storage
space with OS/VS1, OS/VS2, and DOS/VS. If a dump was taken, it was sent to the virtual printer. Issue a CLOSE command to the virtual printer to have the dump print on the real
printer.
The VMDUMP command dumps virtual storage to a specified virtual
machine's reader spool file. Installations that have installed the
VM/Interactive Problem Control System (IPCS) Extensions program product
may use it to process the dump. Other installations may process the
dump with a user-written program.
If you choose to run a standalone dump program to dump the storage in
your virtual machine, be sure to specify the NOCLEAR option when you
issue the CP 1PL command. At any rate, a portion of your virtual
storage is overlaid by CP's virtual IPL simulationa If the problem can be reproduced, it may be helpful to trace the
processing using the CP TRACE command. Also, you can set address stops,
and display and alter registers, control words (such as the PSW), and
data areas. The CP commands can be very helpful in debugging because you
can gather information at various stages in processing. A dump is static
and represents the system at only one particular time. Debugging on a
virtual machine can often be more flexible than debugging on a real
machine. VM/370 may terminate or reset a virtual machine if a non-recoverable
machine check occurs in that virtual machine. Hardware errors usually
cause this type of virtual machine termination. The following message:
DMKMCH616I MACHINE CHECK; USER userid TERMINATED
appears on the processor console.
If the message:
DMKMCT621I MACHINE CHECK; AFFINITY SET OFF appears, then a machine check has occurred on the attached processor,
and the attached processor is no longer being used. The virtual machine
is placed into console function mode and can be made to continue
processing on the main processor by the entry of a BEGIN command.
Channel checks no longer cause the virtual machine to be reset as
they did in earlier releases of VM/370. If the problem appears to be
associated with attempts to recover from a channel check, see the
channel model-dependent functions described in the System Quig.§. Part 1. Debugging with VK/370 23

Aug 1, 1919 UNEXPECTED RESULTS The type of errors classified as unexpected results vary from operating
systems improperly functioning under VM/310 to printed output in the
wrong format.
If an operating system executes properly on a real machine but does not
execute properly with VM/370, a problem exists. Also, if a program
executes properly under control of a particular operating system on a
real machine but does not execute correctly under the same operating
system with VM/370, a problem exists.
First, there are conditions (such as time-dependent programs) that CP does not support. Be sure that one of these conditions is not causing
the unexpected results in CP. Refer to the !M/370 for a list of the restrictions.
Next, be sure that the program and operating system running on the
virtual machine are the same as those that ran on the real machine.
Check for: • The same job stream • The same copy of the operating system (and program) • The same libraries
If the problem still is not found, look for an I/O problem. Try to
reproduce the problem, while tracing all CCWs, SIOs, and interrupts via
the CP TRACE command. Compare the real and virtual CCWs from the trace.
A discrepancy in the CCWs may indicate that one of the CP restrictions
was violated, or that an error occurred in the Control Program. When a program executes correctly under control of a particular
operating system on a real machine but has unexpected results executing
under control of the same operating system with VM/370, a problem
exists. Usually you will find that something was changed. Check that the
job stream, the operating system, and the system libraries are the same.
If unexpected results occur (such as TEXT records interspersed in
printed output), you may wish to examine the contents of the system or
user disk files. Non-CMS users may execute any of the utilities
included in the operating system they are using to examine and rearrange
files. Refer to the utilities publication for the operating system
running in the virtual machine for information on how to use the
utilities.
24 IBM VM/370 System Programmer's Guide