If you contact the z/VSE service team because of a problem, my colleagues may ask to turn "DEBUG ON", which activates the z/VSE debug facility and provides some more information on the behavior of the system.

The Attention Routine (AR) command "DEBUG ON" allocates debug areas in virtual storage - SVA (31 bit) and starts a debug trace. The z/VSE system writes system events into the debug area, which can be helpful to analyze a problem - or just see which partition causes the system events. The z/VSE system may use up to 3 debug areas, each starting with a debug control header, which has pointers to the next free entry within the debug area, the next debug area to be used and the previous debug area. Debug entries are written in wrap around mode. The z/VSE system switches to the next debug area in case of an abnormal task termination (abend). The debug trace can be suspended with the AR command DEBUG OFF or terminated with DEBUG END. DEBUG END also frees the debug areas.

The AR command "DEBUG" shows,  if the debug trace is active.

Mainly the z/VSE Supervisor writes debug trace entries. Trace entries start with the hexadecimal string "EEEE". The first 32 bytes have the same layout and show the entry type, the caller's PSW, task id, CPU state,  time stamp, CPU id, etc. You may see the following debug entries:

• EEEE00IC - Program check (IC = interruption code)
• EEEE0200 - Display registers
• EEEE0300 - Dispatcher exit
• EEEE0400 - I/O interrupt
• EEEE0500 - I/O
• EEEE0600 - External interrupt (CPU timer, clock comparator, emergency signal, external call, ... )
• EEEE0700 - Dispatcher entry
• EEEE0800 - Supervisor call
• EEEE0900 - Task cancel
• EEEE0A00 - Swap debug area
• EEEE0B00 - Display data
• EEEE0C00 - User entry (additional information from system routines)
• EEEE0Fnn - Monitor call (nn = monitor call class)

The z/VSE system calls debug routines via a branch or monitor call interface to write a debug entry.
The z/VSE debug facility may have a performance impact dependent on the system activity, if the facility is activated.

More z/VSE debug details and commands are described in the books "Hints and Tips for z/VSE 5.1" and "z/VSE Supervisor Diagnosis Reference" on the bottom of our documentation page.