Always be aware of the possibility of a storage overlay when analyzing
a dump. System problems in MVS™ are
often caused by storage overlays that destroy data, control blocks,
or executable code. The results of such an overlay vary. For example:
- The system detects an error and issues an abend code, yet the
error can be isolated to an address space. Isolating the error is
important in discovering whether the overlay is in global or local
storage.
- Referencing the data or instructions can cause an immediate error
such as a specification exception (abend X'0C4') or operation
code exception (abend X'0C1').
- The bad data is used to reference a second location, which then
causes another error.
When you recognize that the contents of a storage location are
not valid and subsequently recognize the bit pattern as a certain
control block or piece of data, you generally can identify the erroneous
process/component and start a detailed analysis.