This abend occurs when the combination of the FREESTG amount specified in startup parameters plus the total amount of 24-bit storage needed for the IDMS CV configuration exceeds the maximum amount of 24-bit memory the operating system can give to a private region.
The amount of 24-bit Region needed to startup is reported in startup message
DC390006 REGION NEEDED TO STARTUP.
In the case of one customer we could see in the startup messages prior to the maintenance that caused the U3994 abend, a very small amount of 24-bit memory (164K) was indicated in startup message
DC390008 Vnn STORAGE RETURNED TO OPSYS
Out of the total given to the CV at startup by the opsys, DC390008 tells you the amount of 24-bit storage not used after freeing the FREESTG amount and building the system configuration.
The amount we saw is a very small "cushion".
Opsys maintenance could possibly reduce the maximum 24-bit memory available for a private region, OR, IDMS maintenance that affects the sizes of certain modules or areas of control blocks in 24-bit memory ( or increasing FREESTG or any of the 24-bit storage or program pools), can cause the REGION NEEDED TO STARTUP to increase.
In this particular case the client defined 24-bit storage and program pools that totaled about 4000K. This is not unusual, although we found that Storage Pool 0 was overallocated by at least 1000K, but the item that pushed the CV over the edge was that FREESTG was specified as more than 6000K, which is quite unusual.
We could not determine the reason they had specified such a high FREESTG amount (typically a sufficient amount is 2048K to 3072K), but by reducing it by 1000K as well as reducing 24-stg pool size by 1000K they now have a sufficient cushion against DC390003 and U3994.
Paying attention to the following set of startup messages and noting any changes is recommended as it may give an indication that the CV may be nearing a point when there will not be enough 24-bit stg to start the CV:
DC390005 V1900 RHDCPARM FREESTG RELEASED...
DC390006 V1900 REGION NEEDED TO START UP...
DC390020 V1900 TOTAL AMOUNT OF XA STORAGE..
DC390007 V1900 SYSTEM CONFIGURATION SIZE...
DC390008 V1900 STORAGE RETURNED TO OPSYS...