The REALSTORAGE parameter in the CUNUNIxx parmlib member was introduced to protect the z/OS system against main storage shortage caused by loading a conversion image which exceeds the amount of available real storage. To control the real storage usage, the loading of a new conversion image or individual service request will be rejected when the REALSTORAGE available is less than the amount of storage needed for the complete environment.

The REALSTORAGE parameter value specifies the maximum amount of storage available for page-fixed conversion data. The z/OS Unicode environment will always have 2 pages of paged-fixed control blocks and 20 or more pages of page-fixed control data. The REALSTORAGE parameter does not control the storage used by these control blocks. It only controls and accounts for page-fixed conversion data.

The REALSTORAGE parameter does not have a minimum value. Note however that zero is a special value that does not limit the amount of page-fixed storage available. It is recommended that most installations do not specify a REALSTORAGE limit.

After invoking the image generator program to create the image, message CUN1017I, found in the listing of the //SYSPRINT DD, shows the amount of storage required to store the image in a data set. That same image when loaded into virtual memory will require additional storage. This additional storage is used by the z/OS Unicode Services internally for control structures and boundary alignment.

To calculate the value needed for the REALSTORAGE parameter, use the following formula where X is the value indicated on message CUN1017I.

REALSTORAGE value = (X * 1.10)
where REALSTORAGE value represents the number of pages (1 page = 4K)