VSAM is the only access method that operates in service request block (SRB) or cross-memory mode. The SRB or cross-memory mode enables you to use structures in other address spaces to increase the amount of space available. SRB and cross-memory modes are supervisor modes of operation reserved for authorized users. Cross-memory is a complex concept, and there are several warnings and restrictions associated with it. See z/OS MVS Programming: Authorized Assembler Services Guide.

VSAM operates only in cross-memory or SRB mode for synchronous, supervisor state requests with shared resources or improved control interval (ICI) access. For data sets not processed with ICI, an attempt to invoke VSAM asynchronously, in problem state, or with nonshared resources in either cross-memory or SRB mode results in a logical error. This error is not generated for ICI.

VSAM does not synchronize cross-memory mode requests. For non-ICI processing, the RPL must specify WAITX, and a UPAD exit (user processing exit routine) must be provided in an exit list to handle the wait and post processing for cross-memory requests; otherwise a VSAM error code is returned.

For cross-memory mode requests, VSAM does not do wait processing when a UPAD for wait returns to VSAM. For non-cross-memory task mode, however, if the UPAD taken for wait returns with ECB not posted, VSAM issues a WAIT supervisor call instruction (SVC). For either mode, when a UPAD is taken for post processing returns, VSAM assumes the ECB has been marked complete and does not do post processing.

ICI in cross-memory mode assumes (without checking) the request is synchronous. UPAD is not required. If UPAD routine is not provided, I/O wait and post processing is done by suspend and resume. There is no resource wait/post processing for ICI. See Improved Control Interval Access for information about ICI.

SRB mode does not require UPAD. If a UPAD is provided for an SRB mode request, it is taken only for I/O wait and resource wait processing.

In cross-memory or SRB mode, record management cannot issue any supervisor call instructions (SVCs). Whenever VSAM cannot avoid issuing an SVC, it sets an RPL return code to indicate that you must change processing mode so that you are running under a task control block (TCB) in the address space in which the data set was opened. You cannot be in cross-memory mode. Then reissue the request to permit the SVC to be issued by VSAM. The requirement for VSAM to issue an SVC is kept to a minimum. Areas identified as requiring a TCB not in cross-memory mode are EXCEPTIONEXIT, loaded exits, EOV (end-of-volume), dynamic string addition, and alternate index processing.

If a logical error or an end-of-data condition occurs during cross-memory or SRB processing, VSAM attempts to enter the LERAD (logical error) or EODAD (end-of-data-set) exit routine. If the routine must be loaded, it cannot be taken because loading involves an SVC; VSAM sets the RPL feedback to indicate "invalid TCB". If an I/O error occurs during cross-memory or SRB processing and an EXCEPTIONEXIT or loaded SYNAD (physical error exit) routine is specified, these routines cannot be taken; the RPL feedback indicates an I/O error condition.

See Coding VSAM User-Written Exit Routines for more information.