VTAM® performs normal send and receive processing when an HPDT request (OPTCD=XBUFLST) is received for a session when HPDT services are not available. Under these circumstances, an application program can be written to invoke normal send or receive requests and avoid the steps required to invoke CSM. The application should check the session-information vector in the VTAM-APPCCMD vector list. This vector contains information about the session that the application can use to optimize performance for sending and receiving operations. The following fields in the session-information vector can be examined by the application:

APC19NOF

If this bit setting is on, the route used by the session does not support HPDT services. The application should either not use the HPDT interface or use a CSM data space that is marked eligible to be pagefreed by CSM (BLXEN_PAGEELIG flag is set).

If this bit setting is off, the application should examine the other indicators to make a determination as to what type of CSM storage should be used.

APC19SMB

If this bit setting is on, the session route uses a small maximum RU size and the performance gain for using HPDT is negligible. The application should either not use the HPDT interface or request 4 KB CSM buffers for application data.

APC19PGP

If this bit setting is on, the session is using a connection that does not require fixed buffers. No additional performance can be gained by using CSM fixed storage. The application should use storage marked eligible to be paged.

APC19FXP

If this bit setting is on, maximum performance benefits are realized by providing the data to VTAM in CSM fixed storage.

APC19RUO

This field indicates the maximum outbound RU size.

APC19RUI

This field indicates the maximum inbound RU size.