For direct access devices, specify the seek address in the IOB. The system constructs a CCW chain for a CKD device, The CCW chain issues the seek and the set file mask specified in the DEB, and passes control to the real version of your channel program. You cannot issue the initial seeks or set the file mask CCWs. For an ECKD™ device, the system constructs a Define Extent or Prefix command instead of the seek and set file mask command. The system uses the contents of DEBXDEF, as described in Control Blocks, when building the Define Extent or Prefix command. You can issue a define extent command at the beginning of the channel program. The system copies the data area and replaces the beginning and ending extent addresses and the file mask byte. Your program can issue the DEVTYPE macro to learn whether the device supports ECKD. Use the minimum type of call or the INFOLIST type with INFO=DASD.

For both CKD and ECKD, the file mask is set to prohibit seek-cylinder CCWs, or, if space is not allocated in full cylinders, seek-head commands. If the data set is open for INPUT, write CCWs are also prohibited. For an ECKD device, your channel program can contain Locate-Record CCWs.

The DEVTYPE macro with the INFO=DASD parameter will also tell your program whether the device supports the locate record extended command and it has controller cache. See DEVTYPE—Info Form.