Channel program for HPDT MPC data transfer

Seldom Ending Channel Programs (SECP) is for HPDT MPC read and write devices. Because the read and write channel programs can contain up to 17 CCWs, including eight read or write CCWs, the CCWs and the IDAWs are moved out of the YCNCB and into the M2IO and ALPH, respectively. See Figure 1 for further details.

Figure 1. Basic read Seldom Ending Channel Program structure of HPDT multipath channel

Diagram that illustrates the basic read Seldom Ending Channel Program structure of HPDT multipath channel.

Note:

M2IO always starts on a page boundary plus X'010' bytes. Offsets shown are from the page boundary.
The position of the ALPH pointer corresponds to its CCW set (that is, the ALPH pointer at X'020' contains the address of the ALPH for the prepare/read CCW set at offset X'040'; the ALPH pointer at X'24' contains the address of the ALPH for prepare/read CCW set at offset X'050'; and so on).

Figure 2. Example of Seldom Ending Channel Program structure of HPDT multipath channel