Channel programs for MPC data transfer

MAXBFRU defines a single read buffer. The write buffer in the other host has the same size allocation. PIUs are blocked for transfer, and are written by a single write, and read by a single read.

VTAM® uses three CCWs in its data transfer channel program:

Write CCW
Read CCW
Prepare CCW

MPC unique output control information is in the write buffer and input control information is received in the first 8 bytes of the read buffer.

Normal data transfer for channel programs is a PREP CCW followed by a Write CCW (Write device) or a Prepare CCW followed by a Read CCW (Read device).

For HPDT, sets of these CCWs are chained together in the channel program.

During the XID exchange, each host passes buffer size information for the read portion of its channel programs to other hosts in the network. Each host allocates a write buffer to match the other host read buffer.

Figure 1. MPC transmit buffers used for normal data transfer

Diagram of the MPC transmit buffers used for normal data transfer.

The first 8 bytes of the channel buffer is the control information mapped by ISTBKHDR.

Figure 2. Indirect address word structure of multipath channel programs for normal data flow

Diagram that illustrates the indirect address word structure of multipath channel programs for normal data flow.