The VTAM® MPC function is derived from the VTAM channel-to-channel (CTC) function. Both MPC and CTC communicate with the CTC Adapter, but MPC uses its own set of channel programs.

During normal data transfer, MPC uses the never-ending Read channel program. This means that a subchannel defined as a Read device can have an outstanding Read channel program or can be processing the last channel program to complete. On subchannels defined as Write devices, a Write channel program is generated when data is available to be written to the CTC adapter.

For HPDT, a seldom-ending channel program scheme is implemented. For a read subchannel, there are at least four (and possibly up to eight) read CCWs in the channel program, command-chained together. For a write device, there may be as few as one write in the channel program, but there may be up to seven writes command-chained together, depending on the amount of outbound traffic. Program-controlled interrupt (PCI) is used for both the read and write subchannels. Suspend/Resume is used only for write subchannels.

The information that follows describes the multipath channel programs for activating or deactivating an MPC connection as well as for normal data transfer.