OTMA program switch with express PCB

A P2P message switch with an express PCB can lead to a commit-then-send output message.

Program A uses an express PCB rather than a non-express PCB to perform the P2P message switch. The output from Program B is commit-then-send because using the express PCB forces Program B to be processed asynchronously. When a program is processed asynchronously and inserts back to the I/O PCB, the output message is sent as a commit-then-send message. However, if Program A also switches to Program C using a non-express PCB, Program C then inserts back to the I/O PCB. The output from C will be a send-then-commit message.