The read channel program is a logical ring of at least four and at most eight read CCW sets, followed by a TIC CCW back to the physical start of the ring. (A CCW set, when prepare CCWs are required, is composed of both the prepare and read CCWs; otherwise, the set is just the read CCW alone.) The field CPNCB_index_word contains indexes of the logical first and last CCW sets in the ring and is used to maintain control over the channel program and read completion processing. All CCWs are command-chained together except for the last logical CCW in the channel program, which must have command chaining off. PCI is on in all the prepare CCWs, except the CCW in the channel address word when the channel program is started. As reads complete, PCI interrupts occur. The inbound data is processed and routed internally, the read buffers are replenished, and the CCW set is prepared to be appended to the logical end of the channel program. Preparation is done by ensuring that PCI is on and command chaining is off. The CCW set is appended, or tacked-in, to the logical end of the channel program (probably while the channel program is running) by setting the command-chaining bit in the current logical last CCW in the channel program and updating the index of the last logical CCW set in the ring.

The term missed tack-in is used to describe the condition where the IO processor fetches the logical last CCW before the CPU sets the command-chaining bit to complete the tack-in. In this case the channel program ends with channel end/device end status. It is the responsibility of the interrupt handler to detect a missed tack-in and to restart the channel program.

The suspend/resume function is not implemented in the read channel program because the objective is to keep as many reads outstanding as required to ensure the channel program does not end.