Browse Prior Art Database

I/O Channel and Device Controller with Early Chaining

IP.com Disclosure Number: IPCOM000050343D
Original Publication Date: 1982-Oct-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 2 page(s) / 23K

Publishing Venue

IBM

Related People

Cormier, RL: AUTHOR [+3]

Abstract

The drawing shows a channel 2, an I/O device controller 3, and a sequence of messages between the channel and the controller for controlling an operation of an I/O device 4.

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I/O Channel and Device Controller with Early Chaining

The drawing shows a channel 2, an I/O device controller 3, and a sequence of messages between the channel and the controller for controlling an operation of an I/O device 4.

An operation of the I/O device may begin with an instruction such as Start I/O is executed in an associated processor (not shown) to identify the particular channel, controller, and device of the drawing. In the execution of this instruction, a channel address word is fetched at a predefined location in main storage of the processor and subsequently a channel command word (CCW) is fetched from a storage location identified by the channel address word. The CCW identifies a "command byte" that is to be sent to the device controller; it also has a bit called a command chaining flag that signifies whether the operation of the present CCW is to be followed by fetching a next CCW at a sequential location in the storage.

Conventionally, such an operation of the I/O device then proceeds until the controller presents a status byte on the bus to the channel. The channel may then fetch the next CCW and send the next command byte to the controller.

Certain operations of the device may continue through this interval of the fetch operation by the channel or they may undesirably stop, depending in part on whether the operation to fetch the next CCW takes a longer or shorter time. For example, a card reader or a card punch may begin to stop its mechanical operation, although it would be desirable to continue this operation whenever a next CCW is to be fetched. Conver...