Chapter 43 The structure of SYSTEM/360 599
Fig. 8. Channel status word format.
fetch a channel address word from main-storage location 72. This word specifies the storage-protection key that governs the I/O operation, as well as the location of the first eight bytes of information that the channel fetches from main storage. These 64 bits comprise a channel command word (CCW). Figure 9 shows the CCW format.
Channel program
One or more CCW's make up the channel program that directs channel operations. Each CCW points to the next one to be fetched, except for the last in the chain which so identifies itself.
Six channel commands are provided: read, write, read backward, sense, transfer in channel, and control. The read command defines an area in main storage and causes a read operation from the selected I/O device. The write command causes data to be written by the selected device. The read-backward command is akin to the read command, but the external medium is moved in the opposite direction and bytes read backward are placed in descending main storage locations.
The control command contains information, called an order, that is used to control the selected I/O device. Orders, peculiar to the particular I/O device in use, can specify such functions as rewinding a tape unit, searching for a particular track in disk storage, or line skipping on a printer. In a functional sense, the CPU executes I/O instructions, the channels execute commands, and the control units and devices execute orders.
The sense command specifies a main storage location and transfers one or more bytes of status information from the selected control unit. It provides details concerning the selected I/O device, such as a stacker-full condition of a card reader or a file-protected condition of a magnetic-tape reel.
A channel program normally obtains CCW's from a consecutive string of storage locations. The string can be broken by a transfer-in-channel command that specifies the location of the next CCW to be used by the channel. External documents, such as punched cards or magnetic tape, may carry CCW's that can be used by the channel to govern the reading of the documents.
The input/output interruptions caused by termination of an
Fig. 9. Channel command word format.