Each CCW is 64 bits (8 BYTES) in length and has the following format:

Command Code (Bits 0-7) Specifies the I/O operation to be performed.

Data Address (Bits 8-31) Specifies the location in main storage for the data associated with the I/O operation.

Chain Data Flag (CD) (Bit 32) When set to one indicates the chaining of data.  It causes the storage area of the next CCW to be used with the current I/O operation.  The use of data chaining allows data to be transferred to or from noncontiguous areas of storage in a single I/O operation.

Command Chain (CC) (Bit 33) When set to one, and when the CD flag is set to zero, indicates the chaining of commands.  When the operation of the current CCW is complete and Command Chaining is active, the next CCW will become the new current I/O operation.

Suppress Length Indication (SLI) (Bit 34) When set to one and the CD flag is set to zero, the incorrect length indication is suppressed.

Skip (SKIP) (Bit 35) When set to one the transfer of information is suppressed for read and sense operations.

Program Controlled Interrupt (PCI) (Bit 36) When set to one the channel will generate an interruption condition when the CCW begins execution in the channel.

Indirect Data Address (IDA) (Bit 37) When set to one indicates indirect addressing.  The data address of the CCW points to an Indirect Address List instead of directly to the data.

Count (Bits 48-63) Specifies the number of bytes to be used for the I/O operation.

Bits 38-47 should always be set to zeros.

Channel command codes are specific to each individual device but fall into general categories: Write, Read, Read Backward, Control and Sense.  Write transfers data from the processor storage to the device, read and read backward transfer data from the device to processor storage.  Control operations are specific to each device and generally do not involve data transfer but the data address in the CCW may point to an area in storage related to the control operation.

[Next – Sense Command]