Direct Access Channel Program

The direct access channel program is 24 bytes in length (except when set sector is used for RPS devices) and immediately follows the input/output block. Depending on the type of I/O operation that is specified in the XDAP macro instruction, one of four channel programs can be generated. The three channel command words for each of the four possible channel programs are shown in Table 1.

Table 1. Channel Program Command Words Used by XDAP
Type of I/O Operation	CCW	Command Code
Read by identification	1	Search ID Equal
	2	Transfer in Channel
Verify by identification	3	Read Key and Data
Read by key	1	Search Key Equal
	2	Transfer in Channel
Verify by key	3	Read Data
Write by identification	1	Search ID Equal
	2	Transfer in Channel
	3	Write Key and Data
Write by key	1	Search Key Equal
	2	Transfer in Channel
	3	Write Data
Note: For verifying operations, the third CCW is flagged to suppress the transfer of information to virtual storage.

When a sector address is specified with an RI, VI, or WI operation, the channel program is 32 bytes in length. Each of the channel programs in Table 1 would be preceded by a set sector command.