Read buffer channel program

z/OS Communications Server: SNA Diagnosis Vol 1, Techniques and Procedures

Read buffer channel program

z/OS Communications Server: SNA Diagnosis Vol 1, Techniques and Procedures
GC27-3667-00

The following table shows how CCWs are used in a Read buffer channel program.

Table 1. Read buffer channel program
CCW	Command code (hex)	Flags	Address	Byte count
Select	0B or 1B ¹	CC,SLI	Zero	1
TIC	08	—	Read buffer CCW	—
Read Buffer ² (As many as needed)	02	CD	Input Area	Length of data
TIC	08	—	Read buffer CCW	—
Read Buffer ³	02	CD	Input Area	Length of data
Read Buffer (Skip)	02	SKIP	Zero	X'7FFF'

¹ If the UCB indicates that the device will accept the Select Read, code X'1B', it is used until a command reject is received. Then Select, code X'0B', is used.

² The first Read CCW is pointed to by LDNACREQ.

³ The number of bytes transmitted in the previous Read buffer CCW is stored in the LDNRBLEN field in the LDNCB. For this Read buffer, enough buffers are allocated to hold LDNRBLEN bytes of data. If this is not enough buffers for the device to send all of the pending data, the channel program will end on the Read Skip CCW. The Read Skip CCW reads all of the pending data but does not transmit any of it. The new length is stored in LDNRBLEN. (The length is computed by subtracting the residual byte count in the CSW from X'7FFF'.) Then the channel program is executed again with enough buffers allocated to read and transmit all of the pending data.

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