IBM z13s, Technical Manual

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IBM z13s Technical Guide
򐂰 The data address in the initial IDAW
For example, three IDAWS are required when these events occur:
򐂰 The ORB specifies format-2 IDAWs with 4 KB blocks.
򐂰 The CCW count field specifies 8 KB.
򐂰 The first IDAW designates a location in the middle of a 4 KB block.
CCWs with data chaining can be used to process I/O data blocks that have a more complex
internal structure, in which portions of the data block are directed into separate buffer areas.
This process is sometimes known as scatter-read or scatter-write. However, as technology
evolves and link speed increases, data chaining techniques become less efficient because of
switch fabrics, control unit processing and exchanges, and other issues.
The MIDAW facility is a method of gathering and scattering data from and into discontinuous
storage locations during an I/O operation. The modified IDAW (MIDAW) format is shown in
Figure 7-4. It is 16 bytes long and is aligned on a quadword.
Figure 7-4 MIDAW format
An example of MIDAW usage is shown in Figure 7-5.
Figure 7-5 MIDAW usage
The use of MIDAWs is indicated by the MIDAW bit in the CCW. If this bit is set, the skip flag
cannot be set in the CCW. The skip flag in the MIDAW can be used instead. The data count in
the CCW must equal the sum of the data counts in the MIDAWs. The CCW operation ends