An example of MIDAW usage is shown in Figure 7-4. 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  when the CCW count goes to zero or the last MIDAW (with the last flag) ends.  The combination of the address and count in a MIDAW cannot cross a page boundary.  Therefore, the largest possible count is 4 K. The maximum data count of all the MIDAWs in a  list cannot exceed 64 K, which is the maximum count of the associated CCW. The scatter-read or scatter-write effect of the MIDAWs makes it possible to efficiently send  small control blocks that are embedded in a disk record to separate buffers from those that  are used for larger data areas within the record. MIDAW operations are on a single I/O block,  in the manner of data chaining. Do not confuse this operation with CCW command chaining. z/OS extended format (EF) data sets use internal structures (usually not visible to the  application program) that require a scatter-read (or scatter-write) operation. Therefore, CCW  data chaining is required, which produces less than optimal I/O performance. Because the  most significant performance benefit of MIDAWs is achieved with EF data sets, a brief review  of the EF data sets is included here. VSAM and non-VSAM (DSORG=PS) sets can be defined as EF data sets. For non-VSAM  data sets, a 32-byte suffix is appended to the end of every physical record (that is, block) on  disk. VSAM appends the suffix to the end of every control interval (CI), which normally  corresponds to a physical record.  Extended format data sets Figure 7-4 MIDAW usage 284  IBM z14 (3906) Technical Guide