The supported operating systems are listed in Table 7-6 on page 218 and Table 7-7 on page 219. 

For more information about FICON channel performance, see the performance technical papers that are available at the IBM Z I/O connectivity page of the IBM IT infrastructure website. 

## Modified Indirect Data Address Word facility 

The Modified Indirect Data Address Word (MIDAW) facility improves FICON performance. It provides a more efficient channel command word (CCW)/indirect data address word (IDAW) structure for certain categories of data-chaining I/O operations. 

The MIDAW facility is a system architecture and software feature that is designed to improve FICON performance. This facility was first made available on IBM System z9fi servers, and is used by the Media Manager in z/OS. 

The MIDAW facility provides a more efficient CCW/IDAW structure for certain categories of data-chaining I/O operations. 

MIDAW can improve FICON performance for extended format data sets. Non-extended data sets can also benefit from MIDAW. 

MIDAW can improve channel utilization and I/O response time. It also reduces FICON channel connect time, director ports, and control unit processor usage. 

IBM laboratory tests indicate that applications that use EF data sets, such as Db2, or long chains of small blocks can gain significant performance benefits by using the MIDAW facility. 

MIDAW is supported on FICON channels that are configured as CHPID type FC. The supported operating systems are listed in Table 7-6 on page 218 and Table 7-7 on page 219. 

## MIDAW technical description 

An IDAW is used to specify data addresses for I/O operations in a virtual environment.$^{9}$ The IDAW design allows the first IDAW in a list to point to any address within a page. Subsequent IDAWs in the same list must point to the first byte in a page. Also, IDAWs (except the first and last IDAW) in a list must manage complete 2 K or 4 K units of data. 

