For EXCPVR requests, the channel program provided by the caller can have one or more CCWs with the modified indirect address (MIDA) flag set. When this flag is set, the CCW points to a list of one or more modified indirect address words (MIDAWs). MIDAWs are similar to IDAWs except that the boundary and length requirements are relaxed. For example, each IDAW in a list, except the first, must point to a storage area on either a 2 KB or 4 KB boundary depending on the type of IDAW, and the length includes storage up to the next 2 KB or 4 KB boundary or until the CCW count is exhausted. On the other hand, MIDAWs can point anywhere on a page and contain any length as long as a page boundary is not crossed. MIDAWs can reduce the number of CCWs in your channel program by eliminating the need for data chaining, thereby improving the performance of your channel program.

MIDAWs are supported only for direct access storage devices (DASD) and only when running on an IBM® System z9® or higher processor. Examine bit UCBMIDAW in the UCB to determine whether MIDAWs are supported by the device

If you use MIDAWs, you must specify an IOBE for the EXCPVR request and set flag IOBEMIDA in the IOBE.