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Modified Indirect Data Address Words (MIDAWs) are a channel programming capability of the IBM System z9 processor.[1] The MIDAW facility is an extension to the pre-existing Indirect Data Address Word (IDAW) channel programming capability, providing support for more efficient FICON channel programs.

MIDAWs allow ECKD channel programs to read and write to many storage locations using one channel command, which means fewer signals up and down the channel are required to transfer the same amount of data. This reduction is particularly noticeable for Extended Format data sets, accessed through Media Manager. Examples include Extended Format Sequential data sets, Extended Format VSAM data sets and certain types of DB2 tablespaces. While each of these data set organizations have alternatives, each has a distinct set of advantages, whether in the area of performance, space saving (through hardware-assisted data compression), or scalability (by allowing an individual data set to exceed 4 GiB).


  1. ^ IBM z/Architecture Principles of Operation, publication number SA22-7832