IDAW Requirements for EXCP Requests

For EXCP requests, the virtual channel program provided by the caller can have one or more CCWs with the indirect address (IDA) flag set and the address portion of these CCWs pointing to a single 31-bit or 64-bit IDAW. This EXCP function is referred to as virtual IDAWs.

The 31-bit IDAW can contain a valid virtual address up to the maximum 31-bit address. Virtual IDAWs are supported on all virtual CCWs except:

Transfer in channel (TIC) commands
Read, read backward, and sense commands, with the skip flag set.
All nondata-transfer type commands: for example, recalibrate, rewind, forward space, fold, block data check, no operation, control commands

A virtual IDAW points to a single, contiguous 31-bit or 64-bit virtual storage area. The virtual storage area can be backed by central storage above or below 2 GB. Either type of virtual IDAW can point to storage that is backed above 2 GB. When EXCP translates the channel program, the virtual IDAW is translated into one or more real IDAWs in 31-bit central storage.

Note: 64-bit IDAWs are supported only for direct access storage devices (DASD) and on all IBM-supplied cartridge tape devices. You should examine bit UCBEIDAW in the UCB to determine whether 64-bit IDAWs are supported by the device.