At 13:31 -0500 on 07/06/2011, Blaicher, Chris wrote about Re: Startio:
With EXCP and EXCP/VR you can do more complicated I/O, like a CCW per record and do buffering in the channel program rather than moving data to a buffer. The problem there is all the page-fixing that the system has to do (EXCP) or you have to do (EXCP/VR).
Working with EXCP is fun at times. I once had to write a routine to simulate doing READ BACKWARDS from tape by accessing the file from DASD (to allow reading an archive log so I could do data back-out). I used EXCP to read the CKD field one track at a time until I reached the end of track (which told me how many blocks there were on that track). Using that information, I was able to read the blocks in reverse order via BDAM (and then extract the records in the block in reverse order). I felt reading the CKD field (but not the records) was the simplest solution since BDAM would get the blocks for me.
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