In the late 1970s, I worked at a service bureau where I had to write a tape file conversion program to reblock a tape file with 640KB blocks to a much smaller physical block size that could be handled by QSAM. My program read in one block with an EXCP and 10 CCWs data chained, each of which read in 64KB bytes, all into one buffer of 640KB bytes long. Then I copied the buffer into 20 32KB blocks to an output tape using QSAM. Then I read the next input tape block, etc., until the input found the end-of-file marker. It worked. But I had to run it under MVT running under VM, and VM kept doing funny things with the massive real channel program built by MVT's I/O Supervisor that caused chaining checks at random places in the chain, so I reran the job until I had an execution with no I/O errors.
The CCW architecture allows 64KB per CCW and any number of CCWs chained together. Control unit limitations are smaller. Software limitations are smaller still. I don't think anything good would happen on a DASD controller now if you fed it a CCW with a byte count longer than one full track (ca. 58K bytes). But the CCW architecture would not be the limiting factor. Bill Fairchild ----- Original Message ----- From: "Gerhard Postpischil" <[email protected]> To: [email protected] Sent: Monday, December 9, 2013 11:53:52 AM Subject: Re: "hexadecimal"? On 12/9/2013 11:53 AM, Scott Ford wrote: > Bill, > I thought you could chain ccws I think you misread his message, which started with command chaining. But I would be interested in which control units and controllers allow data chaining beyond 65KiB. Gerhard Postpischil Bradford, Vermont ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
