My point was that one is smaller than 18, and zero is also smaller than 18. The controversy is whether or not a DASD block with a block size smaller than 18 can be written onto a DASD, not over what happens when said block is read back in. An earlier post said that a block size smaller than 18 is considered a noise record. That is true for tapes but not DASD. A unit exception, aka EOF, is not the same as a noise record. EOF is signaled on tape with a tape mark, which is written with a Write Tape Mark CCW command, not with a Write CCW with a length smaller than 18, but an EOF on DASD is signaled with a data length of zero, which is written with a WCKD or WCKD Next Track CCW command, with a byte count of at least 8, and pointing to a count field containing zero in the data field. There has never been a requirement on IBM mainframe DASD, from S/360 through z/whatever, that the data field be no smaller than 18. There is such a requirement for tape I/O.
Does anyone have a copy of the 3081 Channel Characteristics manual? I have been told that this book describes a situation in which a 3380 did not work correctly with a block size smaller than 18 and want to see exactly what it says. Thanks. Bill Fairchild Software Developer Rocket Software 275 Grove Street * Newton, MA 02466-2272 * USA Tel: +1.617.614.4503 * Mobile: +1.508.341.1715 Email: [email protected] Web: www.rocketsoftware.com -----Original Message----- From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf Of Rick Fochtman Sent: Tuesday, August 11, 2009 10:20 AM To: [email protected] Subject: Re: Concatenations and blocksizes ---------------------------<snip>----------------------- You have always been able to write a block of only one byte, or even zero bytes, onto a DASD. ----------------------------<unsnip>---------------------- One byte, yes. But if the device reads a count field with a data length of zero, it signals a Unit Exception, meaning a EOF mark. That's what you might call a "special case". :-) Rick ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
