The block length is indicated by the data length within the count field. A DASD block is written on the track with a Write Count, Key and Data or Write Count, Key, and Data Next Track command. The length of a count field is always 8. If you are to write a block length of zero onto a track, you put the block length into an 8-byte area in which you are building the count field and you set the CCW's byte count to 8, for the number of bytes to be transferred to the controller and written onto the track as a count field. When your software tries to read this block back later, the controller sees the data length of zero and signals unit exception along with the other ending status, then the access method interprets unit exception as end of file and invokes your EOF routine if you have one.
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 Paul Gilmartin Sent: Monday, August 10, 2009 4:18 PM To: [email protected] Subject: Re: Concatenations and blocksizes On Mon, 10 Aug 2009 16:55:31 -0400, Bill Fairchild wrote: >You have always been able to write a block of only one byte, or even zero >bytes, onto a DASD. > I thought a count of zero was prohibited in a CCW. (But there may be other ways.) As for the minimum of 18 for tape, how does the access method deal with a program that does RECFM=VB; OPEN PUT RDW has count of 4; CLOSE; which would generate an 8-byte block. This might easily happen inadvertently if a preceding block was filled and the 8-byte (or up to 17-byte) block was the last block written before CLOSE. If RECFM=VBS, the access method _could_ pad with a null segment, but I don't know that null segments are allowed for non-Spanned RECFM. -- gil ---------------------------------------------------------------------- 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
