In a message dated 3/24/2006 7:13:27 P.M. Central Standard Time, [EMAIL PROTECTED] writes:
Excellent basic primer on records and blocks. A few comments for the more advanced user: >each physical record on tape and DASD ... >... >The hardware limit is established by the Count fields in both Format 0 and Format 1 >CCWs, and is 64K.) The maximum number of bytes that can be transferred by one CCW is 65,535. But you can have any number of CCWs, each transferring 65,535 bytes and using data chaining, to make a much larger block than 64K. Of course, such a block cannot be handled by standard access methods. On tape there is no limit. On DASD the limit these days is one full track, but on DASDs prior to the 3380 there was a feature called Write Special CKD that would let you write a single DASD hardware record (aka a software block) that was larger than one full track. I once wrote a deblocker program to read in 640K tape blocks and break them up into QSAM-friendly chunks of 32,760 bytes or less. It was an interesting exercise, made even more so by having to run it on MVS under VM, which caused a lot of unrepeatable chaining check errors due to the very long channel program to read in 640K in one I/O request. Bill Fairchild ---------------------------------------------------------------------- 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
