If you know the record IDs are "regular format" (meaning the CCHH part always equals the CCHH of the track on which the records are stored and the R part is monotonically increasing), then you can easily read the records on the track backwards by doing direct reads of R(n), R(n-1), ..., R(1). Of course, this method will require n revolutions of the device on ancient non-cached devices. On modern devices, the first access will cause the whole track to be cached, so only one revolution will be necessary, the same as if you read the whole track into storage with a modern command like Read Track and then work backwards through the data in storage. Working backwards will be straightforward if the record lengths are regular (key lengths all the same and data lengths all the same). Otherwise you will first have to scan forward through the track buffer, find the beginning of each record, save its offset in a table, and then process the records in reverse order starting with the ! last entry in the table.
Bill Fairchild Rocket Software -----Original Message----- From: IBM Mainframe Discussion List [mailto:IBM-MAIN@bama.ua.edu] On Behalf Of Shmuel Metz (Seymour J.) Sent: Wednesday, July 13, 2011 1:19 PM To: IBM-MAIN@bama.ua.edu Subject: Re: Startio In <p06240807ca42c258b562@[192.168.1.11]>, on 07/12/2011 at 11:42 PM, "Robert A. Rosenberg" <hal9...@panix.com> said: >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 ITYM the count field. Unless it was a really old device, it would have been easier to just read the entire track. -- Shmuel (Seymour J.) Metz, SysProg and JOAT ISO position; see <http://patriot.net/~shmuel/resume/brief.html> We don't care. We don't have to care, we're Congress. (S877: The Shut up and Eat Your spam act of 2003) ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@bama.ua.edu 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 lists...@bama.ua.edu with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
