The following message is a courtesy copy of an article that has been posted to bit.listserv.ibm-main,alt.folklore.computers as well.
[email protected] (Mike Myers) writes: > ISAM did some pretty interesting channel programming. It was back in > the late '60s and early '70s while teaching the internals of ISAM and > BDAM to Program Support Reps that I learned about self-modifying > channel programs (fun stuff). re: http://www.garlic.com/~lynn/2009r.html#52 360 programs on a z/10 not long after joining the science center ... I got sent out to Denver on cp67 customer support call ... King Resources was trying to get a large ISAM application running under cp67 ... and I spent a week working 3rd shift in their datacenters. This was when machine room was still a "show place" (1st floor downtown office building with outside glass floor-to-ceiling) ... as mentioned here http://www.garlic.com/~lynn/2009r.html#21 Small Server Mob Advantage channel programs built in a "virtual environment" (whether the virtual machine or from application virtual address space) got "translated" into shadow/copy channel programs (with real addresses) that were the ones that really executed. lots of ISAM "self-modifying" was reading/writting BBCCHHR values which were later used by other CCWs in the same channel program. additional challenge for cp67 was "minidisks" ... disk spaces that "virtual cylinder zero" that was other than "real cylinder zero" ... which required translating "CC" field also (so seek BBCCHH fields were copied also & translated also). Since the real seek was then pointing to the copied value ... when a read operation read in a new "BBCCHHR" ... it was into the virtual location ... not the location pointed to by the seek actually executed. After doing all sorts of fiddling ... trying to recognize ISAM channel programs and reads that were fetching BBCCHH field ... the eventual solution was to recognize "full-pack" virtual disks that didn't do the fiddling with seek BBCCHH (and hope noboby really wanted to run ISAM channel programs on minidisks). -- 40+yrs virtualization experience (since Jan68), online at home since Mar1970 ---------------------------------------------------------------------- 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
