David, Not sure if this might be OT - you did specify, "VM Function" - but there were various constraints regarding using non-relocate-0 minidisks (that is, a minidisk whose virtual cylinder zero doesn't correspond to a real cylinder zero) for OS guests 'way back when.
As a youth I was taught that non-relocate-0 OS packs should have one track VTOCs and should not have ISAM files on them. I think that ISAM is now no more than a fond(?) memory for all except Hercules hobbyists and I'm pretty sure that the combination of modern hardware and software has lifted the one track VTOC constraint - but they definitely existed as issues to consider back in '370 days. Maybe this'll be enough to tempt Mr Wheeler out of his shell with some definitive explanations regarding the practical constraints and the architectural basis for them (along, with, no doubt, a few anecdotes for us all to enjoy) :-) Regards, Jeff Gribbin
