e:
http://www.garlic.com/~lynn/2007f.html#20 Historical curiosity question
http://www.garlic.com/~lynn/2007f.html#33 Historical curiosity question
"full-pack" minidisk footnote.
moving to fba-like devices ... you tend to have the device returning the number
of
records on the device ... it eliminates the tight binding between device
geometry
and other characteristics that have been part of the CKD device paradigm.
the place where (ckd) full-pack minidisks came into play was with applications
doing dynamic channel program modifications. they wouldn't actually be
modifying the
channel program instructions but they could be dynamically modifying the
seek ccw channel program argument. CCWTRANS had to "shadow" the channel
program instructions in order to convert virtual address to real address
as part of executing the channel program. CCWTRANS would also shadow
seek ccw arguments ... converting virtual (cylinder/track) seek arguments to
"real" seek arguments. Minidisks that didn't start at real location zero
had the seek arguments appropriately incremented ... and minidisks that
didn't extend to end of device ... might have seeking past end of device
simulated for some seek argument. For full-pack minidisks (and/or dedicated
devices) ... the conversion/translation of seek arguments could be eliminated
(with the seek CCW pointing at the virtual seek argument in the the virtual machine
virtual address space ... instead of a shadow translated value).
This showed up with running ISAM in a virtual machine ... where the ISAM channel
programs could be dynamically modifying seek arguments. w/o full-pack minidisks
... the dynamic modification would be with respect to the copy in virtual
machine
virtual memory ... and not the translated version that the shadow seek CCW was
pointing to. With full-pack minidisks (and/or dedicated devices), the shadow
seek CCW could be pointing at the copy in virtual memory (rather than a
translated
copy) ... that potentially is the target of some read CCW possibly doing dynamic
seek argument modification.
recent posts mentioning "self-modifying" ISAM channel program:
http://www.garlic.com/~lynn/2007e.html#14 Cycles per ASM instruction
http://www.garlic.com/~lynn/2007e.html#19 Cycles per ASM instruction
http://www.garlic.com/~lynn/2007e.html#27 IBM S/360 series operating systems
history
http://www.garlic.com/~lynn/2007f.html#4 ISAM and/or self-modifying channel
programs
