But, all disk CCws also have a module number in the seek address that is
now unused.
Only device I know of that used it was the 'Noodle Snatcher' 2321?
which was supported by MVT.
So isn't it just a matter of reinserting code to increment the module
number ?? :)))
Chris
Bruce Black wrote:
The DCE (UCB DASD Class Extension) has two different halfword fields
indicating capacity. It would appear that the next model larger in
capacity than the model 54 with 65520 cylinders will push the
envelope off its edge.
The problem is not the DCE, it is the CCWs. In all disk CCWs, the
cylinder number is a half-word, 2 bytes, which has a maximum value of
65535 as an unsigned value (X'FFFF').
This is why the large disk support was in two phases. There were many
places in IBM and other code which did signed arithmetic on the
cylinder number. The largest positive signed half-word value is 32767
(X'7FFF') and IBM chose to support 32760 as the largest cylinder
number in the first phase. Both IBM and ISVs had to make some changes
to be sure they could handle values that large, but at least the
arithmetic did not have to change.
The second phase, up to 65520 cylinders, required IBM and others to
review every place that cylinder numbers were manipulated and insure
that unsigned arithmetic was used. This was undoubtedly a more
extensive change.
Disk hardware has always treated the cylinder number as unsigned, so
no CCW processing changes were needed there.
but IBM can't increase the size again without changing CCW protocols
to specify larger cylinder numbers. A number of schemes have been
tossed around, but all would require every piece of IBM and ISV code
which builds or scans disk CCWs to be modified. I haven't heard any
recent talk about doing so, so perhaps it is a dead idea.
The changes on the z990 to support multiple channel subsystems is
beginning to address the maximum number of disk device limitations, so
perhaps this is IBM's solution to the problem.
--
Chris Langford,
Cestrian Software:
Consulting services for: VM, VSE, MVS, z/VM, z/OS, OS/2, P/3x0 etc.
z/FM - A toolbox for VM & MVS at http://zfm.cestrian.com
Deva Woodcrafting:
Furniture creation, House remodeling, Wagon restoration etc.
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