Interesting question, with a number of answers.
The second value only comes into play at PAGEFIX time, be that your program
doing it explicitly, or the system doing it for you, most often as part of an
I/O operation, such as a BSAM READ or WRITE, or a QSAM GET or PUT.
Up until that PAGEFIX happens any real page frame can back any virtual page. I
have seen 24-bit real backing a 64-bit virtual and the reverse. Only at
PAGEFIX does he make sure that the real page backing the virtual is less than
or equal to what you specified in the LOC= parameter.
When you do a GETAMIN, or STORAGE OBTAIN, or whatever, the system notes your
request and sets up some control areas, but no frames are assigned, yet. Once
you touch the page, then a real frame is assigned to the virtual page, and the
system doesn't care where that real frame is.
At PAGEFIX time, then it matters. If you had a LOC=(31,31) and the initial
backing frame is a 24-bit frame, it may do nothing or it may move the data to a
31-bit frame. If the initial backing frame is a 64-bit frame, then it will
move the data to a 24-bit or 31-bit frame.
Now to your question. Unless you are doing something special where you know
you need 24 or 31 bit real, you can use LOC=(24,64), or LOC=(31,64). BSAM,
QSAM and VSAM all support 64-bit backing frames. Since most normal programs
can't do a LRA/LRAG/LRAY, you are probably safe. I can't think of any
non-privileged instructions that are real frame sensitive.
DCB's can be LOC=(24,64)
As with any change, TEST, TEST, TEST. And if I have miss-spoken at any point,
I am sure someone will set the record straight.
HTH
Chris Blaicher
Principal Software Engineer, Software Development
Syncsort Incorporated
50 Tice Boulevard, Woodcliff Lake, NJ 07677
P: 201-930-8260 | M: 512-627-3803
E: cblaic...@syncsort.com
On 9/6/2013 1:52 PM, Manfred Lotz wrote:
I'm maintaining a larger program written in assembler where GETMAINs have
mostly LOC=(BELOW,ANY) or LOC=(ANY,ANY).
This program doesn't use anything like PGSER, EXCPVR or the like.
My question: In order to give the operating system most flexibility wouldn't
it be recommended to change:
LOC=(BELOW,ANY) --- LOC=(24,64)
LOC=(ANY,ANY)--- LOC=(31,64)
Any opinion appreciated.
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