... sorry, wrong list ...
-------- Original-Nachricht --------
Betreff: Fwd: Re: 2GiB <= address < 4GiB
Datum: Wed, 05 Nov 2014 09:32:47 +0100
Von: Bernd Oppolzer <[email protected]>
An: IBM Mainframe Discussion List <[email protected]>
... should be "storage access" instead of "storage address" at the end
of the second
paragraph, before the opening paranthese ...
-------- Original-Nachricht --------
Betreff: Re: 2GiB <= address < 4GiB
Datum: Wed, 05 Nov 2014 09:27:50 +0100
Von: Bernd Oppolzer <[email protected]>
An: IBM Mainframe Assembler List <[email protected]>
Maybe it can be explained this way:
when we migrated from 24 bit to 31 bit, we had lots of problems
when loading 24 bit addresses (for example parameter addresses)
from fullwords and using them in 31 bit mode,
and the addresses had some non zero values in the first (leftmost) byte
of the register (which was no problem in 24 bit mode). There was no way
around this but to clean the register's first byte before doing the storage
address (if the address was a 24 bit address in the beginning).
Now we have the same situation; the leftmost bit of the fullword is
ignored when using the addresses in storage access (in 31 bit mode),
be it 0 or 1 ... and it is 1 sometimes.
When expanding a 31 bit address to 64 bit, simply by adding 32 zero
bits on the left or by loading a fullword into the right half of a 64
bit register,
the 31 bit address will be the wrong address, if the leftmost bit in the
fullword
was set.
But this time we have the chance to flag such errors, simply by
excluding this
address range (0x00000000_80000000 to 0x00000000_FFFFFFFF) from
the range of allowable 64 bit addresses in z/OS. It is, given the overall
address range, not too large.
This is, as I understand it, a design decision of z/OS; Linux and z/VM
deciced
that they don't regard such problems as severe enough, or they even don't
exist, because Linux is 64 bits from the start (but I don't really know
this,
only a guess).
Kind regards
Bernd
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