The discussion suggests that the "dead zone" represented an arbitrary
decision. However it is absolutely necessary to preserve compatibility
with programs dating back to OS/360. If a 24-bit or 31-bit address is
interpreted as or expanded to a 64-bit address and the high-order bit
happens to be on, that would cast the virtual address into the 2-4
gigabyte range and unpredictable effects could ensue.
Use of the high-order bit in an address to signal the end of a parameter
list is common, and no practical means of filtering or converting the
programs is available.
I think the dead zone is necessary in z/VSE for the same reason.
Other operating systems did not use the high order bit in the same way,
so there is no need for the dead zone in virtual addresses.
Has this helped to achieve clarity?
Steve Samson
, IBM Mainframe Discussion List wrote:
In a message dated 11/3/2007 9:49:47 A.M. Central Standard Time,
[EMAIL PROTECTED] writes:
The "bar" is at 2G. But, there is a "dead zone" between 2G and 4G
that will never be allocated by z/OS. For simplicity, some folks like to
think of the z/OS virtual storage "bar" as being 2G thick. And, of
course, there is no equivalent dead zone when working with real storage.
And there is not necessarily any equivalent dead zone when working on a
z/Architecture mainframe processor with any operating system other than z/OS, and
such platforms may even allow virtual addressing in the z/OS-dead zone.
Bill Fairchild
Franklin, TN
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