Steve Samson wrote:
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.
I would not characterize the z/OS (or z/VSE should they ever implement
such a thing) virtual storage "dead" zone as "necessary". Rather, it was
a convenient and "smart" choice. Otherwise, many more bugs would have
gone undetected.
--
Edward E Jaffe
Phoenix Software International, Inc
5200 W Century Blvd, Suite 800
Los Angeles, CA 90045
310-338-0400 x318
[EMAIL PROTECTED]
http://www.phoenixsoftware.com/
----------------------------------------------------------------------
For IBM-MAIN subscribe / signoff / archive access instructions,
send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO
Search the archives at http://bama.ua.edu/archives/ibm-main.html
