At 11:02 -0500 on 07/05/2011, Paul Gilmartin wrote about Re: Lines,
Bars and ... mini-bars???:
Not very special. The 2 GiB thickness of the bar is a product of
the fertile imagination of z/OS. Other OSes routinely permit use
of storage within that range, and routinely support execution above
2GiB and above 4 GiB.
The need for the bar has a number of reasons. One of the major ones
is that in 32Bit Addressing the high bit of the 32bit addresses was
used as a flag to signal end-of-parm-list. Thus the maximum address
that could be passed was 2GiB-1. While a non-zero high word in the
64Bit addresses says that an address was over 4GiB, a zero high word
with the high bit on in the low word could be mean with an address
between 2GiB and 4 GiB-1 OR a flagged address under 2GiB so that 2GiB
range is banned for addresses so that only the flag meaning is valid.
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