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Old May 13th 05, 02:34 PM
CBFalconer
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Tim Anderson wrote:
"Rick" wrote in message

Because *applications* have to be written to be PAE-aware,
and have to be written in 64-bit address space to make use of
anything over 4GB. This was (and still is) very much a niche
market -- very few desktop/workstation users will come close
to using 2GB, let alone 4, which is why MS originally split the
4GB address range between 2GB user space and 2GB
system space.


Thanks for the response. I understand about that; what I'm talking
about is the hardware issue. When you boot with 4GB installed on
one of these motherboards, typically 1GB is not made available to
the OS at all; it is "consumed" before the OS boots (because of
address shadowing). Even with an AMD64 (EM64T) CPU installed.


Because once you assign a group of memory addresses for non-memory
(i.e. i/o) use you have to decode that range. Not only do you have
to designate a range for i/o, you also have to designate the
complementary range for actual memory. The decoders involve added
loads on the address lines, and added propagation delay. So you
want to minimize the bits used in this, and possibly strengthen
their output buffers on the CPU chip.

All of which costs time, power, and performance.

I expect the main problems are due to the added capacitance from
added fan-out. This is similar to the often encountered
difficulties when further memory modules are added to a system.

All of this deals with the physical addresses, not the virtual
addresses that the software sees.

--
Chuck F ) )
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