After no replies from the forum,i started to search for sideport memory in the net and other forums.This is what i got.
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Sideport memory is used by ati tech. as dedicated buffer memory for integrated
graphics,located on the motherboard.It is usually higher clocked ddr2 or ddr3 memory
which is faster than usual system memory providing better performance for integrated graphics.
Motherboards claim for an increased >10% performance by using sideport memory.
This was incorporated in amd motherboards because the IG need not access the memory controller
which is located in the cpu to refresh the buffer memory(So that the cores of the cpu need not
be awaken from their low power states).
For more detailed information:
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http://www.tomshardware.com/reviews/790gx-graphics-sideport,2088.html
And finally coming to the answer for the my question
i think sideport memory wont be of any use if you use external pci e graphics card as they have separate
memory for it's operation.
But still for those who look for a energy efficient system and use most of time
spending on surfing,checking mails or similar non extreme graphics applications
IG is very good by providing hybrid options(ie.by switching between IG and pci e gpu).
where sideport memory out performs comparably slower system memory.
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Pls correct my mistakes(if any) and add extra details.