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Sideport memory is basically what ATI calls the dedicated frame buffer memory for the integrated graphics chip, in this case with the tx2500z, there is 64MB of local memory dedicated to graphics.
Sideport memory is runs considerably faster than system memory, therefore providing noticeable increases in performance over standard IGPs which constantly accesses system memory.
The main reason for the frame buffer is that the IGP need not have to access the memory controller which in AMD systems is located in the CPU to access the memory. In earlier platforms this used to wake the CPU up from low power state just to refresh the buffer. That's why in Griffin the memory controller has its own separate power plane, so even if the IGP needs to refresh it doesn't need to access the CPU at all (if it has sideport memory) or if necessary just uses the memory controller to access the RAM not waking up the CPU cores from their low power state.