This is used automatically by the video hardware.
Quoted from this thread:
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.