Tile data for drawing and desktop buffers in a desktop compositor system is
managed using "copy-on-write" semantics, in which tile data stored in a memory
location is not transferred to another location until the tile data for one of
the buffers is modified. For each tile in drawing buffers and desktop buffers,
an association is maintained with a location in a tile memory, and the number of
buffer tiles associated with each location is tracked. To copy a tile from one
buffer to another, the tile association for the tile in the destination buffer
is modified. New data for a tile of a buffer is written to the tile memory location
associated with the buffer after ensuring that the tile memory location is not
associated with any other tiles of any of the buffers. As a result, memory bandwidth
can be considerably reduced.
