Fast Writes is another newly exploited feature that allows the chipset to "force" a PCI transfer to the AGP system, thus bypassing AGP memory.
Without fast writes, any information the CPU needs to pass to the AGP master must be placed in AGP memory for the controller to read. Fast writes allows the CPU to bypass this, speeding up the data throughput.
Fast writes does supply a small improvement in performance, but some video cards don't support this feature. The bus logic method used to initiate a fast writes transfer is determined by the AGP bus mode (1X, 2X, or 4X). Fast Write operations can only take place under specific system conditions, and may cause stability problems. In cases where stability is compromised, the AGP card manufacturer should take steps to disable problematic features without harming the cards' performance too greatly.