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Mac OS assigns default cache modes to various kinds of memory. Main memory defaults to copyback cache mode; PCI memory space defaults to cache-inhibited mode
With these settings, drivers do not need to perform specific cache flushing. However, drivers may wish to alter a memory section's default cache mode to create the highest performance data transfer rate for their application. For example, the PowerPC processor performs burst bus transactions to memory in copyback or writethrough cache modes.
Drivers may also want to set areas of PCI memory space to a cacheable setting, thereby causing the PowerPC to burst to that space; however, extreme care must be taken to perform appropriate cache flushing when operating on cacheable PCI memory space. Drivers that control PCI master devices may wish to experiment with different cache modes for their DMA buffer spaces to determine the optimal setting.