Cache

CPU cache is a very high-speed memory that sits between the CPU and the system's main memory. I/O cache sits between the device and the system's main memory, as shown in the following figure.

Figure 9–1 CPU and System I/O Caches

When an attempt is made to read data from main memory, the associated cache checks for the requested data. If the data is available, the cache supplies the data quickly. If the cache does not have the data, the cache retrieves the data from main memory. The cache then passes the data on to the requester and saves the data in case of a subsequent request.

Similarly, on a write cycle, the data is stored in the cache quickly. The CPU or device is allowed to continue executing, that is, transferring data. Storing data in a cache takes much less time than waiting for the data to be written to memory.

With this model, after a device transfer is complete, the data can still be in the I/O cache with no data in main memory. If the CPU accesses the memory, the CPU might read the wrong data from the CPU cache. The driver must call a synchronization routine to flush the data from the I/O cache and update the CPU cache with the new data. This action ensures a consistent view of the memory for the CPU. Similarly, a synchronization step is required if data modified by the CPU is to be accessed by a device.

You can create additional caches and buffers between the device and memory, such as bus extenders and bridges. Use ddi_dma_sync(9F) to synchronize all applicable caches.