Cache Effects

Another common technique used to improve execution speed is instruction and/or data caching. The instruction cache is high speed memory internal or external to the processor which is used to hold copies of instructions as they are fetched from memory. The data cache is high speed memory internal or external to the processor which is used to buffer data on its way to or from external memory.

The entire address space maps to the cache in some processor dependent fashion. The cache includes control information which identifies whether each instruction or data element in the cache is valid. When the processor fetches an instruction or data element that is already marked as valid in the cache, the cache copy is used with no time penalty and no external memory interference.

The instruction cache is usually internal to the processor and hence fixed in size. Some processors also include an internal data cache. When an external cache is used, the size of the cache is usually determined by the manner in which it is interfaced to the processor.

Some processors allow the sizes of the internal instruction and data caches to be adjusted to meet the needs of specific applications. Some processors also allow specific blocks of instructions to be copied to the cache and locked in place for guaranteed fast access to particular pieces of code.

Finally, most processors allow the application to dynamically enable or disable the instruction and/or data caches. It is also usually possible to invalidate all or portions of the instruction and/or data caches.