Cache prefetching

Cache prefetching is a technique used by computer processors to boost execution performance by fetching instructions or data from their original storage in slower memory to a faster local memory before it is actually needed (hence the term 'prefetch'). Most modern computer processors have fast and local cache memory in which prefetched data is held until it is required. The source for the prefetch operation is usually main memory. Because of their design, accessing cache memories is typically much faster than accessing main memory, so prefetching data and then accessing it from caches is usually many orders of magnitude faster than accessing it directly from main memory. Prefetching can be done with non-blocking cache control instructions. Cache prefetching can either fetch data or instructions into cache. Data prefetching fetches data before it is needed. Because data access patterns show less regularity than instruction patterns, accurate data prefetching is generally more challenging than instruction prefetching. Instruction prefetching fetches instructions before they need to be executed. The first mainstream microprocessors to use some form of instruction prefetch were the Intel 8086 (six bytes) and the Motorola 68000 (four bytes). In recent years, all high-performance processors use prefetching techniques. Cache prefetching can be accomplished either by hardware or by software. Hardware based prefetching is typically accomplished by having a dedicated hardware mechanism in the processor that watches the stream of instructions or data being requested by the executing program, recognizes the next few elements that the program might need based on this stream and prefetches into the processor's cache. Software based prefetching is typically accomplished by having the compiler analyze the code and insert additional "prefetch" instructions in the program during compilation itself. Stream buffers were developed based on the concept of "one block lookahead (OBL) scheme" proposed by Alan Jay Smith.