Instruction prefetch

In computer architecture, instruction prefetch is a technique used in central processor units to speed up the execution of a program by reducing wait states.

Contents

• Types of prefetching
• Data or instruction prefetching
• Hardware or software prefetching
• Prefetching metrics
• Prefetch degree
• Prefetch distance
• Usefulness
• References

Prefetching occurs when a processor requests an instruction or data block from main memory before it is actually needed. Once the block comes back from memory, it is placed in a cache. When the instruction/data block is actually needed, it can be accessed much more quickly from the cache than if it had to make a request from memory. Thus, prefetching hides memory access latency and hence, it is a useful technique for addressing the memory wall issue.

Since programs are generally executed sequentially, performance is likely to be best when instructions are prefetched in program order. Alternatively, the prefetch may be part of a complex branch prediction algorithm, where the processor tries to anticipate the result of a calculation and fetch the right instructions in advance. In the case of dedicated hardware (like a Graphics Processing Unit) the prefetch can take advantage of the spatial coherence usually found in the texture mapping process. In this case, the prefetched data are not instructions, but texture elements (texels) that are candidates to be mapped on a polygon.

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, many high-performance processors use prefetching techniques.