Instruction prefetch
Encyclopedia
In computer architecture
, instruction prefetch is a technique used in microprocessor
s to speed up the execution of a program by reducing wait state
s.
Modern microprocessors are much faster than the memory where the program is kept, meaning that the program's instructions cannot be read fast enough to keep the microprocessor busy. Adding a cache
can provide faster access to needed instructions.
Prefetching occurs when a processor requests an instruction from main memory before it is actually needed. Once the instruction comes back from memory, it is placed in a cache. When an instruction is actually needed, the instruction can be accessed much more quickly from the cache than if it had to make a request from memory.
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).
Computer architecture
In computer science and engineering, computer architecture is the practical art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals and the formal modelling of those systems....
, instruction prefetch is a technique used in microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
s to speed up the execution of a program by reducing wait state
Wait state
A wait state is a delay experienced by a computer processor when accessing external memory or another device that is slow to respond.As of late 2011, computer microprocessors run at very high speeds, while memory technology does not seem to be able to catch up: typical PC processors like the Intel...
s.
Modern microprocessors are much faster than the memory where the program is kept, meaning that the program's instructions cannot be read fast enough to keep the microprocessor busy. Adding a cache
CPU cache
A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations...
can provide faster access to needed instructions.
Prefetching occurs when a processor requests an instruction from main memory before it is actually needed. Once the instruction comes back from memory, it is placed in a cache. When an instruction is actually needed, the instruction can be accessed much more quickly from the cache than if it had to make a request from memory.
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
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
, 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
Graphics processing unit
A graphics processing unit or GPU is a specialized circuit designed to rapidly manipulate and alter memory in such a way so as to accelerate the building of images in a frame buffer intended for output to a display...
) the prefetch can take advantage of the spatial coherence usually found in the texture mapping
Texture mapping
Texture mapping is a method for adding detail, surface texture , or color to a computer-generated graphic or 3D model. Its application to 3D graphics was pioneered by Dr Edwin Catmull in his Ph.D. thesis of 1974.-Texture mapping:...
process. In this case, the prefetched data are not instructions, but texture elements (texels
Texel (graphics)
A texel, or texture element is the fundamental unit of texture space, used in computer graphics. Textures are represented by arrays of texels, just as pictures are represented by arrays of pixels....
) that are candidates to be mapped on a polygon
Polygon
In geometry a polygon is a flat shape consisting of straight lines that are joined to form a closed chain orcircuit.A polygon is traditionally a plane figure that is bounded by a closed path, composed of a finite sequence of straight line segments...
.
The first mainstream microprocessors to use some form of instruction prefetch were the Intel 8086 (six bytes) and the Motorola
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
68000 (four bytes).