Math Coprocessor

Co-processor

A co-processor is a chip that works side-by-side with the computer's main processor (the chip called the central processing unit, or CPU).The co-processor handles some of the more specialized tasks, such as doing math calculations or displaying graphics on the screen, thereby taking some of the work load off the main processor so it can go on with the business of directing and keeping order over the whole show. A co-processor is installed to reduce the burden on a computer's CPU and thus free it for more general duties such as transferring data and handling multiple tasks.

Math co-processors, for example, are specialized for performing calculations on numbers, and they are much faster at it than the main processor in your computer. So if you have a program that does many math calculations, such as a spreadsheet or a CAD program, then adding a math co-processor to your system can sometimes remarkably improve your computing speed.

There are video co-processors that are used to speed up the display of graphics on your screen. Again, if you use any graphics-based application, including Windows, then adding a video co-processor to your system on an add-in board can speed up your system even more than buying a faster computer.

One catch, however, with any kind of co-processor, is that the software you use must be written so that it knows the co-processor is there, otherwise your system will not recognize its existence and won't be able to use it.

A co-processor may be designed to work just with a particular type of CPU, in which case its instructions can be included in the main program and are passed on to the co-processor by the CPU as it encounters them. In other cases, the co-processor may require its own separate program and program memory, and communicates with the CPU by interrupts or message passing via a shared memory region.

Math Co-processors (Floating-Point Units)

Floating-point unit (FPU) contained in the processor, which was formerly a separate external math co-processor in the 386 and older chips. Older central processing units designed by Intel (and cloned by other companies) used an external math co-processor chip. However, when Intel introduced the 486DX, it included a built-in math co-processor, and every processor built by Intel (and AMD and Cyrix, for that matter) since then includes a math co-processor. Co-processors provide hardware for floating-point math, which otherwise would create an excessive drain on the main CPU. Math chips speed computer's operation only when we are running software designed to take advantage of the co-processor. All the subsequent fifth and sixth generation Intel and compatible processors (such as those from AMD and Cyrix) have featured an integrated floating-point unit, although the Intel ones are known for having the best performance.

Math chips (as co-processors sometimes are called) can perform high-level mathematical operations—long division, trigonometric functions, roots, and logarithms, for example—at 10–100 times the speed of the corresponding main processor. The operations performed by the math chip are all operations that make use of non-integer numbers (numbers that contain digits after the decimal point). The need to process numbers in which the decimal is not always the last character leads to the term floating point because the decimal (point) can move (float), depending on the operation. The integer units in the primary CPU work with integer numbers, so they perform addition, subtraction, and multiplication operations. The primary CPU is designed to handle such computations; these operations are not offloaded to the math chip.

The instruction set of the math chip is different from that of the primary CPU. A program must detect the existence of the co-processor and then execute instructions written explicitly for that co-processor; otherwise, the math co-processor draws power and does nothing else. Fortunately, most modern programs that can benefit from the use of the co-processor correctly detect and use the co-processor. These programs usually are math intensive: spreadsheet programs, database applications, statistical programs, and graphics programs, such as computer-aided design (CAD) software. Word processing programs do not benefit from a math chip and therefore are not designed to use one. Table summarizes the co-processors available for the Intel family of processors.

Math Co-processor Summary

Processor	Co-processor
8086	8087
8088	8087
286	287
386SX	387SX
386DX	387DX
486SX	487SX, DX2/OverDrive
487SX1	Built-in FPU
486SX2	DX2/OverDrive
486DX	Built-in FPU
486DX2	Built-in FPU
486DX4/5x86	Built-in FPU
Intel Pentium/Pentium MMX	Built-in FPU
Cyrix 6x86/MI/MII	Built-in FPU
AMD K5/K6/Athlon/Duron	Built-in FPU
Pentium II/III/Celeron/Xeon	Built-in FPU

                                                                          

FPU = Floating-point unit

Although virtually all processors since the 486 series have built-in floating-point units, they vary in performance. Historically the Intel processor FPUs have dramatically outperformed those from AMD and Cyrix, although AMD and Cyrix are achieving performance parity in their newer offerings.

Within each of the original 8087 group, the maximum speed of the math chips varies. A suffix digit after the main number, as shown in Table indicates the maximum speed at which a system can run a math chip.

Maximum Math Chip Speeds

Part	Speed	Part	Speed
8087	5MHz	287	6MHz
8087-3	5MHz	287-6	6MHz
8087-2	8MHz	287-8	8MHz
8087-1	10MHz	287-10	10MHz

The 387 math coprocessors, and the 486 or 487 and Pentium processors, always indicate their maximum speed rating in MHz in the part number suffix. A 486DX2-66, for example, is rated to run at 66MHz. Some processors incorporate clock multiplication, which means that they can run at different speeds compared with the rest of the system.

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