# Microprocessor Types and Specifications

## Math Coprocessors (Floating-Point Units)

This section covers the floating-point unit (FPU) contained in the processor, which was formerly a separate external math coprocessor in the 386 and older chips. Older central processing units designed by Intel (and cloned by other companies) used an external math coprocessor chip. However, when Intel introduced the 486DX, it included a built-in math coprocessor, and every processor built by Intel (and AMD and Cyrix, for that matter) since then includes a math coprocessor. Coprocessors provide hardware for floating-point math, which otherwise would create an excessive drain on the main CPU. Math chips speed your computer's operation only when you are running software designed to take advantage of the coprocessor. 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 coprocessors 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 noninteger 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 coprocessor and then execute instructions written explicitly for that coprocessor; otherwise, the math coprocessor draws power and does nothing else. Fortunately, most modern programs that can benefit from the use of the coprocessor correctly detect and use the coprocessor. 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 3.15 summarizes the coprocessors available for the Intel family of processors.

#### Table 3.15 Math Coprocessor Summary

 Processor Coprocessor 8086 8087 8088 8087 286 287 386SX 387SX 386DX 387DX 486SX 487SX, DX2/OverDrive9 487SX1 Built-in FPU 486SX2 DX2/OverDrive10 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 3.16, indicates the maximum speed at which a system can run a math chip.

#### Table 3.16 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.

TIP

The performance increase in programs that use the math chip can be dramatic—usually, a geometric increase in speed occurs. If the primary applications that you use can take advantage of a math coprocessor, you should upgrade your system to include one.

Most systems that use the 386 or earlier processors are socketed for a math coprocessor as an option, but they do not include a coprocessor as standard equipment. A few systems on the market don't even have a socket for the coprocessor because of cost and size considerations. These systems are usually low-cost or portable systems, such as older laptops, the IBM PS/1, and the PCjr. For more specific information about math coprocessors, see the discussions of the specific chips—8087, 287, 387, and 487SX—in the later sections. Table 3.17 shows the specifications of the various math coprocessors.

#### Table 3.17 Older Intel Math Coprocessor Specifications

 Name Power Consumption Case Minimum Temperature Case Maximum Temperature No. of Transistors Date Introduced 8087 3 watts 0°C, 32°F 85°C, 185°F 45,000 1980 287 3 watts 0°C, 32°F 85°C, 185°F 45,000 1982 287XL 1.5 watts 0°C, 32°F 85°C, 185°F 40,000 1990 387SX 1.5 watts 0°C, 32°F 85°C, 185°F 120,000 1988 387DX 1.5 watts 0°C, 32°F 85°C, 185°F 120,000 1987

Most often, you can learn what CPU and math coprocessor are installed in a particular system by checking the markings on the chip.

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