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Upgrading & Repairing PCs, Eighth Edition

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Author: Scott Mueller
Retail Price: $49.99
Publisher: Que
ISBN: 0789712954
Publication Date: 9/16/97
Pages: 1168


Chapter 6 - Microprossor Types and Specifications

Award-winning author Scott Mueller patiently retraces the evolution of microprocessors (including math coprocessors) from the 8088 onwards providing detailed specs for each.

 
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The brain of the PC is the processor, or Central Processing Unit (CPU). The CPU performs the system's calculating and processing--except for special math-intensive processing in systems that have a math coprocessing unit chip. The processor is easily the most expensive chip in the system. All the PC-compatibles use processors that are compatible with the Intel family of chips, although the processors themselves may have been manufactured or designed by various companies, including AMD, IBM, Cyrix, and others.

The following sections cover the processor chips that have been used in personal computers since the first PC was introduced almost two decades ago. These sections provide a great deal of technical detail about these chips and explain why one type of CPU chip can do more work than another in a given period of time. First, however, you learn about two important components of the processor: the data bus and the address bus.

Processor Specifications

Many confusing specifications often are quoted in discussions of processors. The following sections discuss some of these specifications, including the data bus, address bus, and speed. The next section includes a table that lists the specifications of virtually all PC processors.

Data Bus

One of the most common ways to describe a processor is by the size of the processor's data bus and address bus. A bus is simply a series of connections that carry common signals. Imagine running a pair of wires from one end of a building to another. If you connect a 110v AC power generator to the two wires at any point and place outlets at convenient locations along the wires, you have constructed a power bus. No matter which outlet you plug the wires into, you have access to the same signal, which in this example is 110v AC power.

Any transmission medium that has more than one outlet at each end can be called a bus. A typical computer system has several buses, and a typical processor has two important buses for carrying data and memory--addressing information: the data bus and the address bus.

The processor bus discussed most often is the data bus--the bundle of wires (or pins) used to send and receive data. The more signals that can be sent at the same time, the more data can be transmitted in a specified interval and, therefore, the faster the bus.

Data in a computer is sent as digital information consisting of a time interval in which a single wire carries 5v to signal a 1 data bit, or 0v to signal a 0 data bit. The more wires you have, the more individual bits you can send in the same time interval. A chip such as the 286, which has 16 wires for transmitting and receiving such data, has a 16-bit data bus. A 32-bit chip, such as the 486, has twice as many wires dedicated to simultaneous data transmission as a 16-bit chip and can send twice as much information in the same time interval as a 16-bit chip.

A good way to understand this flow of information is to consider a highway and the traffic it carries. If a highway has only one lane for each direction of travel, only one car at a time can move in a certain direction. If you want to increase traffic flow, you can add another lane so that twice as many cars pass in a specified time. You can think of an 8-bit chip as being a single-lane highway because with this chip, one byte flows through at a time. (One byte equals eight individual bits.) The 16-bit chip, with two bytes flowing at a time, resembles a two-lane highway. To move a large number of automobiles, you may have four lanes in each direction. This structure corresponds to a 32-bit data bus, which has the capability to move four bytes of information at a time.

Just as you can describe a highway by its lane width, you can describe a chip by the width of its data bus. When you read an advertisement that describes a computer system as being a 16-bit or 32-bit system, the ad usually is referring to the data bus of the CPU. This number provides a rough idea of the performance potential of the chip (and, therefore, the system).

Table 6.1 lists the specifications, including the data-bus sizes, for the Intel family of processors used in IBM and compatible PCs.

Table 6.1  Intel Processor Specifications

Processor CPU Clock Std. Voltage Internal Register Size Data-Bus Width Address-Bus Width Maximum Memory Integral Cache Cache Type Burst Mode Integral FPU No. of Transistors Date Introduced
8088 1x 5v 16-bit 8-bit 20-bit 1M No - No No 29,000 June '79
8086 1x 5v 16-bit 16-bit 20-bit 1M No - No No 29,000 June '78
286 1x 5v 16-bit 16-bit 24-bit 16M No - No No 134,000 Feb. '82
386SX 1x 5v 32-bit 16-bit 24-bit 16M No - No No 275,000 June '88
386SL 1x 3.3v 32-bit 16-bit 24-bit 16M 0K* WT No No 855,000 Oct. '90
386DX 1x 5v 32-bit 32-bit 32-bit 4G No - No No 275,000 Oct. '85
486SX 1x 5v 32-bit 32-bit 32-bit 4G 8K WT Yes No 1,185,000 April '91
486SX2 2x 5v 32-bit 32-bit 32-bit 4G 8K WT Yes No 1,185,000 April '94
487SX 1x 5v 32-bit 32-bit 32-bit 4G 8K WT Yes Yes 1,200,000 April '91
486DX 1x 5v 32-bit 32-bit 32-bit 4G 8K WT Yes Yes 1,200,000 April '89
486SL** 1x 3.3v 32-bit 32-bit 32-bit 4G 8K WT Yes Optional 1,400,000 Nov. '92
486DX2 2x 5v 32-bit 32-bit 32-bit 4G 8K WT Yes Yes 1,100,000 March '92
486DX4 2-3x 3.3v 32-bit 32-bit 32-bit 4G 16K WT Yes Yes 1,600,000 Feb. '94
Pentium OD 2.5x 5v 32-bit 32-bit 32-bit 4G 2x16K WB Yes Yes 3,100,000 Jan. '95
Pentium 60/66 1x 5v 32-bit 64-bit 32-bit 4G 2x8K WB Yes Yes 3,100,000 March '93
Pentium 75+ 1.5-3x 3.3v*** 32-bit 64-bit 32-bit 4G 2x8K WB Yes Yes 3,300,000 March '94
Pentium Pro 2-3x 2.9v 32-bit 64-bit 36-bit 64G 2x8K WB Yes Yes 5,500,000 Sept. '95

The 386SL contains an integral-cache controller, but the cache memory must be provided outside the chip.
**There are several different voltage variations of Pentium processors, including what Intel calls VRE (3.465v), and VR (3.3v).
***These figures do not include the optional 256K or 512K Level 2 cache built-in to the CPU packages. The L2 cache contains an additional 15.5 million or 31 million transistors!
FPU = Floating-Point Unit (math coprocessor)
WT = Write-Through cache (caches reads only)
WB = Write-Back cache (caches both reads and writes)
Note that the Pentium Pro processor includes 256K of L2 cache in a separate die within the chip.

Internal Registers

The size of the internal register is a good indication of how much information the processor can operate on at one time. Most advanced processors today--all the chips from the 386 to the Pentium--use 32-bit internal registers.

Some processors have an internal data bus (made up of data paths and of storage units called registers) that is different from the external data bus. The 8088 and 386SX are examples of this structure. Each chip has an internal data bus twice the width of the external bus. These designs, which sometimes are called hybrid designs, usually are low-cost versions of a "pure" chip. The 386SX, for example, can pass data around internally with a full 32-bit register size; for communications with the outside world, however, the chip is restricted to a 16-bit-wide data path. This design enables a systems designer to build a lower-cost motherboard with a 16-bit bus design and still maintain compatibility with the full 32-bit 386.

Internal registers often are larger than the data bus, which means that the chip requires two cycles to fill a register before the register can be operated on. For example, both the 386SX and 386DX have internal 32-bit registers, but the 386SX has to "inhale" twice (figuratively) to fill them, whereas the 386DX can do the job in one "breath." The same thing would happen when the data is passed from the registers back out to the system bus.

The Pentium is an example of the opposite situation. This chip has a 64-bit data bus but only 32-bit registers--a structure that may seem to be a problem until you understand that the Pentium has two internal 32-bit pipelines for processing information. In many ways, the Pentium is like two 32-bit chips in one. The 64-bit data bus provides for very efficient filling of these multiple registers.

Address Bus

The address bus is the set of wires that carry the addressing information used to describe the memory location to which the data is being sent, or from which the data is being retrieved. As with the data bus, each wire in an address bus carries a single bit of information. This single bit is a single digit in the address. The more wires (digits) used in calculating these addresses, the greater the total number of address locations. The size (or width) of the address bus indicates the maximum amount of RAM that a chip can address.

The highway analogy can be used to show how the address bus fits in. If the data bus is the highway, and if the size of the data bus is equivalent to the number of lanes, the address bus relates to the house number or street address. The size of the address bus is equivalent to the number of digits in the house address number. For example, if you live on a street in which the address is limited to a two-digit (base 10) number, no more than 100 distinct addresses (00 to 99) can exist for that street (10 to the power of 2). Add another digit, and the number of available addresses increases to 1,000 (000 to 999), or 10 to the third power.

Computers use the binary (base 2) numbering system, so a two-digit number provides only four unique addresses (00, 01, 10, and 11) calculated as 2 to the power of 2; and a three-digit number provides only eight addresses (000 to 111) which is 2 to the 3rd power. For example, the 8086 and 8088 processors use a 20-bit address bus that calculates as a maximum of 2 to the 20th power or 1,048,576 bytes (1M) of address locations. Table 6.2 describes the memory-addressing capabilities of Intel processors.

Table 6.2  Intel Processor Memory-Addressing Capabilities

Processor Family Address Bus Bytes Kilobytes Megabytes Gigabytes
8088/8086 20-bit 1,048,576 1,024 1 none
286/386SX 24-bit 16,777,216 16,384 16 none
386DX-Pentium Pro 32-bit 4,294,967,296 4,194,304 4,096 4
Pentium II 36-bit 68,719,476,736 67,108,864 65,536 64

The data bus and address bus are independent, and chip designers can use whatever size they want for each. Usually, however, chips with larger data buses have larger address buses. The sizes of the buses can provide important information about a chip's relative power, measured in two important ways. The size of the data bus is an indication of the information-moving capability of the chip, and the size of the address bus tells you how much memory the chip can handle.

Processor Speed Ratings

A common misunderstanding about processors is their different speed ratings. This section covers processor speed in general and then provides more specific information about Intel processors.

A computer system's clock speed is measured as a frequency, usually expressed as a number of cycles per second. A crystal oscillator controls clock speeds, using a sliver of quartz in a small tin container. As voltage is applied to the quartz, it begins to vibrate (oscillate) at a harmonic rate dictated by the shape and size of the crystal (sliver). The oscillations emanate from the crystal in the form of a current that alternates at the harmonic rate of the crystal. This alternating current is the clock signal. A typical computer system runs millions of these cycles per second, so speed is measured in megahertz (MHz). (One hertz is equal to one cycle per second.)


NOTE: The hertz was named for the German physicist Heinrich Rudolph Hertz. In 1885, Hertz confirmed through experimentation the electromagnetic theory, which states that light is a form of electromagnetic radiation and is propagated as waves.

A single cycle is the smallest element of time for the processor. Every action requires at least one cycle and usually multiple cycles. To transfer data to and from memory, for example, an 8086 chip needs four cycles plus wait states. (A wait state is a clock tick in which nothing happens to ensure that the processor isn't getting ahead of the rest of the computer.) A 286 needs only two cycles plus any wait states for the same transfer.

The time required to execute instructions also varies. The original 8086 and 8088 processors take an average of 12 cycles to execute a single instruction. The 286 and 386 processors improve this rate to about 4.5 cycles per instruction; the 486 drops the rate further to two cycles per instruction. The Pentium includes twin instruction pipelines and other improvements that provide for operation at 1 cycle per average instruction.

Different instruction execution times (in cycles) make it difficult to compare systems based purely on clock speed, or number of cycles per second. One reason the 486 is so fast is that it has an average instruction-execution time of 2 clock cycles. Therefore, a 100MHz Pentium is about equal to a 200MHz 486, which is about equal to a 400MHz 386 or 286, which is about equal to a 1,000MHz 8088. As you can see, you have to be careful in comparing systems based on pure MHz alone; many other factors affect system performance.

How can two processors that run at the same clock rate perform differently, with one running "faster" than the other? The answer is simple: efficiency.

Intel has devised a specific series of benchmarks that can be run against Intel chips to produce a relative gauge of performance. It has recently been updated to reflect performance on 32-bit systems, and is called the iCOMP 2.0 (intel COmparative Microprocessor Performance) index. Table 6.3 shows the relative power, or iCOMP 2.0 index, for several processors.

Table 6.3  Intel iCOMP 2.0 Index Ratings

Processor iCOMP 2.0 Index
Pentium 75 67
Pentium 100 90
Pentium 120 100
Pentium 133 111
Pentium 150 114
Pentium 166 127
Pentium 200 142
Pentium-MMX 166 160
Pentium-MMX 200 182
Pentium-MMX 233 203
Pentium Pro 180 197
Pentium Pro 200 220
Pentium II 233 267
Pentium II 266 303
Pentium II 300 N/A*

* As of this writing, the Pentium II 300 has not yet been rated. The iCOMP 2.0 index is derived from several independent benchmarks and is a stable indication of relative processor performance. The benchmarks balance integer with floating point and multimedia performance.

Modern systems use a variable frequency synthesizer circuit usually found in the main motherboard chipset to control the motherboard speed and CPU speed. Most Pentium motherboards will have 3 or 4 speed settings. The processors used today are available in a variety of versions that run at different frequencies based on a given motherboard speed. For example, most of the Pentium chips run at a speed that is some multiple of the true motherboard speed. For example, Pentium processors and motherboards run at the speeds shown in Table 6.4.

Table 6.4  Intel Processor and Motherboard Speeds

CPU Type/Speed CPU Clock Motherboard Speed
Pentium 60 1x 60
Pentium 66 1x 66
Pentium 75 1.5x 50
Pentium 90 1.5x 60
Pentium 100 1.5x 66
Pentium 120 2x 60
Pentium 133 2x 66
Pentium 150 2.5x 60
Pentium/Pentium Pro/MMX 166 2.5x 66
Pentium/Pentium Pro 180 3x 60
Pentium/Pentium Pro/MMX 200 3x 66
Pentium-MMX/Pentium II 233 3.5x 66
Pentium II 266 4x 66
Pentium II 300 4.5x 66

If all other variables are equal--including the type of processor, the number of wait states (empty cycles) added to different types of memory accesses, and the width of the data bus--you can compare two systems by their respective clock rates. However, the construction and design of the memory subsystem can have an enormous effect on a system's final execution speed.

In building a processor, a manufacturer tests it at different speeds, temperatures, and pressures. After the processor is tested, it receives a stamp indicating the maximum safe speed at which the unit will operate under the wide variation of temperatures and pressures encountered in normal operation. The rating system usually is simple. For example, the top of the processor in one of my systems is marked like this: A80486DX2-66 The A is Intel's indicator that this chip has a Ceramic Pin Grid Array form factor, or an indication of the physical packaging of the chip. The 80486DX2 is the part number, which identifies this processor as a clock-doubled 486DX processor. The -66 at the end indicates that this chip is rated to run at a maximum speed of 66MHz. Because of the clock doubling, the maximum motherboard speed is 33MHz. This chip would be acceptable for any application in which the chip runs at 66MHz or slower. For example, you could use this processor in a system with a 25MHz motherboard, in which case the processor would happily run at 50MHz.

Most 486 motherboards also have a 40MHz setting, in which case the DX2 would run at 80MHz internally. Because this is 14MHz beyond its rated speed, many would not work; or if they worked at all, it would only be for a short time. On the other hand, I have found that most of the newer chips marked with -66 ratings seem to run fine (albeit somewhat hotter!) at the 40/80MHz settings. This is called overclocking, and can end up being a simple, cost-effective way to speed up your system. However, I would not recommend this for mission-critical applications where the system reliability is of the utmost importance, because a system pushed beyond specification like this can often exhibit erratic behavior under stress.

One good source of online overclocking information is located at

http://www.sysopt.com/overc.html

It includes, among other things, fairly thorough overclocking FAQs, and an ongoing survey of users that have successfully (and sometimes unsuccessfully) overclocked their CPUs.

Sometimes, however, the markings don't seem to indicate the speed directly. In the older 8086, for example, -3 translates to 6MHz operation. This marking scheme is more common in some of the older chips manufactured before some of the marking standards used today were standardized.

A manufacturer sometimes places the CPU under a heat sink, which prevents you from reading the rating printed on the chip. (A heat sink is a metal device that draws heat away from an electronic device.) Most of the processors running at 50MHz and higher should have a heat sink installed to prevent the processor from overheating.

Intel Processors

PC-compatible computers use processors manufactured primarily by Intel. Some other companies, such as Cyrix and AMD, have reverse-engineered the Intel processors and made their own compatible versions. IBM also manufactures processors for some of its own systems as well as for installation in boards and modules sold to others. The x86 series of IBM microprocessors was developed in conjunction with Cyrix, and is essentially identical to that company's popular 6x86 units.

Knowing the processors used in a system can be very helpful in understanding the capabilities of the system, as well as in servicing it. To fully understand the capabilities of a system and perform any type of servicing, you must know at least the type of processor that the system uses.

8088 and 8086 Processors

The original IBM PC used an Intel CPU chip called the 8088. The original 8088 CPU chip ran at 4.77MHz, which means that the computer's circuitry drove the CPU at a rate of 4,770,000 ticks, or computer heartbeats, per second. Each tick represents a small amount of work--the CPU executing an instruction or part of an instruction--rather than a period of elapsed time.

Computer users sometimes wonder why a 640K conventional-memory barrier exists if the 8088 chip can address 1M of memory. The conventional-memory barrier exists because IBM reserved 384K of the upper portion of the 1,024K (1M) address space of the 8088 for use by adapter cards and system BIOS (a computer program permanently "burned into" the ROM chips in the PC). The lower 640K is the conventional memory in which DOS and software applications execute.

In 1976, before the 8088 chip, Intel made a slightly faster chip named the 8086. The 8086, which was one of the first 16-bit chips on the market, addressed 1M of RAM. The design failed to catch on, however, because both the chip and a motherboard designed for the chip were costly. The cost was high because the system needed a 16-bit data bus rather than the less expensive 8-bit bus. Systems available at that time were 8-bit, and users apparently weren't willing to pay for the extra performance of the full 16-bit design. Therefore, Intel introduced the 8088 in 1978. Both the 8086 and the 8088 CPU chips are quite slow by today's standards.

80186 and 80188 Processors

After Intel produced the 8086 and 8088 chips, it turned its sights toward producing a more powerful chip with an increased instruction set. The company's first efforts along this line--the 80186 and 80188--were unsuccessful. But incorporating system components into the CPU chip was an important idea for Intel, because it led to faster, better chips, such as the 286.

The relationship between the 80186 and 80188 is the same as that of the 8086 and 8088; one is a slightly more advanced version of the other. Compared CPU to CPU, the 80186 is almost the same as the 8088 and has a full 16-bit design. The 80188 (like the 8088) is a hybrid chip that compromises the 16-bit design with an 8-bit external communications interface. The advantage of the 80186 and 80188 is that they combine on a single chip 15 to 20 of the 8086-8088 series system components, a fact that can greatly reduce the number of components in a computer design. The 80186 and 80188 chips are used for highly intelligent peripheral adapter cards, such as network adapters.

286 Processors

The Intel 80286 (normally abbreviated as 286) processor did not suffer from the compatibility problems that damned the 80186 and 80188. The 286 chip, introduced in 1981, is the CPU behind the IBM AT. Other computer makers manufactured what came to be known as IBM clones, many of these manufacturers calling their systems AT-compatible or AT-class computers.

When IBM developed the AT, it selected the 286 as the basis for the new system because the chip provided compatibility with the 8088 used in the PC and the XT, which means that software written for those chips should run on the 286. The 286 chip is many times faster than the 8088 used in the XT, and it offered a major performance boost to PCs used in businesses. The processing speed, or throughput, of the original AT (which ran at 6 MHz) was five times greater than that of the PC running at 4.77 MH Hz.

For several reasons, 286 systems are faster than their predecessors. The main reason is that 286 processors are much more efficient in executing instructions. An average instruction takes 12 clock cycles on the 8086 or 8088, but an average 4.5 cycles on the 286 processor. Additionally, the 286 chip can handle up to 16 bits of data at a time through an external data bus twice the size of the 8088.

The 286 chip has two modes of operation: real mode and protected mode. The two modes are distinct enough to make the 286 resemble two chips in one. In real mode, a 286 acts essentially the same as an 8086 chip and is fully object-code compatible with the 8086 and 8088. (A processor with object-code compatibility can run programs written for another processor without modification and execute every system instruction in the same manner.)

In the protected mode of operation, the 286 was truly something new. In this mode, a program designed to take advantage of the chip's capabilities believes that it has access to 1G of memory (including virtual memory). The 286 chip, however, can address only 16M of hardware memory. A significant failing of the 286 chip is that it cannot switch from protected mode to real mode without a hardware reset (a warm reboot) of the system. (It can, however, switch from real mode to protected mode without a reset.) A major improvement of the 386 over the 286 is that software can switch the 386 from real mode to protected mode, and vice versa.

Only a small amount of software that took advantage of the 286 chip was sold until Windows 3.0 offered Standard Mode for 286 compatibility; and by that time, the hottest-selling chip was the 386. Still, the 286 was Intel's first attempt to produce a CPU chip that supported multitasking, in which multiple programs run at the same time. The 286 is designed so that if one program locks up or fails, the entire system doesn't need a warm boot (reset) or cold boot (power off or on). Theoretically, what happens in one area of memory doesn't affect other programs. Before multitasked programs are "safe" from one another, however, the 286 chip (and subsequent chips) needs an operating system that works cooperatively with the chip to provide such protection.

386 Processors

The Intel 80386 (normally abbreviated as 386) caused quite a stir in the PC industry because of the vastly improved performance that it brought to the personal computer. Compared with 8088 and 286 systems, the 386 chip offers greater performance in almost all areas of operation.

The 386 is a full 32-bit processor optimized for high-speed operation and multitasking operating systems. Intel introduced the chip in 1985, but the 386 appeared in the first systems in late 1986 and early 1987. The Compaq Deskpro 386 and systems made by several other manufacturers introduced the chip; somewhat later, IBM used the chip in its PS/2 Model 80. For several years, the 386 chip rose in popularity, which peaked around 1991. Since then, the popularity of the 386 has waned to the point that it is virtually nonexistent on the market today.

The 386 can execute the real-mode instructions of an 8086 or 8088, but in fewer clock cycles. The 386 was as efficient as the 286 in executing instructions, which means that the average instruction takes about 4.5 clock cycles. In raw performance, therefore, the 286 and 386 actually seemed to be about at equal clock rates. Many 286 system manufacturers were touting their 16MHz and 20 MHz 286 systems as being just as fast as 16MHz and 20MHz 386 systems, and they were right! The 386 offered greater performance in other ways, mainly due to additional software capability (modes) and a greatly enhanced Memory Management Unit (MMU).

The 386 can switch to and from protected mode under software control without a system reset, a capability that makes using protected mode more practical. In addition, the 386 has a new mode, called virtual real mode, which enables several real-mode sessions to run simultaneously under protected mode.

Other than raw speed, probably the most important feature of this chip is its available modes of operation, which are:

  • Real mode

  • Protected mode

  • Virtual real mode (sometimes called virtual 86 mode)

Real mode on a 386 chip, as on a 286 chip, is 8086-compatible mode. In real mode, the 386 essentially is a much faster "turbo PC" with 640K of conventional memory, just like systems based on the 8088 chip. DOS and any software written to run under DOS requires this mode to run.

The protected mode of the 386 is fully compatible with the protected mode of the 286. The protected mode for both chips often is called their native mode of operation, because these chips are designed for advanced operating systems such as OS/2 and Windows NT, which run only in protected mode. Intel extended the memory-addressing capabilities of 386 protected mode with a new MMU that provides advanced memory paging and program switching. These features are extensions of the 286 type of MMU, so the 386 remains fully compatible with the 286 at the system-code level.

The 386 chip's virtual real mode is new. In virtual real mode, the processor can run with hardware memory protection while simulating an 8086's real-mode operation. Multiple copies of DOS and other operating systems, therefore, can run simultaneously on this processor, each in a protected area of memory. If the programs in one segment crash, the rest of the system is protected. Software commands can reboot the blown partition.

Numerous variations of the 386 chip exist, some of which are less powerful and some of which are less power-hungry. The following sections cover the members of the 386-chip family and their differences.

386DX Processors

The 386DX chip was the first of the 386-family members that Intel introduced. The 386 is a full 32-bit processor with 32-bit internal registers, a 32-bit internal data bus, and a 32-bit external data bus. The 386 contains 275,000 transistors in a VLSI (Very Large Scale Integration) circuit. The chip comes in a 132-pin package and draws approximately 400 milliamperes (ma), which is less power than even the 8086 requires. The 386 has a smaller power requirement because it is made of CMOS (Complementary Metal Oxide Semiconductor) materials. The CMOS design enables devices to consume extremely low levels of power.

The Intel 386 chip was available in clock speeds ranging from 16MHz to 33MHz; other manufacturers, primarily AMD and Cyrix, offered comparable versions with speeds up to 40MHz. In general, these "clones" were fully functional with Intel chips, meaning that they could run any software designed for the Intel 386 chips.

The 386DX can address 4G of physical memory. Its built-in virtual memory manager enables software designed to take advantage of enormous amounts of memory to act as though a system has 64T of memory. (A terabyte (T) is 1,099,511,627,776 bytes of memory.)

386SX Processors

The 386SX, code-named the P9 chip during its development, was designed for systems designers who were looking for 386 capabilities at 286-system prices. Like the 286, the 386SX is restricted to only 16 bits when communicating with other system components such as memory. Internally, however, the 386SX is identical to the DX chip; the 386SX has 32-bit internal registers, and can therefore run 32-bit software. The 386SX uses a 24-bit memory-addressing scheme like that of the 286, rather than the full 32-bit mem-ory address bus of the standard 386. The 386SX, therefore, can address a maximum 16M of physical memory rather than the 4G of physical memory that the 386DX can address. Before it was discontinued, the 386SX was available in clock speeds ranging from 16 to 33MHz.

The 386SX signaled the end of the 286 because of the 386SX chip's superior MMU and the addition of the virtual real mode. Under a software manager such as Windows or OS/2, the 386SX can run numerous DOS programs at the same time. The capability to run 386-specific software is another important advantage of the 386SX over any 286 or older design. For example, Windows 3.1 runs nearly as well on a 386SX as it does on a 386DX.


NOTE: One common fallacy about the 386SX is that you can plug one into a 286 system and give the system 386 capabilities. This is not true; the 386SX chip is not pin-compatible with the 286 and does not plug into the same socket. Several upgrade products, however, have been designed to adapt the chip to a 286 system. In terms of raw speed, converting a 286 system to a 386 CPU chip results in little performance gain because 286 motherboards are built with a restricted 16-bit interface to memory and peripherals. A 16MHz 386SX is not markedly faster than a 16MHz 286, but it does offer improved memory-management capabilities on a motherboard designed for it, as well as the capability to run 386-specific software.

386SL Processors

Another variation on the 386 chip is the 386SL. This low-power CPU has the same capabilities as the 386SX, but it is designed for laptop systems in which low power consumption is needed. The SL chips offer special power-management features that are important to systems that run on batteries. The SL chip offers several sleep modes that conserve power.

The chip includes an extended architecture that includes a System Management Interrupt (SMI), which provides access to the power-management features. Also included in the SL chip is special support for LIM (Lotus Intel Microsoft) expanded memory functions and a cache controller. The cache controller is designed to control a 16-64K external processor cache.

These extra functions account for the higher transistor count in the SL chips (855,000) compared with even the 386DX processor (275,000). The 386SL is available in 25MHz clock speed.

Intel offered a companion to the 386SL chip for laptops called the 82360SL I/O subsystem. The 82360SL provides many common peripheral functions, such as serial and parallel ports, a direct memory access (DMA) controller, an interrupt controller, and power-management logic for the 386SL processor. This chip subsystem works with the processor to provide an ideal solution for the small size and low power-consumption requirements of portable and laptop systems.

486 Processors

In the race for more speed, the Intel 80486 (normally abbreviated as 486) was another major leap forward. The additional power available in the 486 fueled tremendous growth in the software industry. Tens of millions of copies of Windows, and millions of copies of OS/2, have been sold largely because the 486 finally made the graphical user interface (GUI) of Windows and OS/2 a realistic option for people who work on their computers every day.

Four main features make a given 486 processor roughly twice as fast as an equivalent MHz 386 chip. These features are:

  • Reduced instruction-execution time. Instructions in the 486 take an average of only two clock cycles to complete, compared with an average of more than four cycles on the 386.

  • Internal (Level 1) cache. The built-in cache has a hit ratio of 90 to 95 percent, which describes how often zero-wait-state read operations will occur. External caches can improve this ratio further.

  • Burst-mode memory cycles. A standard 32-bit (4-byte) memory transfer takes two clock cycles. After a standard 32-bit transfer, more data up to the next 12 bytes (or three transfers) can be transferred with only one cycle used for each 32-bit (4-byte) transfer. Thus, up to 16 bytes of contiguous, sequential memory data can be transferred in as little as five cycles instead of eight cycles or more. This effect can be even greater when the transfers are only 8 bits or 16 bits each.

  • Built-in (synchronous) enhanced math coprocessor (some versions). The math co- processor runs synchronously with the main processor and executes math instructions in fewer cycles than previous designs did. On average, the math coprocessor built into the DX-series chips provides two to three times greater math performance than an external 387 chip.

The 486 chip is about twice as fast as the 386, which means that a 386DX-40 is about as fast as a 486SX-20. This made the 486 a much more desirable option, primarily because it could more easily be upgraded to a DX2 or DX4 processor at a later time. You can see why the arrival of the 486 rapidly killed off the 386 in the marketplace.

Before the 486, many people avoided GUIs because they didn't have time to sit around waiting for the hourglass, which indicates that the system is performing behind-the-scenes operations that the user cannot interrupt. The 486 changed that situation. Many people believe that the 486 CPU chip spawned the widespread acceptance of GUIs.

With the release of its faster Pentium CPU chip, Intel began to cut the price of the 486 line to entice the industry to shift over to the 486 as the mainstream system. Intel later did the same thing with its Pentium chips, spelling the end of the 486 line. The 486 is now offered by Intel only for use in embedded microprocessor applications, used primarily in expansion cards.

Most of the 486 chips were offered in a variety of maximum speed ratings, varying from 16MHz all the way up to 120MHz. Additionally, 486 processors have slight differences in overall pin configurations. The DX, DX2, and SX processors have a virtually identical 168-pin configuration, whereas the OverDrive chips have either the standard 168-pin configuration or a specially modified 169-pin OverDrive (sometimes also called 487SX) configuration. If your motherboard has two sockets, the primary one likely supports the standard 168-pin configuration, and the secondary (OverDrive) socket supports the 169-pin OverDrive configuration. Most newer motherboards with a single ZIF (Zero Insertion Force) socket support any of the 486 processors except the DX4. The DX4 is different because it requires 3.3v to operate instead of 5v, like most other chips up to that time.

A processor rated for a given speed always functions at any of the lower speeds. A 100MHz-rated 486DX4 chip, for example, runs at 75MHz if it is plugged into a 25MHz motherboard. Note that the DX2/OverDrive processors operate internally at two times the motherboard clock rate, whereas the DX4 processors operate at two, two-and-a-half, or three times the motherboard clock rate. Table 6.5 shows the different speed combinations that can result from using the DX2 or DX4 processors with different motherboard clock speeds.

Table 6.5  Intel DX2 and DX4 Operating Speeds versus Motherboard
Clock Speeds

<TD ALIGN

Motherboard Clock Speed 16MHz 20MHz 25MHz 33MHz 40MHz 50MHz
DX2 processor speed 32MHz 40MHz 50MHz 66MHz 80MHz N/A
DX4 (2x mode) speed 32MHz 40MHz 50MHz 66MHz 80MHz 100MHz
DX4 (2.5x mode) speed 40MHz 50MHz 63MHz 83MHz 100MHz N/A
DX4 (3x mode) speed 48MHz 60MHz