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P4 (486) Fourth-Generation Processors

The third generation had been a large change from the previous generations of processors. With the fourth generation, more refinement than complete redesign was accomplished. Even so, Intel, AMD, and others managed to literally double processor performance with their fourth-generation processors. The following section defines the fourth-generation processors from Intel, AMD, and others.

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 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. A single instruction in the 486 takes an average of only two clock cycles to complete, compared with an average of more than four cycles on the 386. Clock-multiplied versions such as the DX2 and DX4 further reduced this to about two cycles per instruction.

  • Internal (Level 1) cache. The built-in cache has a hit ratio of 90–95 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.

  • See "Burst EDO."

  • Built-in (synchronous) enhanced math coprocessor (some versions). The math coprocessor 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 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 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-one-half, or three times the motherboard clock rate. Table 3.21 shows the different speed combinations that can result from using the DX2 or DX4 processors with different motherboard clock speeds.

Table 3.21 Intel DX2 and DX4 Operating Speeds Versus Motherboard Clock Speeds

Processor speed

DX2 (2x mode) Speed

DX4 (2.5x mode) Speed

DX4 (3x mode) Speed

DX4

16MHz Motherboard

32MHz

32MHz

40MHz

48MHz

40MHz Motherboard

80MHz

80MHz

100MHz

120MHz

20MHz Motherboard

40MHz

40MHz

50MHz

60MHz

50MHz Motherboard

n/a

100MHz

n/a

n/a

25MHz Motherboard

50MHz

50MHz

63MHz

75MHz

33MHz Motherboard

66MHz

66MHz

83MHz

100MHz


The internal core speed of the DX4 processor is controlled by the CLKMUL (Clock Multiplier) signal at pin R-17 (Socket 1) or S-18 (Socket 2, 3, or 6). The CLKMUL input is sampled only during a reset of the CPU and defines the ratio of the internal clock to the external bus frequency CLK signal at pin C-3 (Socket 1) or D-4 (Socket 2, 3, or 6). If CLKMUL is sampled low, the internal core speed will be two times the external bus frequency. If driven high or left floating (most motherboards would leave it floating), triple speed mode is selected. If the CLKMUL signal is connected to the BREQ (Bus Request) output signal at pin Q-15 (Socket 1) or R-16 (Socket 2, 3, or 6), the CPU internal core speed will be two and a half times the CLK speed. To summarize, here is how the socket has to be wired for each DX4 speed selection:

CPU Speed

CLKMUL (Sampled Only at CPU Reset)

2x

Low

2.5x

Connected to BREQ

3x

High or Floating


You will have to determine how your particular motherboard is wired and whether it can be changed to alter the CPU core speed in relation to the CLK signal. In most cases, there would be one or two jumpers on the board near the processor socket. The motherboard documentation should cover these settings if they can be changed.

One interesting capability here is to run the DX4-100 chip in a doubled mode with a 50MHz motherboard speed. This would give you a very fast memory bus, along with the same 100MHz processor speed, as if you were running the chip in a 33/100MHz tripled mode.

NOTE

One caveat is that if your motherboard has VL-Bus slots, they will have to be slowed down to 33 or 40MHz to operate properly.

Many VL-Bus motherboards can run the VL-Bus slots in a buffered mode, add wait states, or even selectively change the clock only for the VL-Bus slots to keep them compatible. In most cases, they will not run properly at 50MHz. Consult your motherboard—or even better, your chipset documentation—to see how your board is set up.

CAUTION

If you are upgrading an existing system, be sure that your socket will support the chip that you are installing. In particular, if you are putting a DX4 processor in an older system, you need some type of adapter to regulate the voltage down to 3.3v. If you put the DX4 in a 5v socket, you will destroy the chip! See the earlier section on processor sockets for more information.

The 486-processor family is designed for greater performance than previous processors because it integrates formerly external devices, such as cache controllers, cache memory, and math coprocessors. Also, 486 systems were the first designed for true processor upgradability. Most 486 systems can be upgraded by simple processor additions or swaps that can effectively double the speed of the system.

486DX Processors

The original Intel 486DX processor was introduced on April 10, 1989, and systems using this chip first appeared during 1990. The first chips had a maximum speed rating of 25MHz; later versions of the 486DX were available in 33MHz- and 50MHz-rated versions. The 486DX originally was available only in a 5v, 168-pin PGA version, but now is also available in 5v, 196-pin PQFP (Plastic Quad Flat Pack), and 3.3v, 208-pin SQFP (Small Quad Flat Pack). These latter form factors are available in SL Enhanced versions, which are intended primarily for portable or laptop applications in which saving power is important.

Two main features separate the 486 processor from older processors:

  • The 486DX integrates functions such as the math coprocessor, cache controller, and cache memory into the chip.

  • The 486 also was designed with upgradability in mind; double-speed OverDrive are upgrades available for most systems.

The 486DX processor is fabricated with low-power CMOS (complementary metal oxide semiconductor) technology. The chip has a 32-bit internal register size, a 32-bit external data bus, and a 32-bit address bus. These dimensions are equal to those of the 386DX processor. The internal register size is where the "32-bit" designation used in advertisements comes from. The 486DX chip contains 1.2 million transistors on a piece of silicon no larger than your thumbnail. This figure is more than four times the number of components on 386 processors and should give you a good indication of the 486 chip's relative power. The die for the 486 is shown in Figure 3.32.

Figure 3.32 486 processor die. Photograph used by permission of Intel Corporation.

The standard 486DX contains a processing unit, a floating-point unit (math coprocessor), a memory-management unit, and a cache controller with 8KB of internal-cache RAM. Due to the internal cache and a more efficient internal processing unit, the 486 family of processors can execute individual instructions in an average of only two processor cycles. Compare this figure with the 286 and 386 families, both of which execute an average 4.5 cycles per instruction. Compare it also with the original 8086 and 8088 processors, which execute an average 12 cycles per instruction. At a given clock rate (MHz), therefore, a 486 processor is roughly twice as efficient as a 386 processor; a 16MHz 486SX is roughly equal to a 33MHz 386DX system; and a 20MHz 486SX is equal to a 40MHz 386DX system. Any of the faster 486s are way beyond the 386 in performance.

The 486 is fully instruction-set–compatible with previous Intel processors, such as the 386, but offers several additional instructions (most of which have to do with controlling the internal cache).

Like the 386DX, the 486 can address 4GB of physical memory and manage as much as 64TB of virtual memory. The 486 fully supports the three operating modes introduced in the 386: real mode, protected mode, and virtual real mode.

  • In real mode, the 486 (like the 386) runs unmodified 8086-type software.

  • In protected mode, the 486 (like the 386) offers sophisticated memory paging and program switching.

  • In virtual real mode, the 486 (like the 386) can run multiple copies of DOS or other operating systems while simulating an 8086's real mode operation. Under an operating system such as Windows or OS/2, therefore, both 16-bit and 32-bit programs can run simultaneously on this processor with hardware memory protection. If one program crashes, the rest of the system is protected, and you can reboot the blown portion through various means, depending on the operating software.

The 486DX series has a built-in math coprocessor that sometimes is called an MCP (math coprocessor) or FPU (floating-point unit). This series is unlike previous Intel CPU chips, which required you to add a math coprocessor if you needed faster calculations for complex mathematics. The FPU in the 486DX series is 100 percent software-compatible with the external 387 math coprocessor used with the 386, but it delivers more than twice the performance. It runs in synchronization with the main processor and executes most instructions in half as many cycles as the 386.

486SL

The 486SL was a short-lived, standalone chip. The SL enhancements and features became available in virtually all the 486 processors (SX, DX, and DX2) in what are called SL enhanced versions. SL enhancement refers to a special design that incorporates special power-saving features.

The SL enhanced chips originally were designed to be installed in laptop or notebook systems that run on batteries, but they found their way into desktop systems, as well. The SL-enhanced chips featured special power-management techniques, such as sleep mode and clock throttling, to reduce power consumption when necessary. These chips were available in 3.3v versions, as well.

Intel designed a power-management architecture called system management mode (SMM). This mode of operation is totally isolated and independent from other CPU hardware and software. SMM provides hardware resources such as timers, registers, and other I/O logic that can control and power down mobile-computer components without interfering with any of the other system resources. SMM executes in a dedicated memory space called system management memory, which is not visible and does not interfere with operating system and application software. SMM has an interrupt called system management interrupt (SMI), which services power-management events and is independent from, and higher priority than, any of the other interrupts.

SMM provides power management with flexibility and security that were not available previously. For example, an SMI occurs when an application program tries to access a peripheral device that is powered down for battery savings, which powers up the peripheral device and reexecutes the I/O instruction automatically.

Intel also designed a feature called Suspend/Resume in the SL processor. The system manufacturer can use this feature to provide the portable computer user with instant-on-and-off capability. An SL system typically can resume (instant on) in one second from the suspend state (instant off) to exactly where it left off. You do not need to reboot, load the operating system, load the application program, and then load the application data. Simply push the Suspend/Resume button and the system is ready to go.

The SL CPU was designed to consume almost no power in the suspend state. This feature means that the system can stay in the suspend state possibly for weeks and yet start up instantly right where it left off. An SL system can keep working data in normal RAM memory safe for a long time while it is in the suspend state, but saving to a disk still is prudent.

486SX

The 486SX, introduced in April 1991, was designed to be sold as a lower-cost version of the 486. The 486SX is virtually identical to the full DX processor, but the chip does not incorporate the FPU or math coprocessor portion.

As you read earlier in this chapter, the 386SX was a scaled-down (some people would say crippled) 16-bit version of the full-blown 32-bit 386DX. The 386SX even had a completely different pinout and was not interchangeable with the more powerful DX version. The 486SX, however, is a different story. The 486SX is, in fact, a full-blown 32-bit 486 processor that is basically pin-compatible with the DX. A few pin functions are different or rearranged, but each pin fits into the same socket.

The 486SX chip is more a marketing quirk than new technology. Early versions of the 486SX chip actually were DX chips that showed defects in the math-coprocessor section. Instead of being scrapped, the chips were packaged with the FPU section disabled and sold as SX chips. This arrangement lasted for only a short time; thereafter, SX chips got their own mask, which is different from the DX mask. (A mask is the photographic blueprint of the processor and is used to etch the intricate signal pathways into a silicon chip.) The transistor count dropped to 1.185 million (from 1.2 million) to reflect this new mask.

The 486SX chip is twice as fast as a 386DX with the same clock speed. Intel marketed the 486SX as being the ideal chip for new computer buyers, because fewer entry-level programs of that day used math-coprocessor functions.

The 486SX was normally available in 16, 20, 25, and 33MHz-rated speeds, and there was also a 486 SX/2 that ran at up to 50 or 66MHz. The 486SX normally comes in a 168-pin version, although other surface-mount versions are available in SL-enhanced models.

Despite what Intel's marketing and sales information implies, no technical provision exists for adding a separate math coprocessor to a 486SX system; neither is a separate math coprocessor chip available to plug in. Instead, Intel wanted you to add a new 486 processor with a built-in math unit and disable the SX CPU that already was on the motherboard. If this situation sounds confusing, read on, because this topic brings you to the most important aspect of 486 design: upgradability.

487SX

The 487SX math coprocessor, as Intel calls it, really is a complete 25MHz 486DX CPU with an extra pin added and some other pins rearranged. When the 487SX is installed in the extra socket provided in a 486SX CPU-based system, the 487SX turns off the existing 486SX via a new signal on one of the pins. The extra key pin actually carries no signal itself and exists only to prevent improper orientation when the chip is installed in a socket.

The 487SX takes over all CPU functions from the 486SX and also provides math coprocessor functionality in the system. At first glance, this setup seems rather strange and wasteful, so perhaps further explanation is in order. Fortunately, the 487SX turned out to be a stopgap measure while Intel prepared its real surprise: the OverDrive processor. The DX2/OverDrive speed- doubling chips, which are designed for the 487SX 169-pin socket, have the same pinout as the 487SX. These upgrade chips are installed in exactly the same way as the 487SX; therefore, any system that supports the 487SX also supports the DX2/OverDrive chips.

Although in most cases you can upgrade a system by removing the 486SX CPU and replacing it with a 487SX (or even a DX or DX2/OverDrive), Intel originally discouraged this procedure. Instead, Intel recommended that PC manufacturers include a dedicated upgrade (OverDrive) socket in their systems, because several risks were involved in removing the original CPU from a standard socket. (The following section elaborates on those risks.) Now Intel recommends—or even insists on—the use of a single processor socket of a ZIF design, which makes upgrading an easy task physically.

See "Zero Insertions Force (ZIF) Sockets."

Very few early 486 systems had a socket for the Weitek 4167 coprocessor chip for 486 systems that existed in November 1989.

DX2/OverDrive and DX4 Processors

On March 3, 1992, Intel introduced the DX2 speed-doubling processors. On May 26, 1992, Intel announced that the DX2 processors also would be available in a retail version called OverDrive. Originally, the OverDrive versions of the DX2 were available only in 169-pin versions, which meant that they could be used only with 486SX systems that had sockets configured to support the rearranged pin configuration.

On September 14, 1992, Intel introduced 168-pin OverDrive versions for upgrading 486DX systems. These processors could be added to existing 486 (SX or DX) systems as an upgrade, even if those systems did not support the 169-pin configuration. When you use this processor as an upgrade, you install the new chip in your system, which subsequently runs twice as fast.

The DX2/OverDrive processors run internally at twice the clock rate of the host system. If the motherboard clock is 25MHz, for example, the DX2/OverDrive chip runs internally at 50MHz; likewise, if the motherboard is a 33MHz design, the DX2/OverDrive runs at 66MHz. The DX2/OverDrive speed doubling has no effect on the rest of the system; all components on the motherboard run the same as they do with a standard 486 processor. Therefore, you do not have to change other components (such as memory) to accommodate the double-speed chip. The DX2/OverDrive chips have been available in several speeds. Three different speed-rated versions have been offered:

  • 40MHz DX2/OverDrive for 16MHz or 20MHz systems

  • 50MHz DX2/OverDrive for 25MHz systems

  • 66MHz DX2/OverDrive for 33MHz systems

Notice that these ratings indicate the maximum speed at which the chip is capable of running. You could use a 66MHz-rated chip in place of the 50MHz- or 40MHz-rated parts with no problem, although the chip will run only at the slower speeds. The actual speed of the chip is double the motherboard clock frequency. When the 40MHz DX2/OverDrive chip is installed in a 16MHz 486SX system, for example, the chip will function only at 32MHz—exactly double the motherboard speed. Intel originally stated that no 100MHz DX2/OverDrive chip would be available for 50MHz systems—which technically has not been true, because the DX4 could be set to run in a clock-doubled mode and used in a 50MHz motherboard (see the discussion of the DX4 processor in this section).

The only part of the DX2 chip that doesn't run at double speed is the bus interface unit, a region of the chip that handles I/O between the CPU and the outside world. By translating between the differing internal and external clock speeds, the bus interface unit makes speed doubling transparent to the rest of the system. The DX2 appears to the rest of the system to be a regular 486DX chip, but one that seems to execute instructions twice as fast.

DX2/OverDrive chips are based on the 0.8 micron circuit technology that was first used in the 50MHz 486DX. The DX2 contains 1.1 million transistors in a three-layer form. The internal 8KB cache, integer, and floating-point units all run at double speed. External communication with the PC runs at normal speed to maintain compatibility.

Besides upgrading existing systems, one of the best parts of the DX2 concept was the fact that system designers could introduce very fast systems by using cheaper motherboard designs, rather than the more costly designs that would support a straight high-speed clock. This means that a 50MHz 486DX2 system was much less expensive than a straight 50MHz 486DX system. The system board in a 486DX-50 system operates at a true 50MHz. The 486DX2 CPU in a 486DX2-50 system operates internally at 50MHz, but the motherboard operates at only 25MHz.

You may be thinking that a true 50MHz DX processor–based system still would be faster than a speed-doubled 25MHz system, and this generally is true. But, the differences in speed actually are very slight—a real testament to the integration of the 486 processor and especially to the cache design.

When the processor has to go to system memory for data or instructions, for example, it has to do so at the slower motherboard operating frequency (such as 25MHz). Because the 8KB internal cache of the 486DX2 has a hit rate of 90–95 percent, however, the CPU has to access system memory only 5–10 percent of the time for memory reads. Therefore, the performance of the DX2 system can come very close to that of a true 50MHz DX system and cost much less. Even though the motherboard runs only at 33.33MHz, a system with a DX2 66MHz processor ends up being faster than a true 50MHz DX system, especially if the DX2 system has a good L2 cache.

Many 486 motherboard designs also include a secondary cache that is external to the cache integrated into the 486 chip. This external cache allows for much faster access when the 486 chip calls for external-memory access. The size of this external cache can vary anywhere from 16KB to 512KB or more. When you add a DX2 processor, an external cache is even more important for achieving the greatest performance gain. This cache greatly reduces the wait states that the processor will have to add when writing to system memory or when a read causes an internal cache miss. For this reason, some systems perform better with the DX2/OverDrive processors than others, usually depending on the size and efficiency of the external-memory cache system on the motherboard. Systems that have no external cache will still enjoy a near-doubling of CPU performance, but operations that involve a great deal of memory access will be slower.

This brings us to the DX4 processor. Although the standard DX4 technically was not sold as a retail part, it could be purchased from several vendors, along with the 3.3v voltage adapter needed to install the chip in a 5v socket. These adapters have jumpers that enable you to select the DX4 clock multiplier and set it to 2x, 2.5x, or 3x mode. In a 50MHz DX system, you could install a DX4/voltage-regulator combination set in 2x mode for a motherboard speed of 50MHz and a processor speed of 100MHz! Although you may not be able to take advantage of certain VL-Bus adapter cards, you will have one of the fastest 486-class PCs available.

Intel also sold a special DX4 OverDrive processor that included a built-in voltage regulator and heat sink that are specifically designed for the retail market. The DX4 OverDrive chip is essentially the same as the standard 3.3v DX4 with the main exception that it runs on 5v because it includes an on-chip regulator. Also, the DX4 OverDrive chip will run only in the tripled speed mode, and not the 2x or 2.5x modes of the standard DX4 processor.

NOTE

As of this writing, Intel has discontinued all 486 and DX2/DX4/OverDrive processors, including the so-called Pentium OverDrive processor.

Pentium OverDrive for 486SX2 and DX2 Systems

The Pentium OverDrive Processor became available in 1995. An OverDrive chip for 486DX4 systems had been planned, but poor marketplace performance of the SX2/DX2 chip meant that it never saw the light of day. One thing to keep in mind about the 486 Pentium OverDrive chip is that although it is intended primarily for SX2 and DX2 systems, it should work in any upgradable 486SX or DX system that has a Socket 2 or Socket 3. If in doubt, check Intel's online upgrade guide for compatibility.

The Pentium OverDrive processor is designed for systems that have a processor socket that follows the Intel Socket 2 specification. This processor also will work in systems that have a Socket 3 design, although you should ensure that the voltage is set for 5v rather than 3.3v. The Pentium OverDrive chip includes a 32KB internal L1 cache, and the same superscalar (multiple instruction path) architecture of the real Pentium chip. Besides a 32-bit Pentium core, these processors feature increased clock-speed operation due to internal clock multiplication and incorporate an internal write-back cache (standard with the Pentium). If the motherboard supports the write-back cache function, increased performance will be realized. Unfortunately, most motherboards, especially older ones with the Socket 2 design, only support write-through cache.

Most tests of these OverDrive chips show them to be only slightly ahead of the DX4-100 and behind the DX4-120 and true Pentium 60, 66, or 75. Unfortunately, these are the only solutions still offered by Intel for upgrading the 486. Based on the relative affordability of low-end "real" Pentiums (in their day), it was hard not to justify making the step up to a Pentium system. At the time, I did not recommend the 486 Pentium OverDrive chips as a viable solution for the future.

"Vacancy"—Secondary OverDrive Sockets

Perhaps you saw the Intel advertisements—both print and television—that featured a 486SX system with a neon Vacancy sign pointing to an empty socket next to the CPU chip. Unfortunately, these ads were not very informative, and they made it seem that only systems with the extra socket could be upgraded. I was worried when I first saw these ads because I had just purchased a 486DX system, and the advertisements implied that only 486SX systems with the empty OverDrive socket were upgradable. This, of course, was not true, but the Intel advertisements did not communicate that fact very well.

I later found that upgradability does not depend on having an extra OverDrive socket in the system and that virtually any 486SX or DX system can be upgraded. The secondary OverDrive socket was designed to make upgrading easier and more convenient. Even in systems that have the second socket, you can actually remove the primary SX or DX CPU and plug the OverDrive processor directly into the main CPU socket, rather than into the secondary OverDrive socket.

In that case, you would have an upgraded system with a single functioning CPU installed; you could remove the old CPU from the system and sell it or trade it in for a refund. Unfortunately, Intel does not offer a trade-in or core-charge policy; it does not want your old chip. For this reason, some people saw the OverDrive socket as being a way for Intel to sell more CPUs. Some valid reasons exist, however, to use the OverDrive socket and leave the original CPU installed.

One reason is that many PC manufacturers void the system warranty if the CPU has been removed from the system. Also, most manufacturers require that the system be returned with only the original parts when systems are serviced; you must remove all add-in cards, memory modules, upgrade chips, and similar items before sending the system in for servicing. If you replace the original CPU when you install the upgrade, returning the system to its original condition will be much more difficult.

Another reason for using the upgrade socket is that the system will not function if the main CPU socket is damaged when you remove the original CPU or install the upgrade processor. By contrast, if a secondary upgrade socket is damaged, the system still should work with the original CPU.

80487 Upgrade

The Intel 80486 processor was introduced in late 1989, and systems using this chip appeared during 1990. The 486DX integrated the math coprocessor into the chip.

The 486SX began life as a full-fledged 486DX chip, but Intel actually disabled the built-in math coprocessor before shipping the chip. As part of this marketing scheme, Intel marketed what it called a 487SX math coprocessor. Motherboard manufacturers installed an Intel-designed socket for this so-called 487 chip. In reality, however, the 487SX math chip was a special 486DX chip with the math coprocessor enabled. When you plugged this chip into your motherboard, it disabled the 486SX chip and gave you the functional equivalent of a full-fledged 486DX system.

AMD 486 (5x86)

AMD makes a line of 486-compatible chips that install into standard 486 motherboards. In fact, AMD makes the fastest 486 processor available, which it calls the Am5x86(TM)-P75. The name is a little misleading, as the 5x86 part makes some people think that this is a fifth-generation Pentium-type processor. In reality, it is a fast clock-multiplied (4x clock) 486 that runs at four times the speed of the 33MHz 486 motherboard you plug it into.

The 5x85 offers high-performance features such as a unified 16KB write-back cache and 133MHz core clock speed; it is approximately comparable to a Pentium 75, which is why it is denoted with a P-75 in the part number. It is the ideal choice for cost-effective 486 upgrades, where changing the motherboard is difficult or impossible.

Not all motherboards support the 5x86. The best way to verify that your motherboard supports the chip is by checking with the documentation that came with the board. Look for keywords such as "Am5X86," "AMD-X5," "clock-quadrupled," "133MHz," or other similar wording. Another good way to determine whether your motherboard supports the AMD 5x86 is to look for it in the listed models on AMD's Web site.

There are a few things to note when installing a 5x86 processor into a 486 motherboard:

  • The operating voltage for the 5x86 is 3.45v +/- 0.15v. Not all motherboards may have this setting, but most that incorporate a Socket 3 design should. If your 486 motherboard is a Socket 1 or 2 design, you cannot use the 5x86 processor directly. The 3.45 volt processor will not operate in a 5-volt socket and may be damaged. To convert a 5-volt motherboard to 3.45 volts, adapters can be purchased from several vendors including Kingston, Evergreen, and AMP. In fact, Kingston and Evergreen sell the 5x86 complete with a voltage regulator adapter attached in an easy-to-install package. These versions are ideal for older 486 motherboards that don't have a Socket 3 design.

  • It is generally better to purchase a new motherboard with Socket 3 than to buy one of these adapters; however, 486 motherboards are hard to find these days, and your old board may be in a proprietary form factor for which it is impossible to find a replacement. Buying a new motherboard is also better than using an adapter because the older BIOS may not understand the requirements of the processor as far as speed is concerned. BIOS updates are often required with older boards.

  • Most Socket 3 motherboards have jumpers, allowing you to set the voltage manually. Some boards don't have jumpers, but have voltage autodetect instead. These systems check the VOLDET pin (pin S4) on the microprocessor when the system is powered on.

  • The VOLDET pin is tied to ground (Vss) internally to the microprocessor. If you cannot find any jumpers for setting voltage, you can check the motherboard as follows: Switch the PC off, remove the microprocessor, connect pin S4 to a Vss pin on the ZIF socket, power on, and check any Vcc pin with a voltmeter. This should read 3.45 (± 0.15) volts. See the previous section on CPU sockets for the pinout.

  • The 5x86 requires a 33MHz motherboard speed, so be sure the board is set to that frequency. The 5x86 operates at an internal speed of 133MHz. Therefore, the jumpers must be set for "clock-quadrupled" or "4x clock" mode. By setting the jumpers correctly on the motherboard, the CLKMUL pin (pin R17) on the processor will be connected to ground (Vss). If there is no 4x clock setting, the standard DX2 2x clock setting should work.

  • Some motherboards have jumpers that configure the internal cache in either write-back (WB) or write-through (WT) mode. They do this by pulling the WB/WT pin (pin B13) on the microprocessor to logic High (Vcc) for WB or to ground (Vss) for WT. For best performance, configure your system in WB mode; however, reset the cache to WT mode if there are problems running applications or the floppy drive doesn't work right (DMA conflicts).

  • The 5x86 runs hot, so a heat sink is required; it normally must have a fan.

In addition to the 5x86, the AMD-enhanced 486 product line includes 80MHz; 100MHz; and 1,20MHz CPUs. These are the A80486DX2-80SV8B (40MHzx2), A80486DX4-100SV8B (33MHzx3), and the A80486DX4-120SV8B (40MHzx3).

Cyrix/TI 486

The Cyrix 486DX2/DX4 processors were available in 100MHz, 80MHz, 75MHz, 66MHz, and 50MHz versions. Like the AMD 486 chips, the Cyrix versions are fully compatible with Intel's 486 processors and work in most 486 motherboards.

The Cx486DX2/DX4 incorporates an 8KB write-back cache, an integrated floating-point unit, advanced power management, and SMM, and was available in 3.3v versions.

NOTE

TI originally made all the Cyrix-designed 486 processors, and under the agreement it also sold them under the TI name. Eventually, TI and Cyrix had a falling out, and then IBM made most of the Cyrix chips for a while. That changed in 1999 when first National Semiconductor bought Cyrix, and took over production from IBM. Then National sold Cyrix to VIA Technologies, although VIA still uses National to manufacture the chips.

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