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P3 (386) Third-Generation Processors

The third generation represents perhaps the most significant change in processors since the first PC. The big deal was the migration from processors that handled 16-bit operations to true 32-bit chips. The third-generation processors were so far ahead of their time, it took fully 10 years before 32-bit operating systems and software became mainstream, and by that time the third-generation chips had become a memory. The following section details the third-generation processors.

386 Processors

The Intel 80386 (normally abbreviated as 386) caused quite a stir in the PC industry because of the vastly improved performance it brought to the personal computer. Compared with 8088 and 286 systems, the 386 chip offered 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. The 386 chip rose in popularity for several years, which peaked around 1991. Obsolete 386 processor systems are mostly retired or scrapped, having been passed down the user chain. If they are in operating condition, they can be useful for running old DOS or Windows 3.x–based applications, which they can do quite nicely.

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 took about 4.5 clock cycles. In raw performance, therefore, the 286 and 386 actually seemed to be at almost equal clock rates. Many 286 system manufacturers were touting their 16MHz and 20MHz 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 because of additional software capability (modes) and a greatly enhanced memory management unit (MMU). The die for the 386 is shown in Figure 3.31.

Figure 3.31 386 processor die. Photograph used by permission of Intel Corporation.

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.

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 provided advanced memory paging and program switching. These features were extensions of the 286 type of MMU, so the 386 remained fully compatible with the 286 at the system-code level.

The 386 chip's virtual real mode was new. In virtual real mode, the processor could run with hardware memory protection while simulating an 8086's real-mode operation. Multiple copies of DOS and other operating systems, therefore, could run simultaneously on this processor, each in a protected area of memory. If the programs in one segment crashed, the rest of the system was protected. Software commands could 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 16–33MHz; other manufacturers, primarily AMD and Cyrix, offered comparable versions with speeds up to 40MHz.

The 386DX can address 4GB 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 64TB of memory. (A terabyte, or TB, is 1,099,511,627,776 bytes of memory, or about 1,000GB.)

386SX Processors

The 386SX 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 memory address bus of the standard 386. The 386SX, therefore, can address a maximum 16MB of physical memory rather than the 4GB 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.


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—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, and the capability to run 386-specific software.

386SL Processors

The 386SL is another variation on the 386 chip. This low-power CPU had the same capabilities as the 386SX, but it was designed for laptop systems in which low power consumption was needed. The SL chips offered special power-management features that were important to systems that ran on batteries. The SL chip also offered several sleep modes to conserve power.

The chip included an extended architecture that contained a System Management Interrupt (SMI), which provided access to the power-management features. Also included in the SL chip was special support for LIM (Lotus Intel Microsoft) expanded memory functions and a cache controller. The cache controller was designed to control a 16–64KB 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 was available in 25MHz clock speed.

Intel offered a companion to the 386SL chip for laptops called the 82360SL I/O subsystem. The 82360SL provided 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 worked with the processor to provide an ideal solution for the small size and low power-consumption requirements of portable and laptop systems.

80387 Coprocessor

Although the 80387 chips ran asynchronously, 386 systems were designed so that the math chip runs at the same clock speed as the main CPU. Unlike the 80287 coprocessor, which was merely an 8087 with different pins to plug into the AT motherboard, the 80387 coprocessor was a high-performance math chip designed specifically to work with the 386.

All 387 chips used a low power-consumption CMOS design. The 387 coprocessor had two basic designs: the 387DX coprocessor, which was designed to work with the 386DX processor, and the 387SX coprocessor, which was designed to work with the 386SX, SL, or SLC processors.

Intel originally offered several speeds for the 387DX coprocessor. But when the company designed the 33MHz version, a smaller mask was required to reduce the lengths of the signal pathways in the chip. This increased the performance of the chip by roughly 20 percent.


Because Intel lagged in developing the 387 coprocessor, some early 386 systems were designed with a socket for a 287 coprocessor. Performance levels associated with that union, however, leave much to be desired.

Installing a 387DX is easy, but you must be careful to orient the chip in its socket properly; otherwise, the chip will be destroyed. The most common cause of burned pins on the 387DX is incorrect installation. In many systems, the 387DX was oriented differently from other large chips. Follow the manufacturer's installation instructions carefully to avoid damaging the 387DX; Intel's warranty does not cover chips that are installed incorrectly.

Several manufacturers developed their own versions of the Intel 387 coprocessors, some of which were touted as being faster than the original Intel chips. The general compatibility record of these chips was very good.

Weitek Coprocessors

In 1981, several Intel engineers formed the Weitek Corporation. Weitek developed math coprocessors for several systems, including those based on Motorola processor designs. Intel originally contracted Weitek to develop a math coprocessor for the Intel 386 CPU, because Intel was behind in its own development of the 387 math coprocessor. The result was the Weitek 1167, a custom math coprocessor that uses a proprietary Weitek instruction set, which is incompatible with the Intel 387.

To use the Weitek coprocessor, your system must have the required additional socket, which was different from the standard Intel coprocessor sockets.

80386 Bugs

Some early 16MHz Intel 386DX processors had a small bug that appeared as a software problem. The bug, which apparently was in the chip's 32-bit multiply routine, manifested itself only when you ran true 32-bit code in a program such as OS/2 2.x, UNIX/386, or Windows in enhanced mode. Some specialized 386 memory-management software systems also may invoke this subtle bug, but 16-bit operating systems (such as DOS and OS/2 1.x) probably will not.

The bug usually causes the system to lock up. Diagnosing this problem can be difficult because the problem generally is intermittent and software-related. Running tests to find the bug is difficult; only Intel, with proper test equipment, can determine whether your chip has a bug. Some programs can diagnose the problem and identify a defective chip, but they cannot identify all defective chips. If a program indicates a bad chip, you certainly have a defective one; if the program passes the chip, you still might have a defective one.

Intel requested that its 386 customers return possibly defective chips for screening, but many vendors did not return them. Intel tested returned chips and replaced defective ones. The defective chips later were sold to bargain liquidators or systems houses that wanted chips that would not run 32-bit code. The defective chips were stamped with a 16-bit SW Only logo, indicating that they were authorized to run only 16-bit software.

Chips that passed the test, and all subsequent chips produced as bug-free, were marked with a double-sigma code (SS). 386DX chips that are not marked with either of these designations have not been tested by Intel and might be defective.

The following marking indicates that a chip has not yet been screened for the defect; it might be either good or bad.


The following marking indicates that the chip has been tested and has the 32-bit multiply bug. The chip works with 16-bit software (such as DOS) but not with 32-bit, 386-specific software (such as Windows or OS/2).

16-bit SW Only

The following mark on a chip indicates that it has been tested as defect-free. This chip fulfills all the capabilities promised for the 80386.


This problem was discovered and corrected before Intel officially added DX to the part number. So, if you have a chip labeled as 80386DX or 386DX, it does not have this problem.

Another problem with the 386DX can be stated more specifically. When 386-based versions of XENIX or other UNIX implementations are run on a computer that contains a 387DX math coprocessor, the computer locks up under certain conditions. The problem does not occur in the DOS environment, however. For the lockup to occur, all the following conditions must be in effect:

  • Demand page virtual memory must be active.

  • A 387DX must be installed and in use.

  • DMA (direct memory access) must occur.

  • The 386 must be in a wait state.

When all these conditions are true at the same instant, the 386DX ends up waiting for the 387DX and vice versa. Both processors will continue to wait for each other indefinitely. The problem is in certain versions of the 386DX, not in the 387DX math coprocessor.

Intel published this problem (Errata 21) immediately after it was discovered to inform its OEM customers. At that point, it became the responsibility of each manufacturer to implement a fix in its hardware or software product. Some manufacturers, such as Compaq and IBM, responded by modifying their motherboards to prevent these lockups from occurring.

The Errata 21 problem occurs only in the B-stepping version of the 386DX and not in the later D-stepping version. You can identify the D-stepping version of the 386DX by the letters DX in the part number (for example, 386DX-20). If DX is part of the chip's part number, the chip does not have this problem.

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