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P2 (286) Second-Generation Processors

The second generation of PC processors allowed for a great leap in system speed and processing efficiency. With these chips we went from moving 8 bits of data around to moving 16 bits at a time. The following section details the second-generation PC processor, the 286.

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, first introduced in 1981, is the CPU behind the original 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. That 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 6MHz) was five times greater than that of the PC running at 4.77MHz. The die for the 286 is shown in Figure 3.30.

Figure 3.30 286 Processor die. Photograph used by permission of Intel Corporation.

286 systems are faster than their predecessors for several reasons. 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 1GB of memory (including virtual memory). The 286 chip, however, can address only 16MB 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. See the section "Processor Modes," earlier in this chapter for more information.

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; 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 was designed so that if one program locked up or failed, the entire system didn't need a warm boot (reset) or cold boot (power off, then back on). Theoretically, what happened in one area of memory didn't affect other programs. Before multitasked programs could be "safe" from one another, however, the 286 chip (and subsequent chips) needed an operating system that worked cooperatively with the chip to provide such protection.

80287 Coprocessor

The 80287, internally, is the same math chip as the 8087, although the pins used to plug them into the motherboard are different. Both the 80287 and the 8087 operate as though they were identical.

In most systems, the 80286 internally divides the system clock by two to derive the processor clock. The 80287 internally divides the system-clock frequency by three. For this reason, most AT-type computers run the 80287 at one-third the system clock rate, which also is two-thirds the clock speed of the 80286. Because the 286 and 287 chips are asynchronous, the interface between the 286 and 287 chips is not as efficient as with the 8088 and 8087.

In summary, the 80287 and the 8087 chips perform about the same at equal clock rates. The original 80287 is not better than the 8087 in any real way—unlike the 80286, which is superior to the 8086 and 8088. In most AT systems, the performance gain that you realize by adding the coprocessor is much less substantial than the same type of upgrade for PC- or XT-type systems or for the 80386.

286 Processor Problems

After you remove a math coprocessor from an AT-type system, you must rerun your computer's Setup program. Some AT-compatible Setup programs do not properly unset the math coprocessor bit. If you receive a POST error message because the computer cannot find the math chip, you might have to unplug the battery from the system board temporarily. All Setup information will be lost, so be sure to write down the hard drive type, floppy drive type, and memory and video configurations before unplugging the battery. This information is critical in reconfiguring your computer correctly.

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