Processor Types and Specifications - Upgrading and Repairing PCs - page 105

The 386 is a full 32-bit processor optimized for high-speed operation and multitasking
OSs. 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 man-
ufacturers introduced the chip; somewhat later, IBM used the chip in its PS/2 Model 80.
The 386 can execute the real-mode instructions of an 8086 or 8088, but in fewer clock
cycles.The386wasasefficientasthe286inexecutinginstructions—the averageinstruc-
tion took about 4.5 clock cycles. In raw performance, therefore, the 286 and 386 actually
seemed to be at almost equal clock rates. 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 3.19 .
Figure 3.19 386 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
includes 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. In-
telextendedthememory-addressing capabilities of386protected modewithanewMMU
that provided advanced memory paging and program switching. These features were ex-