The Intel 80486 Processor

Some of the most recent PS/2 machines use the latest Intel microprocessor, the 80486. The 80486 offers more processing power and also some functional extensions over the 80386. The 80486 microprocessor subsystem has the following characteristics:

32-bit addressing

32-bit data interface

Extensive instruction set, including string I/O

Hardware fixed-point multiply and divide

Three operating modes:

Real address mode
Protected virtual address mode
Virtual 8086 mode

4GB physical address space

Eight general-purpose 32-bit registers

64TB virtual address space

Internal 8KB, set-associative instruction cache with controller

Note: The 80486SX is an exception, since it does not provide a built-in 80387 numeric coprocessor

The 80486 microprocessor is compatible with the 80386 in the following areas:

Real address mode

Protected virtual address mode

Virtual 8086 mode

80386 paging mechanism

All published 80386 instructions

All published 80387 instructions.

The major differences between the 80386 and 80486 processors are in the execution speed of instructions, and the fact that the 80486 has six new processor instructions to control the operation of the internal 8KB instruction cache.

In addition, the 80486 processor performs certain operations in a different manner. For example, flushing the transaction lookaside buffer in an 80386 processor may only be done for the entire TLB in a single operation. The 80486 provides a facility for selectively flushing the TLB.

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