Intel 486
486
486DX
80486
i486
Intel 80486

    (Or "i486", "iAPX 80486", and "Intel DX4" but
   usually just "486").  A range of Intel CISC
   microprocessors which is part of the Intel 80x86 family of
   processors.

   The 486s are very similar to their immediate predecessor, the
   Intel 80386DX.  The main differences are that the 486 has an
   optimised instruction set, has an on-chip unified
   instruction and data cache, an optional on-chip
   floating-point unit (FPU), and an enhanced bus interface
   unit.  These improvements yield a rough doubling in
   performance over an Intel 80386 at the same clock rate.

   There are several suffixes and variants including:

   Intel 486SX - a 486DX with a faulty FPU that has been
   disabled in the factory.

   Intel 486DX - 486SX with a working FPU.

   486DX-2 - runs at twice the external clock rate.

   486SX-2 - runs at twice the external clock rate.

   486SL - 486DX with power conservation circuitry.

   486SL-NM - 486SX with power conservation circuitry; SL
   enhanced suffix, denotes a 486 with special power conservation
   circuitry similar to that in the 486SL processors.

   487 - 486DX with a slightly different pinout for use in 486SX
   systems.

   OverDrive - 486DX-2 with a slightly different pinout for use
   in 486SX systems.

   RapidCAD - 486DX in a special package with a companion FPU
   dummy package for use in Intel 80386 systems.

   Intel DX4, Cyrix Cy486SLC.

   External clock rates include 16MHz, 20MHz, 25MHz, 33MHz,
   40MHz, although 16Mhz is rare now, and the 20MHz processors
   are often clock doubled.

   The 486 processor has been licensed or reverse engineered by
   other companies such as IBM, AMD, Cyrix, and Chips &
   Technologies.  Some are almost exact duplicates in
   specications and performance, some aren't.

   The successor to the 486 is the Pentium.

   (1995-02-21)