INTEL 80486

The Intel 80486 (i486, 486) is a range of Intel CISC microprocessors which is part of the Intel x86 family of processors.

The 486s are very similar to their immediate predecessor, the Intel 80386. 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. However, some low-end 486 models were actually slower than the faster 386s, especially so with the 'SX' 486s.

There are several suffixes and variants including:

• Intel 80486SX - a 486DX with its FPU disabled.
• Intel 80486DX - same as above, with a working FPU.
• Intel 80486DX2 - runs at twice the external clock rate.
• Intel 80486SX2 - runs at twice the external clock rate, FPU disabled.
• Intel 80486SL - 486DX with power conservation circuitry.
• Intel 80486SL-NM - 486SX with power conservation circuitry; SL enhanced suffix, denotes a 486 with special power conservation circuitry similar to that in the 486SL processors.
• Intel 80487 - 486DX with a slightly different pinout for use in 486SX systems as a FPU.
• Intel 80486 OverDrive - 486SX, 486SX2, 486DX2 or 486DX4. Marked as upgrade processors, some models had different pinouts or voltage handling abilities from 'standard' chips of the same speed stepping.
• Intel 80486DX4 - designed to run at triple clock rate (not quadruple as often believed).

External clock rates include 16MHz, 20MHz, 25MHz, 33MHz, 40MHz and 50MHz. Some 486 motherboards provided unofficial and undocumented support for 60 and 66MHz, however.

The 486 processor has been licensed or reverse engineered by other companies such as IBM, Texas Instruments, 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.

See also List of Intel microprocessors

Based on material from FOLDOC, used with permission.

External Links

• http://users.erols.com/chare/486.htm