Intel 80486

Encyclopedia : I : IN : INT : Intel 80486

Contents

1 Overview
2 Models
3 Competitive alternatives
4 Platform
5 See also
6 References
7 External links

Overview

The exposed die of an Intel 80486DX2 microprocessor.

The 486DX2 architecture.

The Intel i486 (also called 486 or 80486) is a range of 32-bit scalar Intel CISC microprocessors which is part of the Intel x86 family of processors. The i486's predecessor was the Intel 80386 processor. The first line of 486 processors were introduced in 1989. The i486 was so named without the usual 80-prefix, because of a court ruling that prohibited trademarking numbers (like 80486). Intel dropped number-based naming altogether with the successor to the i486 – the Pentium processor.

From a software point of view, the instruction set of the i486 family is very similar to its predecessor, the Intel 80386, with the addition of only a few extra instructions.

From a hardware point of view, however, the architecture of the i486 is a vast improvement. It has an on-chip unified instruction and data cache, an optional on-chip floating-point unit (FPU) (DX models only), and an enhanced bus interface unit. In addition, under optimal conditions, the processor core can sustain an execution rate of one instruction per clock cycle. These improvements yield a rough doubling in performance over an Intel 80386 at the same clock rate. However, some low-end i486 models were actually slower than the highest-speed 386s, especially so with the 'SX' i486s.

Differences between the 80386 and 80486 include:

Data/Instruction Cache - An 8192-byte (8 kB) SRAM built into the processor core, designed to store the most commonly used instructions. The 386 supported an off-chip cache, but this was much slower.
Pipelining - This allows the processor to handle a LocateFetchExecute each clock cycle. The pipeline is offset meaning the execute step required information from the previous two clock cycles. A locate would be to feed the next fetch, the fetch would be to feed the next execute. The 386 needs to do each step separately.
Improvements to MMU performance
Integrated FPU- Added accelerated high end math functions.

The 486 has a 32-bit data bus and a 32-bit address bus. This requires either four matched 30-pin SIMMs or one 72-pin SIMM. The 32-bit address bus limits it 4 GB of RAM.

The Intel project manager for the 80486 was Patrick Gelsinger.

In May 2006 Intel announced that production of the 80486 would cease at the end of September 2007. [link] Although the chip had long been obsolete for personal computer applications, Intel had continued production for use in embedded systems.

Models

An Intel 80486DX-33 microprocessor

There are several suffixes and variants including:

Intel 80486SX - a i486DX with its FPU disabled, although the earlier variants were simply normal i486s with defective FPUs. In later versions, the FPU was removed from the die to reduce its area and thus reduce cost.
Intel 80486DX - same as above, with a working FPU.
Intel 80486DX2 - the internal processor clock runs at twice the clock rate of the external bus clock.
Intel 80486SX2 - same as the i486DX2, but with the FPU disabled.
Intel 80486SL - i486SX with power conservation circuitry. Mainly for use in portable computers.
Intel 80486SL-NM - i486SX with power conservation circuitry; SL enhanced suffix, denotes a i486 with special power conservation circuitry similar to that in the i486SL processors.
Intel 80487 - i486DX with a slightly different pinout for use in i486SX systems as a FPU.
Intel 80486 OverDrive - i486SX, i486SX2, i486DX2 or i486DX4. 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; the DX3, which was meant to run at 2.5x the clock speed, was never released).

Internal clock rates included 16, 20, 25, 33, 40, 50, 66, 75 and 100 MHz, although the 100 MHz versions could be somewhat unstable. The 486DX2 66 MHz was the most widespread high-end 486 chip, while more powerful iterations such as the OverDrive and DX4 were less used in favour of the succeeding Pentium. The only 486 that ran a 50 MHz bus, the 486DX 50 MHz chip, had compatibility problems with boards and components because of this high bus speed requirement. 486DX 50 MHz was a rather unpopular chip and was quickly replaced by the clock-doubled 486 DX2 chips which ran the bus at half of the CPU clock speed.

Competitive alternatives

486 compatible processors have been produced by other companies such as IBM, Texas Instruments, AMD, Cyrix, and Chips and Technologies. Some are almost exact duplicates in specifications and performance, some are not. The 486 was, however, covered by many of Intel's 386 patents as well as some of its own. Intel and IBM have broad cross-licenses of these patents, and AMD was granted rights to the relevant patents in the 1995 settlement of a lawsuit between the companies.[link]

Platform

With regards to the 486 system platform, early 486 machines were equipped with only 16-bit and 8-bit ISA slots. Later motherboards combined ISA with the high-speed VESA Local Bus (VLB), primarily for video cards and hard drive controllers. The final 486 boards came equipped with PCI and ISA, and sometimes VLB as well (though in this configuration VLB suffered performance-wise). Bus speed was determined by multipliers for ISA, but PCI and VLB bus clocks were often equal to the clock of the 486 bus (some boards had multipliers for these as well).

Later 486 boards also supported Plug-and-play, the Microsoft technology that began as a part of Windows 95 designed to make component installation easier for consumers.

References

This article was originally based on material from the Free On-line Dictionary of Computing, which is [Foldoc licenselicensed] under the GFDL.

External links

http://users.erols.com/chare/486.htm
[Intel 80486 images and descriptions at cpu-collection.de]
[CPU-INFO: 80486, indepth processor history]

**List of Intel microprocessors | List of Intel CPU slots, sockets**
Intel processors 4004 \| 4040 \| 8008 \| 8080 \| 8085 \| 8086 \| 8088 \| iAPX 432 \| 80186 \| 80188 \| 80286 \| 80386 \| 80486 \| i860 \| i960 \| Pentium \| Pentium Pro \| Pentium II \| Celeron \| Pentium III \| XScale \| Pentium 4 \| Pentium M \| Pentium D \| Pentium Extreme Edition \| Xeon \| Core \| Core 2 \| Itanium \| Itanium 2 (italics indicate non-x86 processors)

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