What is 80486?

486 Central processing unit
The exposed die of an Intel 80486DX2 microprocessor.
Produced:	From 1989 to 2007
Common Manufacturers: Intel AMD Texas Instruments
CPU Speeds:	16 MHz to 133 MHz
FSB Speeds:	16  MHz to 50  MHz
Process: (MOSFET channel length)	0.8 m
Instruction Set:	x86 (i386)
Number of cores:	1
Sockets: Socket 1 Socket 2 Socket 3

The Intel486^[1] brand refers to Intel's family of i486 (incl. i487) CPUs - the second generation of 32-bit x86 microprocessors, and the first truly pipelined x86 design. Their predecessor was the Intel386, the very first 32-bit x86 processor. The first line of 486 processors was introduced in 1989 containing 1.2 million transistors (800 nanometer technology). 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.

Improvements

From a software point of view, the instruction set of the i486 CPUs is very similar to its predecessor, the Intel386, with the addition of only a few extra instructions, such as CMPXCHG which executes the Compare-and-swap atomic operation and the XADD which executes the Fetch-and-add atomic operation. Though many atomic instructions have existed since the 8086/8088 because of the nature of the x86 instruction set, they did not correspond to the atomic instructions implemented in many other processors, particularly RISC processors, which made it harder to port applications from these processors, and sometimes they were simply not adequate.

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) (original and DX models), 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 Intel386 at the same clock rate. A 386 (or 286) chip therefore has to reach 50 MHz to be comparable with low end parts in the 486 series.

Differences between the 386 and 486

• Data/Instruction Cache — An 8192-byte 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 Locate-Fetch-Execute each clock cycle. The pipeline is offset meaning the execute step required information from the previous two clock cycles. A locate would feed the next fetch, the fetch would feed the next execute. The 386 needs to do each step separately.
• Improvements to MMU performance.
• Integrated FPU — (DX models only) 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. A 32-bit address bus means that 4 GiB of memory can be directly addressed.

The Intel project manager for the 80486 was Pat Gelsinger.

In May 2006 Intel announced that production of the 80486 would cease at the end of September 2007. [1] Although the chip had long been obsolete for personal computer applications, Intel had continued production for use in embedded systems. The 80486 was able to compute at up to 41 million instructions per second. [2]

Models

There are several suffixes and variants including:

• i486 — The original chip (without any clock doubling).
• i486SX — an i486DX with the FPU part disabled or missing. Early variants were parts with disabled (defective) FPUs, later versions has the FPU removed from the die to reduce area and hence cost.
• i486DX — newer versions of the original i486.
• i486DX2 — the internal processor clock runs at twice the clock rate of the external bus clock.
• i486SX2 — i486DX2 with the FPU disabled.
• i486SL — i486DX with power conservation circuitry. Mainly for use in portable computers.
• i486SL-NM — i486DX with power conservation circuitry; SL enhanced suffix, denotes a i486 with special power conservation circuitry similar to that in the i486SL processors.
• i487 — i486DX with a slightly different pinout sold as FPU to i486SX systems; it was widely documented that i487 when installed completely disables the existing i486SX on mother board.
• i486 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.
• i486DX4 — 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. 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 i486DX 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 i486DX2 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, UMC, and Chips and Technologies. Some are near duplicates in terms of 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.[3]

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. Prior to this some motherboards came equipped with 32 bit versions of the ISA standard called: EISA. These were supplanted with VLB and later PCI. 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).

One of the earliest complete systems to use the 80486 chip was the Apricot VX FT, produced by United Kingdom hardware manufacturer Apricot Computers. Even overseas in the United States it drew attention as "The World's First 486" in a popular September 1989 issue of Byte magazine (shown right).

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

See also

• List of Intel microprocessors
• Motorola 68040, although not compatible, often considered the Motorola equivalent to the Intel 80486 in terms of performance.
• "80486 System Architecture" published by Mindshare (pdf)

Notes

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

External links

• Intel 80486 images and descriptions at cpu-collection.de
• CPU-INFO: 80486, indepth processor history

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