Intel 80486

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Intel 80486
The exposed die of an Intel 80486DX2 microprocessor
General information
LaunchedApril 1989
DiscontinuedSeptember 28, 2007
Common manufacturer(s)
Max. CPU clock rate 16 MHz to 100 MHz
FSB speeds16 MHz to 50 MHz
Data width32 bits [1]
Address width32 bits [1]
Virtual address width32 bits (linear); 46 bits (logical) [1]
Architecture and classification
Min. feature size 1 µm to 0.6 µm
Instruction set x86 including x87 (except for "SX" models)
Physical specifications
Co-processor Intel 80487SX
Predecessor Intel 80386
Successor Pentium (P5)

The Intel 80486, officially named i486 and also known as 486, is a higher-performance follow-up to the Intel 80386 microprocessor. The 80486 was introduced in 1989 and was the first tightly pipelined x86 design as well as the first x86 chip to use more than a million transistors, due to a large on-chip cache and an integrated floating-point unit. It represents a fourth generation of binary compatible CPUs since the original 8086 of 1978.


A 50 MHz 80486 executes around 40 million instructions per second on average and is able to reach 50 MIPS peak performance, approximately twice as fast as the 80386 or 80286 per clock cycle, thanks to its five-stage pipeline with all stages bound to a single cycle. The on chip enhanced FPU unit was also significantly faster than the 80387 per cycle.


The 80486 was announced at Spring Comdex in April 1989. At the announcement, Intel stated that samples would be available in the third quarter of 1989 and production quantities would ship in the fourth quarter of 1989. [2] The first 80486-based PCs were announced in late 1989, but some advised that people wait until 1990 to purchase an 80486 PC because there were early reports of bugs and software incompatibilities. [3]


The 486DX2 architecture 80486DX2 arch.svg
The 486DX2 architecture
Intel 80486 registers
31...15...07...00(bit position)
Main registers(8/16/32 bits)
EAXAHALA register
EBXBHBLB register
ECXCHCLC register
EDXDHDLD register
Index registers(16/32 bits)
ESISISource Index
EDIDIDestination Index
EBPBPBase Pointer
ESPSPStack Pointer
Program counter(16/32 bits)
EIPIPInstruction Pointer
Segment selectors(16 bits)
 CSCode Segment
 DSData Segment
 ESExtra Segment
 SSStack Segment
Status register
 171615141312111009080706050403020100(bit position)
  V R0N IOPL O D I T S Z 0 A 0 P 1 C EFlags
Floating-point registers(80 bits)
79...00(bit position)
ST0STack register 0
ST1STack register 1
ST2STack register 2
ST3STack register 3
ST4STack register 4
ST5STack register 5
ST6STack register 6
ST7STack register 7

The instruction set of the i486 is very similar to its predecessor, the Intel 80386, with the addition of only a few extra instructions, such as CMPXCHG which implements a compare-and-swap atomic operation and XADD, a fetch-and-add atomic operation returning the original value (unlike a standard ADD which returns flags only).

From a performance point of view, the architecture of the i486 is a vast improvement over the 80386. It has an on-chip unified instruction and data cache, an on-chip floating-point unit (FPU) and an enhanced bus interface unit. Due to the tight pipelining, sequences of simple instructions (such as ALU reg,reg and ALU reg,im) could sustain a single-clock-cycle throughput (one instruction completed every clock). These improvements yielded a rough doubling in integer ALU performance over the 386 at the same clock rate. A 16-MHz 80486 therefore had a performance similar to a 33-MHz 386, and the older design had to reach 50 MHz to be comparable with a 25-MHz 80486 part. [lower-alpha 1]

Differences between i386 and i486

Just as in the 80386, a simple flat 4 GB memory model could be implemented by setting all "segment selector" registers to a neutral value in protected mode, or setting (the same) "segment registers" to zero in real mode, and using only the 32-bit "offset registers" (x86-terminology for general CPU registers used as address registers) as a linear 32-bit virtual address bypassing the segmentation logic. Virtual addresses were then normally mapped onto physical addresses by the paging system except when it was disabled. (Real mode had no virtual addresses.) Just as with the 80386, circumventing memory segmentation could substantially improve performance in some operating systems and applications.

On a typical PC motherboard, either four matched 30-pin (8-bit) SIMMs or one 72-pin (32-bit) SIMM per bank were required to fit the 80486's 32-bit data bus. The address bus used 30-bits (A31..A2) complemented by four byte-select pins (instead of A0,A1) to allow for any 8/16/32-bit selection. This meant that the limit of directly addressable physical memory was 4  gigabytes as well (23032-bit words = 2328-bit words).


There are several suffixes and variants. (see Table). Other variants include:

The specified maximal internal clock frequency (on Intel's versions) ranged from 16 to 100 MHz. The 16 MHz i486SX model was used by Dell Computers.

One of the few 80486 models specified for a 50 MHz bus (486DX-50) initially had overheating problems and was moved to the 0.8-micrometre fabrication process. However, problems continued when the 486DX-50 was installed in local-bus systems due to the high bus speed, making it rather unpopular with mainstream consumers, as local-bus video was considered a requirement at the time, though it remained popular with users of EISA systems. The 486DX-50 was soon eclipsed by the clock-doubled i486DX2, which although running the internal CPU logic at twice the external bus speed (50 MHz), was nevertheless slower due to the external bus running at only 25 MHz. The i486DX2 at 66 MHz (with 33 MHz external bus) was faster than the 486DX-50, overall.

More powerful 80486 iterations such as the OverDrive and DX4 were less popular (the latter available as an OEM part only), as they came out after Intel had released the next-generation P5 Pentium processor family. Certain steppings of the DX4 also officially supported 50 MHz bus operation, but it was a seldom-used feature.

clock speed
Voltage L1 cache*IntroducedNotes
Intel i486 DX 25MHz SX328.jpg
Intel i486 DX-33.jpg
Intel i486 dx 50mhz 2007 03 27.jpg
i486DX (P4)20, 25 MHz
33 MHz
50 MHz
5 V8 KB WTApril 1989
May 1990
June 1991
The original chip without clock multiplier
KL Intel 486SL.jpg
i486SL20, 25, 33 MHz5 V or 3.3 V8 KB WTNovember 1992Low-power version of the i486DX, reduced VCore, SMM (System Management Mode), stop clock, and power-saving features — mainly for use in portable computers
Intel i486 sx 33mhz 2007 03 27.jpg
i486SX (P23)16, 20, 25 MHz
33 MHz
5 V8 KB WTSeptember 1991
September 1992
An i486DX with the FPU part disabled or missing. Early variants were parts with disabled (defective) FPUs. [4] Later versions had the FPU removed from the die to reduce area and hence cost.
Intel i486 dx2 66mhz 2007 03 27.jpg
i486DX2 (P24)40/20, 50/25 MHz
66/33 MHz
5 V8 KB WTMarch 1992
August 1992
The internal processor clock runs at twice the clock rate of the external bus clock
i486DX-S (P4S)33 MHz; 50 MHz5 V or 3.3 V8 KB WTJune 1993SL Enhanced 486DX
KL Intel i486DX2 PQFP.jpg
i486DX2-S (P24S)40/20 MHz,
50/25 MHz,
(66/33 MHz)
5 V or 3.3 V8 KB WTJune 1993
KL Intel i486SX PQFP.jpg
i486SX-S (P23S)25, 33 MHz5 V or 3.3 V8 KB WTJune 1993SL Enhanced 486SX
KL intel i486SX2.jpg
i486SX250/25, 66/33 MHz5 V8 KB WTMarch 1994i486DX2 with the FPU disabled
FC80486DX4-75 AK SX883 USA 1995 01 WT.jpg
Intel i486 DX4 100 MHz SK051.jpeg
IntelDX4 (P24C)75/25, 100/33 MHz3.3 V16 KB WTMarch 1994Designed to run at triple clock rate (not quadruple, as often believed; the DX3, which was meant to run at 2.5× the clock speed, was never released). DX4 models that featured write-back cache were identified by an "&EW" laser-etched into their top surface, while the write-through models were identified by "&E".
Intel i486 DX2 66 CPU SX955.jpg
i486DX2WB (P24D)50/25 MHz,
66/33 MHz
5 V8 KB WBOctober 1994Enabled write-back cache.
Intel i486 dx4 100mhz 2007 03 27.jpg
IntelDX4WB100/33 MHz3.3 V16 KB WBOctober 1994
i486DX2 (P24LM)90/30 MHz,
100/33 MHz
2.5–2.9 V8 KB WT1994
KL Intel i486GX.jpg
i486GXup to 33 MHz3.3 V8 KB WTEmbedded ultra-low-power CPU with all features of the i486SX and 16-bit external data bus. This CPU is for embedded battery-operated and hand-held applications.

*WT = write-through cache strategy, WB = write-back cache strategy

Other makers of 80486-like CPUs

STMicroelectronics' ST ST486DX2-40 KL STMicroelectronics ST486DX2-40.jpg
STMicroelectronics' ST ST486DX2-40
Cyrix Cx486DRx2 KL Cyrix 486DRx2.jpg
Cyrix Cx486DRx²

Processors compatible with the 80486 have been produced by other companies such as IBM, Texas Instruments, AMD, Cyrix, UMC, and STMicroelectronics (formerly SGS-Thomson). Some were clones (identical at the microarchitectural level), others were clean room implementations of the Intel instruction set. (IBM's multiple-source requirement is one of the reasons behind its x86 manufacturing since the 80286.) The 80486 was, however, covered by many of Intel's patents covering new R&D as well as that of the prior 80386. 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. [5]

AMD produced several clones of the 80486 using a 40 MHz bus (486DX-40, 486DX/2-80, and 486DX/4-120) which had no equivalent available from Intel, as well as a part specified for 90 MHz, using a 30 MHz external clock, that was sold only to OEMs. The fastest running 80486 CPU, the Am5x86, ran at 133 MHz and was released by AMD in 1995. 150 MHz and 160 MHz parts were planned but never officially released.

Cyrix made a variety of 80486-compatible processors, positioned at the cost-sensitive desktop and low-power (laptop) markets. Unlike AMD's 80486 clones, the Cyrix processors were the result of clean-room reverse engineering. Cyrix's early offerings included the 486DLC and 486SLC, two hybrid chips which plugged into 386DX or SX sockets respectively, and offered 1 KB of cache (versus 8 KB for the then-current Intel/AMD parts). Cyrix also made "real" 80486 processors, which plugged into the i486's socket and offered 2 or 8 KB of cache. Clock-for-clock, the Cyrix-made chips were generally slower than their Intel/AMD equivalents, though later products with 8 KB caches were more competitive, if late to market.

The Motorola 68040, while not compatible with the 80486, was often positioned as the 80486's equivalent in features and performance. Clock-for-clock basis the Motorola 68040 could significantly outperform the Intel 80486 chip. [6] [7] However, the 80486 had the ability to be clocked significantly faster without suffering from overheating problems. The Motorola 68040 performance lagged behind the later production 80486 systems.[ citation needed ]

Motherboards and buses

The first 80486 system from the UK on the cover of BYTE, September 1989 Byte magazine September 1989 cover.png
The first 80486 system from the UK on the cover of BYTE, September 1989

Early 80486 machines were equipped with several ISA slots (using an emulated PC/AT-bus) and sometimes one or two 8-bit-only slots (compatible with the PC/XT-bus). [lower-alpha 3] Many motherboards enabled overclocking of these up from the default 6 or 8 MHz to perhaps 16.7 or 20 MHz (half the i486 bus clock) in a number of steps, often from within the BIOS setup. Especially older peripheral cards normally worked well at such speeds as they often used standard MSI chips instead of slower (at the time) custom VLSI designs. This could give significant performance gains (such as for old video cards moved from a 386 or 286 computer, for example). However, operation beyond 8 or 10 MHz could sometimes lead to stability problems, at least in systems equipped with SCSI or sound cards.

Some motherboards came equipped with a 32-bit bus called EISA that was backward compatible with the ISA-standard. EISA offered a number of attractive features such as increased bandwidth, extended addressing, IRQ sharing, and card configuration through software (rather than through jumpers, DIP switches, etc.) However, EISA cards were expensive and therefore mostly employed in servers and workstations. Consumer desktops often used the simpler but faster VESA Local Bus (VLB), unfortunately somewhat prone to electrical and timing-based instability; typical consumer desktops had ISA slots combined with a single VLB slot for a video card. VLB was gradually replaced by PCI during the final years of the 80486 period. Few Pentium class motherboards had VLB support as VLB was based directly on the i486 bus; it was no trivial matter adapting it to the quite different P5 Pentium-bus. ISA persisted through the P5 Pentium generation and was not completely displaced by PCI until the Pentium III era.

Late 80486 boards were normally equipped with both PCI and ISA slots, and sometimes a single VLB slot as well. In this configuration VLB or PCI throughput suffered depending on how buses were bridged. Initially, the VLB slot in these systems was usually fully compatible only with video cards (quite fitting as "VESA" stands for Video Electronics Standards Association ); VLB-IDE, multi I/O, or SCSI cards could have problems on motherboards with PCI slots. The VL-Bus operated at the same clock speed as the i486-bus (basically being a local 80486-bus) while the PCI bus also usually depended on the i486 clock but sometimes had a divider setting available via the BIOS. This could be set to 1/1 or 1/2, sometimes even 2/3 (for 50 MHz CPU clocks). Some motherboards limited the PCI clock to the specified maximum of 33 MHz and certain network cards depended on this frequency for correct bit-rates. The ISA clock was typically generated by a divider of the CPU/VLB/PCI clock (as implied above).

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

Later 80486 boards also supported Plug-And-Play, a specification designed by Microsoft that began as a part of Windows 95 to make component installation easier for consumers.


The 486DX2 66 MHz processor was popular on home-oriented PCs during the early to mid 1990s, toward the end of the MS-DOS gaming era. It was often coupled with a VESA Local Bus video card.

The introduction of 3D computer graphics spelled the end of the 80486's reign, because 3D graphics make heavy use of floating-point calculations and require a faster CPU cache and more memory bandwidth. Developers began to target the P5 Pentium processor family almost exclusively with x86 assembly language optimizations (e.g., Quake ) which led to the usage of terms like "Pentium-compatible processor" for software requirements. Many of these games required the speed of the P5 Pentium processor family's double-pipelined architecture.


The AMD Am5x86, up to 133 MHz, and Cyrix Cx5x86, up to 120 MHz, were the last 80486 processors that were often used in late generation 80486 motherboards with PCI slots and 72-pin SIMMs that are designed to be able to run Windows 95, and also often used as upgrades for older 80486 motherboards. While the Cyrix Cx5x86 faded quite quickly when the Cyrix 6x86 took over, the AMD Am5x86 was important during the time when the AMD K5 was delayed.

Machines based on the 80486 remained popular through the late 1990s, serving as low-end processors for entry-level PCs. Production for traditional desktop and laptop systems ceased in 1998, when Intel introduced the Celeron brand as an modern replacement for the aging chip, though it continued to be produced for embedded systems through the late 2000s.

In the general-purpose desktop computer role, 80486-based machines remained in use into the early 2000s, especially as Windows 95, Windows 98, and Windows NT 4.0 were the latest Microsoft operating systems to officially support installation on an 80486-based system. [8] [9] However, as Windows 95/98 and Windows NT 4.0 were eventually overtaken by newer operating systems, 80486 systems likewise fell out of use. Still, a number of 80486 machines remained in use, mostly for backward compatibility with older programs (most notably games), especially since many of them have problems running on newer operating systems. However, DOSBox is also available for current operating systems and provides emulation of the 80486 instruction set, as well as full compatibility with most DOS-based programs. [10]

Although the 80486 was eventually overtaken by the Pentium for personal computer applications, Intel had continued production for use in embedded systems. In May 2006 Intel announced that production of the 80486 would stop at the end of September 2007. [11]

See also


  1. The "low-end" 16 and 25 MHz 80486 parts did not use a clock multiplier and are therefore comparable to a 386/286 clock by clock.
  2. The 386, 286, and even the 8086 all had overlapping fetch, decode, execution (calculation), and write back; however, tightly pipelined usually means that all stages perform their respective duties within the same length time slot. In contrast loosely pipelined implies that some kind of buffering is used to decouple the units and allow them to work more independently. Both the original 8086 and the x86-chips of today are "loosely pipelined" in this sense, while the 80486 and the original Pentium worked in a "tightly pipelined" manner for typical instructions. This included most "CISC" type instructions as well as the simple load/store-free "RISC-like" ones, although the most complex also used some dedicated microcode control.
  3. In general, 8-bit ISA slots in these systems were implemented just by leaving off the shorter "C"/"D" connector of the slot, though the copper traces for a 16-bit slot were still there on the motherboard; the computer could tell no difference between an 8-bit ISA adapter in such a slot and the same adapter in a 16-bit slot, and there were still enough 8-bit adapters in circulation that vendors figured they could save money on a few connectors this way. Also, leaving off the 16-bit extension to the ISA connector allowed use of some early 8-bit ISA cards that otherwise could not be used due to the PCB "skirt" hanging down into that 16-bit extension space. IBM was the first to do this in the IBM AT.

Related Research Articles

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Intel 80386 32-bit microprocessor by Intel

The Intel 80386, also known as i386 or just 386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistors and were the CPU of many workstations and high-end personal computers of the time. As the original implementation of the 32-bit extension of the 80286 architecture, the 80386 instruction set, programming model, and binary encodings are still the common denominator for all 32-bit x86 processors, which is termed the i386-architecture, x86, or IA-32, depending on context.

P5 (microarchitecture) Intel microporocessor

The original Pentium microprocessor was introduced by Intel on March 22, 1993. It was instruction set compatible with the 80486 but was a new and very different microarchitecture design. The P5 Pentium was the first superscalar x86 microarchitecture and the world’s first superscalar microprocessor to be in mass production. It included dual integer pipelines, a faster floating-point unit, wider data bus, separate code and data caches as well as many other techniques and features to enhance performance and support security, encryption, and multiprocessing for workstations and servers.

VESA Local Bus Expansion bus for 486 PCs

The VESA Local Bus is a short-lived expansion bus introduced during the i486 generation of x86 IBM-compatible personal computers. Created by VESA, the VESA Local Bus works alongside the then-dominant ISA bus to provide a standardized high-speed conduit intended primarily to accelerate video (graphics) operations. VLB provides a standardized "fast path" that add-in (video) card makers could tap for greatly accelerated memory-mapped I/O and DMA, while still using the familiar ISA bus to handle basic device duties such as interrupts and port-mapped I/O.

Intel 80486DX2

The Intel i486DX2, rumored as 80486DX2 is a CPU produced by Intel that was introduced in 1992. The i486DX2 was nearly identical to the i486DX, but it had additional clock multiplier circuitry. It was the first chip to use clock doubling, whereby the processor runs two internal logic clock cycles per external bus cycle. An i486 DX2 was thus significantly faster than an i486 DX at the same bus speed thanks to the 8K on-chip cache shadowing the slower clocked external bus.

Cyrix American microprocessor developer

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Cyrix 5x86

The Cyrix 5x86 was a x86 microprocessor designed by Cyrix. Released in August 1995, four months before the more famous Cyrix 6x86, the Cyrix 5x86 was one of the fastest CPUs ever produced for Socket 3 computer systems. With better performance in most applications than an Intel Pentium processor at 75 MHz, the Cyrix Cx5x86 filled a gap by providing a medium-performance processor option for 486 Socket 3 motherboards.


The Am5x86 processor is an x86-compatible CPU introduced in 1995 by AMD for use in 486-class computer systems. It is one of the fastest, and most universally compatible upgrade paths for 486 systems.

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The Am486 is a 80486-class family of computer processors that was produced by AMD in the 1990s. Intel beat AMD to market by nearly four years, but AMD priced its 40 MHz 486 at or below Intel's price for a 33 MHz chip, offering about 20% better performance for the same price.


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Cyrix Cx486SLC

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Cyrix Cx486DLC

The Cyrix Cx486DLC was an early 486 CPU from Cyrix, intended to compete with the Intel 486SX and DX. Texas Instruments, who manufactured the 486DLC for Cyrix, later released its own version of the chip, the TI486SXL, with 8 kB internal cache vs 1 kB of the original Cyrix design. The similarly named IBM 486DLC, 486DLC2, 486DLC3 are often confused with the Cyrix chips, but are not related and are instead based on Intel's i486 design.


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The UMC Green CPU was an x86-compatible microprocessor produced by UMC, a Taiwanese semiconductor company, in the early- to mid-1990s. It was offered as an alternative to the Intel 80486 with which it was pin compatible, enabling it to be installed in most 80486 motherboards. All models had power management features intended to reduce electricity consumption.

Cyrix Cx486

The Cyrix Cx486 was an x86 microprocessor designed by Cyrix. It primarily competed with the Intel 486 with which it was software compatible, would operate in the same motherboards provided proper support by the BIOS was available and generally showed comparable performance. The chip also competed with parts from AMD and UMC.


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