Intel 80486
Encyclopedia
The Intel 80486 microprocessor
(alias i486 or Intel486) was a higher performance follow up on the Intel 80386
. Introduced in 1989, it 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 executed around 40 million instructions per second
on average and was able to reach 50 MIPS peak.
The i486 was without the usual 80-prefix because of a court ruling that prohibited trademarking numbers (such as 80486). Later, with the introduction of the Pentium brand, Intel began branding its chips with words rather than numbers.
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 executes the compare-and-swap
atomic operation and the XADD which executes the fetch-and-add
atomic operation returning the original value, unlike the ADD instruction that only returned some flags.
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), except in the SX and SL models, and an enhanced bus
interface unit. Simple instructions (such as ALU reg, reg) execute in one clock cycle. These improvements yield a rough doubling in ALU performance over the 386 at the same clock rate
. A 16-MHz 486 therefore has a performance similar to a 33-MHz 386
, and the older design has to reach 50 MHz to be comparable with a 25-MHz 486 part.
The 486 has a 32-bit data bus and a 32-bit address bus
. This required either four matched 30-pin (8-bit) SIMM
s or one 72-pin (32-bit) SIMM on a typical PC motherboard
. Just like the 80386, the 32-bit address bus of the 80486 enabled up to 4 gigabyte
s of memory to be directly addressed using a flat memory model with 32-bit linear addresses in protected mode
. Just as with the 80386, the ability to use memory directly without segmentation helped performance in compliant operating systems and applications.
The specified maximum 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 486 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 instead ran the CPU logic at twice the external bus speed.
More powerful 486 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 was a seldom used feature.
WT = Write-Through cache strategy, WB = Write-Back cache strategy
, AMD, Cyrix
, UMC
, and SGS Thompson
. 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 486 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.
AMD produced several clones of the 486 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 486 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 486-compatible processors, positioned at the cost-sensitive desktop and low-power (laptop) markets. Unlike AMD's 486 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" 486 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.
(best known for its use in the Macintosh Quadra
series), while not compatible with the 486, was often positioned as the 486's equivalent in features and performance. While the 68040 outperformed the 486 significantly on a clock for clock basis, the 486 had the ability to be clocked significantly faster without suffering from overheating problems. For a time Apple attempted to compete with Intel's clock doubling 486DX2 systems by publicizing doubled clock rates for its '040-based Macintosh Performa
systems, despite the lack of any clock doubling.
Early 486 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). Many motherboard
s enabled overclocking of these up from the default 6 or 8 MHz to perhaps 16.5 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 486 boards were normally equipped with both PCI- and ISA-slots, and sometimes a single VLB slot as well. In this configuration VLB or PCI thoughput suffered depending on how buses were bridged. The VLB slot in these systems was usually only fully compatible 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 486-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 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
, a specification designed by Microsoft
to make component installation easier for consumers that began as a part of Windows 95
.
gaming era. It was often coupled with a VESA Local Bus
video card.
The introduction of 3D computer graphics
spelled the end of the 486's reign, because 3D graphics make heavy use of floating point
calculations, need 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.
were the last 486 processors that were often used in late generation 486 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 486 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.
In the general purpose desktop computer role, the 486s were used as budget machines for people who could not afford the latest computers, until around 2001, when Windows 95 support ended and Windows 98, ME, 2000, and XP required much more powerful computers to perform well. In fact, the Windows XP
kernel could not run on a 486 at all due to the use of new instruction set extensions introduced with the P5
Pentium processor family.
Although the 486 became obsolete for personal computer
applications by the mid-1990s, 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.
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
(alias i486 or Intel486) was a higher performance follow up on the Intel 80386
Intel 80386
The Intel 80386, also known as the i386, or just 386, was a 32-bit microprocessor introduced by Intel in 1985. The first versions had 275,000 transistors and were used as the central processing unit of many workstations and high-end personal computers of the time...
. Introduced in 1989, it was the first tightly pipelined
Instruction pipeline
An instruction pipeline is a technique used in the design of computers and other digital electronic devices to increase their instruction throughput ....
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
Intel 8086
The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released. The 8086 gave rise to the x86 architecture of Intel's future processors...
of 1978.
A 50 MHz 80486 executed around 40 million instructions per second
Instructions per second
Instructions per second is a measure of a computer's processor speed. Many reported IPS values have represented "peak" execution rates on artificial instruction sequences with few branches, whereas realistic workloads typically lead to significantly lower IPS values...
on average and was able to reach 50 MIPS peak.
The i486 was without the usual 80-prefix because of a court ruling that prohibited trademarking numbers (such as 80486). Later, with the introduction of the Pentium brand, Intel began branding its chips with words rather than numbers.
Background
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. 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.Improvements
The instruction setInstruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
of the i486 is very similar to its predecessor, the Intel 80386
Intel 80386
The Intel 80386, also known as the i386, or just 386, was a 32-bit microprocessor introduced by Intel in 1985. The first versions had 275,000 transistors and were used as the central processing unit of many workstations and high-end personal computers of the time...
, with the addition of only a few extra instructions, such as CMPXCHG which executes the compare-and-swap
Compare-and-swap
In computer science, the compare-and-swap CPU instruction is a special instruction that atomically compares the contents of a memory location to a given value and, only if they are the same, modifies the contents of that memory location to a given new value...
atomic operation and the XADD which executes the fetch-and-add
Fetch-and-add
In computer science, the fetch-and-add CPU instruction is a special instruction that atomically modifies the contents of a memory location. It is used to implement mutual exclusion and concurrent algorithms in multiprocessor systems, a generalization of semaphores.In uniprocessor systems, it is...
atomic operation returning the original value, unlike the ADD instruction that only returned some flags.
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
CPU cache
A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations...
, an on-chip floating-point unit (FPU), except in the SX and SL models, and an enhanced bus
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
interface unit. Simple instructions (such as ALU reg, reg) execute in one clock cycle. These improvements yield a rough doubling in ALU performance over the 386 at the same clock rate
Clock rate
The clock rate typically refers to the frequency that a CPU is running at.For example, a crystal oscillator frequency reference typically is synonymous with a fixed sinusoidal waveform, a clock rate is that frequency reference translated by electronic circuitry into a corresponding square wave...
. A 16-MHz 486 therefore has a performance similar to a 33-MHz 386
Intel 80386
The Intel 80386, also known as the i386, or just 386, was a 32-bit microprocessor introduced by Intel in 1985. The first versions had 275,000 transistors and were used as the central processing unit of many workstations and high-end personal computers of the time...
, and the older design has to reach 50 MHz to be comparable with a 25-MHz 486 part.
Differences between i386 and i486
- An 8 KBKilobyteThe kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...
on-chip SRAMStatic random access memoryStatic random-access memory is a type of semiconductor memory where the word static indicates that, unlike dynamic RAM , it does not need to be periodically refreshed, as SRAM uses bistable latching circuitry to store each bit...
cacheCacheIn computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
stores the most recently used instructions and data (16 KB and/or write-back on some later models). The 386 had no such internal cache but supported a slower off-chip cache. - Tightly coupled pipeliningInstruction pipelineAn instruction pipeline is a technique used in the design of computers and other digital electronic devices to increase their instruction throughput ....
allows the 486 to complete a simple instruction like ALU reg,reg or ALU reg,im every clock cycle. The 386 needed two clock cycles for this. - Integrated FPUFloating point unitA floating-point unit is a part of a computer system specially designed to carry out operations on floating point numbers. Typical operations are addition, subtraction, multiplication, division, and square root...
(disabled or absent in SX models) with a dedicated local bus; together with faster algorithms on more extensive hardware than in the i387, this gives faster floating point calculations compared to the i386+i387 combination. - Improved MMUMemory management unitA memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
performance.
The 486 has a 32-bit data bus and a 32-bit address bus
Address bus
An address bus is a computer bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus...
. This required either four matched 30-pin (8-bit) SIMM
SIMM
A SIMM, or single in-line memory module, is a type of memory module containing random access memory used in computers from the early 1980s to the late 1990s. It differs from a dual in-line memory module , the most predominant form of memory module today, in that the contacts on a SIMM are redundant...
s or one 72-pin (32-bit) SIMM on a typical PC motherboard
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
. Just like the 80386, the 32-bit address bus of the 80486 enabled up to 4 gigabyte
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...
s of memory to be directly addressed using a flat memory model with 32-bit linear addresses in protected mode
Protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...
. Just as with the 80386, the ability to use memory directly without segmentation helped performance in compliant operating systems and applications.
Models
There are several suffixes and variants including:- i486DX: The original chip (without any clock doubling).
- i486DX-S: SL Enhanced 486DX
- Intel RapidCADRapidCADRapidCAD is a specially packaged Intel 486DX and a dummy floating point unit designed as pin-compatible replacements for an Intel 80386 processor and 80387 FPU. Because the i486DX has a working on-chip FPU, a dummy FPU package is supplied to go in the Intel 387 FPU socket...
: a specially packaged Intel 486DX and a dummy floating point unitFloating point unitA floating-point unit is a part of a computer system specially designed to carry out operations on floating point numbers. Typical operations are addition, subtraction, multiplication, division, and square root...
(FPU) designed as pin-compatible replacements for an Intel 80386Intel 80386The Intel 80386, also known as the i386, or just 386, was a 32-bit microprocessor introduced by Intel in 1985. The first versions had 275,000 transistors and were used as the central processing unit of many workstations and high-end personal computers of the time...
processor and 80387 FPU. - i486SXIntel 80486SXThe Intel's i486SX was a modified Intel 486DX microprocessor with its floating-point unit disconnected. All early 486SX chips were actually i486DX chips with a defective FPU...
: an i486DX with the FPU part disabled or missing. Early variants were parts with disabled (defective) FPUs, later versions had the FPU removed from the dieIntegrated circuitAn integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
to reduce area and hence cost. - i486SX-S: SL Enhanced 486SX
- i486DX2Intel 80486DX2The Intel 80486DX2 is a CPU produced by Intel that was introduced in 1992. The i486DX2 was nearly identical to the i486DX but for the addition of 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...
: the internal processor clock runs at twice the clock rateClock rateThe clock rate typically refers to the frequency that a CPU is running at.For example, a crystal oscillator frequency reference typically is synonymous with a fixed sinusoidal waveform, a clock rate is that frequency reference translated by electronic circuitry into a corresponding square wave...
of the external bus clock. - i486SX2: i486DX2 with the FPU disabled.
- i486SLIntel 80486SLThe Intel's i486SL is the power-saving variant of the i486DX microprocessor. The SL was designed for use in mobile computers. It was produced between November 1992 and June 1993. Clock speeds available were 20, 25 and 33 MHz...
: low power version of the i486DX, reduced VCore, SMM (System Management Mode), stop clock, and power saving features - mainly for use in portable computers. - i486SL-NM: i486SL based on i486SX
- i487SX: i486DX with a slightly different pinout sold as an FPU upgrade to i486SX systems; it was widely documented that an i487SX when installed completely disabled the existing i486SX on the motherboardMotherboardIn personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
, replacing it. - i486 OverDriveIntel 80486 OverDriveThe Intel's i486 OverDrive processors are a category of various Intel 80486s that were produced with the designated purpose of being used to upgrade personal computers. The OverDrives typically possessed qualities different from 'standard' i486s with the same speed steppings...
: 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. Fitted to a coprocessor or "OverDrive" socket on the motherboard, worked the same as the i487SX. - i486DX4Intel 80486DX4The IntelDX4 is a clock-tripled i486 microprocessor with 16 kB L1 cache. Intel named it DX4 as a consequence of litigation with AMD over trademarks...
: 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). 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". - i486GX: Embedded 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.
The specified maximum 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 486 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
Intel 80486DX2
The Intel 80486DX2 is a CPU produced by Intel that was introduced in 1992. The i486DX2 was nearly identical to the i486DX but for the addition of 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...
which instead ran the CPU logic at twice the external bus speed.
More powerful 486 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
P5 (microarchitecture)
The original Pentium microprocessor was introduced on March 22, 1993. Its microarchitecture, deemed P5, was Intel's fifth-generation and first superscalar x86 microarchitecture. As a direct extension of the 80486 architecture, it included dual integer pipelines, a faster FPU, wider data bus,...
Pentium processor family. Certain steppings of the DX4 also officially supported 50 MHz bus operation but was a seldom used feature.
| Model | Specified max clock Clock rate The clock rate typically refers to the frequency that a CPU is running at.For example, a crystal oscillator frequency reference typically is synonymous with a fixed sinusoidal waveform, a clock rate is that frequency reference translated by electronic circuitry into a corresponding square wave... |
Voltage | L1-Cache | Introduced | |
 |
i486DX (P4) | 20,25,33 MHz; 50 MHz | 5V | 8 KB WT | April 1989; April 1989; May 1990; June 1991 |
 |
i486SL | 20,25,33 MHz | 5V or 3.3V | 8 KB WT | Nov 1992 |
 |
i486SX (P23) | 16,20,25 MHz (33 MHz) | 5V | 8 KB WT | September 1991 (September 1992) |
 |
i486DX2 (P24) | 40/20, 50/25 MHz (66/33 MHz) | 5V | 8 KB WT | March 1992 (August 1992) |
| i486DX-S (P4S) | 33 MHz; 50 MHz | 5V or 3.3V | 8 KB WT | June 1993 | |
| i486DX2-S (P24S) | 40/20, 50/25 MHz (66/33 MHz) | 5V or 3.3V | 8 KB WT | June 1993 | |
| i486SX-S (P23S) | 25,33 MHz | 5V or 3.3V | 8 KB WT | June 1993 | |
 |
i486SX2 | 50/25, 66/33 MHz | 5V | 8 KB WT | March 1994 |
| IntelDX4 (P24C) | 75/25, 100/33 MHz | 3.3V | 16 KB WT | March 1994 | |
 |
IntelDX4WB | 100/33 MHz | 3.3V | 16 KB WB | October 1994 |
| i486DX2WB (P24D) | 50/25, 66/33 MHz | 5V | 8 KB WB | October 1994 | |
| i486DX2 (P24LM) | 90/30 MHz; 100/33 MHz | 2.5-2.9V | 8 KB WT | 1994 | |
| i486GX | up to 33 MHz | 3.3V | 8 KB WT |
WT = Write-Through cache strategy, WB = Write-Back cache strategy
Other makers of 486-like CPUs
486 compatible processors have been produced by other companies such as IBM, Texas InstrumentsTexas Instruments
Texas Instruments Inc. , widely known as TI, is an American company based in Dallas, Texas, United States, which develops and commercializes semiconductor and computer technology...
, AMD, Cyrix
Cyrix
Cyrix Corporation was a microprocessor developer that was founded in 1988 in Richardson, Texas as a specialist supplier of high-performance math coprocessors for 286 and 386 microprocessors. The company was founded by former Texas Instruments staff members and had a long but troubled relationship...
, UMC
United Microelectronics Corporation
UMC was founded as Taiwan's first semiconductor company in 1980 as a spin-off of the government-sponsored Industrial Technology Research Institute .-Overview:...
, and SGS Thompson
STMicroelectronics
STMicroelectronics is an Italian-French electronics and semiconductor manufacturer headquartered in Geneva, Switzerland.While STMicroelectronics corporate headquarters and the headquarters for EMEA region are based in Geneva, the holding company, STMicroelectronics N.V. is registered in Amsterdam,...
. 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 486 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.
AMD produced several clones of the 486 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 486 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 486-compatible processors, positioned at the cost-sensitive desktop and low-power (laptop) markets. Unlike AMD's 486 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" 486 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.
Competitive alternatives
The Motorola 68040Motorola 68040
The Motorola 68040 is a microprocessor from Motorola, released in 1990. It is the successor to the 68030 and is followed by the 68060. There was no 68050. In keeping with general Motorola naming, the 68040 is often referred to as simply the '040 ....
(best known for its use in the Macintosh Quadra
Macintosh Quadra
The Macintosh Quadra series was Apple Computer's product family of professional high-end Apple Macintosh personal computers built using the Motorola 68040 CPU. The first two models in the Quadra line were introduced in 1991, and the name was used until the Power Mac was introduced in 1994...
series), while not compatible with the 486, was often positioned as the 486's equivalent in features and performance. While the 68040 outperformed the 486 significantly on a clock for clock basis, the 486 had the ability to be clocked significantly faster without suffering from overheating problems. For a time Apple attempted to compete with Intel's clock doubling 486DX2 systems by publicizing doubled clock rates for its '040-based Macintosh Performa
Macintosh Performa
The Macintosh Performa series was Apple Computer's consumer product family of Apple Macintosh personal computers sold through department stores and mass-market retailers from 1992 until 1997, when it was superseded by the Power Macintosh 5x00 series...
systems, despite the lack of any clock doubling.
Motherboards and buses
Industry Standard Architecture
Industry Standard Architecture is a computer bus standard for IBM PC compatible computers introduced with the IBM Personal Computer to support its Intel 8088 microprocessor's 8-bit external data bus and extended to 16 bits for the IBM Personal Computer/AT's Intel 80286 processor...
slots (using an emulated PC/AT-bus) and sometimes one or two 8-bit
8-bit
The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 and the Motorola 6800 were also used in similar computers...
-only slots (compatible with the PC/XT-bus). Many motherboard
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
s enabled overclocking of these up from the default 6 or 8 MHz to perhaps 16.5 or 20 MHz (half the i486 bus clock) in a number of steps, often from within the BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
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
Extended Industry Standard Architecture
The Extended Industry Standard Architecture is a bus standard for IBM PC compatible computers...
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
VESA Local Bus
The VESA Local Bus was mostly used in personal computers. VESA Local Bus worked alongside the ISA bus; it acted as a high-speed conduit for memory-mapped I/O and DMA, while the ISA bus handled interrupts and port-mapped I/O.-Historical overview:In the early 1990s, the I/O bandwidth of...
(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
Peripheral Component Interconnect
Conventional PCI is a computer bus for attaching hardware devices in a computer...
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 486 boards were normally equipped with both PCI- and ISA-slots, and sometimes a single VLB slot as well. In this configuration VLB or PCI thoughput suffered depending on how buses were bridged. The VLB slot in these systems was usually only fully compatible with video cards (quite fitting as "VESA" stands for Video Electronics Standards Association
VESA
VESA is an international standards body for computer graphics founded in 1989 by NEC Home Electronics and eight other video display adapter manufacturers.VESA's initial goal was to produce a standard for 800×600 SVGA resolution video displays...
); 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 486-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 United Kingdom hardware manufacturer Apricot Computers
Apricot Computers
Apricot Computers is a British manufacturer of business personal computers, originally founded in 1965 as "Applied Computer Techniques" , changing its name to Apricot Computers, Ltd. in the 1980s...
. Even overseas in the United States it drew attention as "The World's First 486" in a popular September 1989 issue of Byte magazine
Byte (magazine)
BYTE magazine was a microcomputer magazine, influential in the late 1970s and throughout the 1980s because of its wide-ranging editorial coverage...
(shown right).
Later 486 boards also supported Plug-And-Play
Plug-and-play
In computing, plug and play is a term used to describe the characteristic of a computer bus, or device specification, which facilitates the discovery of a hardware component in a system, without the need for physical device configuration, or user intervention in resolving resource conflicts.Plug...
, a specification designed by Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
to make component installation easier for consumers that began as a part of Windows 95
Windows 95
Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...
.
Gaming
The 486DX2 66Mhz processor was popular with many players of video games during the early to mid 1990s, toward the end of the MS-DOSMS-DOS
MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
gaming era. It was often coupled with a VESA Local Bus
VESA Local Bus
The VESA Local Bus was mostly used in personal computers. VESA Local Bus worked alongside the ISA bus; it acted as a high-speed conduit for memory-mapped I/O and DMA, while the ISA bus handled interrupts and port-mapped I/O.-Historical overview:In the early 1990s, the I/O bandwidth of...
video card.
The introduction of 3D computer graphics
3D computer graphics
3D computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering 2D images...
spelled the end of the 486's reign, because 3D graphics make heavy use of floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
calculations, need faster CPU cache
CPU cache
A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations...
and more memory bandwidth
Memory bandwidth
Memory bandwidth is the rate at which data can be read from or stored into a semiconductor memory by a processor. Memory bandwidth is usually expressed in units of bytes/second, though this can vary for systems with natural data sizes that are not a multiple of the commonly used 8-bit bytes.Memory...
. Developers began to target the P5 Pentium processor family almost exclusively with x86 assembly language
X86 assembly language
x86 assembly language is a family of backward-compatible assembly languages, which provide some level of compatibility all the way back to the Intel 8008. x86 assembly languages are used to produce object code for the x86 class of processors, which includes Intel's Core series and AMD's Phenom and...
optimizations (e.g., Quake) which led to the usage of terms like "Pentium compatible processor
Pentium compatible processor
A Pentium compatible processor is a 32-bit processor computer chip which supports the instructions in the IA-32 instruction set that were implemented by the Intel P5 Pentium processor family...
" for software requirements. Many of these games required the speed of the P5 Pentium processor family's double-pipelined architecture.
Obsolescence
The AMD Am5x86 and Cyrix Cx5x86Cyrix Cx5x86
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...
were the last 486 processors that were often used in late generation 486 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 486 motherboards. While the Cyrix Cx5x86
Cyrix Cx5x86
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...
faded quite quickly when the Cyrix 6x86
Cyrix 6x86
The Cyrix 6x86 is a sixth-generation, 32-bit 80x86-compatible microprocessor designed by Cyrix and manufactured by IBM and SGS-Thomson. It was originally released in 1996.-Architecture:...
took over, the AMD Am5x86 was important during the time when the AMD K5
AMD K5
The K5 was AMD's first x86 processor to be developed entirely in-house. Introduced in March 1996, its primary competition was Intel's Pentium microprocessor. The K5 was an ambitious design, closer to a Pentium Pro than a Pentium regarding technical solutions and internal architecture...
was delayed.
In the general purpose desktop computer role, the 486s were used as budget machines for people who could not afford the latest computers, until around 2001, when Windows 95 support ended and Windows 98, ME, 2000, and XP required much more powerful computers to perform well. In fact, the Windows XP
Windows XP
Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...
kernel could not run on a 486 at all due to the use of new instruction set extensions introduced with the P5
P5 (microarchitecture)
The original Pentium microprocessor was introduced on March 22, 1993. Its microarchitecture, deemed P5, was Intel's fifth-generation and first superscalar x86 microarchitecture. As a direct extension of the 80486 architecture, it included dual integer pipelines, a faster FPU, wider data bus,...
Pentium processor family.
Although the 486 became obsolete for personal computer
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
applications by the mid-1990s, 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.
See also
- List of Intel microprocessors
- Motorola 68040Motorola 68040The Motorola 68040 is a microprocessor from Motorola, released in 1990. It is the successor to the 68030 and is followed by the 68060. There was no 68050. In keeping with general Motorola naming, the 68040 is often referred to as simply the '040 ....
, although not compatible, was often positioned as the MotorolaMotorolaMotorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
equivalent to the Intel 80486 in terms of performance and features.