Intel 80486

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Intel 80486
80486dx2-large.jpg
The exposed die of an Intel 80486DX2 microprocessor
ProducedFrom April 1989 to 2007
Common manufacturer(s)
Max. CPU clock rate 16 MHz to 100 MHz
FSB speeds16 MHz to 50 MHz
Min. feature size 1 µm to 0.6 µm
Instruction set x86 including x87 (except for "SX" models)
Data width32 bits [1]
Address width32 bits [1]
Virtual address width32 bits (linear); 46 bits (logical) [1]
Predecessor Intel 80386
Successor Pentium (P5)
Co-processor Intel 80487SX
Package(s)

The Intel 80486, also known as the i486 or 486, is the successor model of 32-bit x86 microprocessor to the Intel 80386. Introduced in 1989, the 80486 improved on the performance of the 80386DX thanks to on-die L1 cache and floating-point unit, as well as an improved, five-stage tightly-coupled pipelined design. It was the first x86 chip to use more than a million transistors. It represents the fourth generation of binary compatible CPUs since the original 8086 of 1978.

Intel American semiconductor chip manufacturer

Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, in the Silicon Valley. It is the world's second largest and second highest valued semiconductor chip manufacturer based on revenue after being overtaken by Samsung Electronics, and is the inventor of the x86 series of microprocessors, the processors found in most personal computers (PCs). Intel ranked No. 46 in the 2018 Fortune 500 list of the largest United States corporations by total revenue. Intel is incorporated in Delaware.

In computer architecture, 32-bit integers, memory addresses, or other data units are those that are 32 bits wide. Also, 32-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. 32-bit microcomputers are computers in which 32-bit microprocessors are the norm.

x86 type of instruction set architecture

x86 is a family of instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was introduced in 1978 as a fully 16-bit extension of Intel's 8-bit 8080 microprocessor, with memory segmentation as a solution for addressing more memory than can be covered by a plain 16-bit address. The term "x86" came into being because the names of several successors to Intel's 8086 processor end in "86", including the 80186, 80286, 80386 and 80486 processors.

Contents

The initial model, the 80486DX, was introduced with 25 and 33 MHz models. Later a 50 MHz part was added, then clock-doubled DX2/50 and DX2/66 parts, and later still, clock-tripled DX4/75 and DX4/100 ones.

The 486DX was later supplemented with the cheaper 80486SX, which was also available in 16 and 20 MHz variants. The "SX" and "DX" designations matched those of the 80386DX and 80386SX, but had different meanings. For the 486, the SX designation indicated no on-chip FPU. In early 486SX units, the FPU was present but disabled; later models removed it entirely. A supplementary 80487DX upgrade chip was also offered, but this was not an FPU; it was an entire replacement processor that disabled the original SX part.

Floating-point unit part of a computer system specially designed to carry out operations on floating point numbers

A 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, square root, and bitshifting. Some systems can also perform various transcendental functions such as exponential or trigonometric calculations, though in most modern processors these are done with software library routines.

A 50 MHz 80486 executes around 40 million instructions per second on average and is able to reach 50 MIPS peak performance.

Instructions per second (IPS) is a measure of a computer's processor speed. For CISC computers different instructions take different amounts of time, so the value measured depends on the instruction mix; even for comparing processors in the same family the IPS measurement can be problematic. Many reported IPS values have represented "peak" execution rates on artificial instruction sequences with few branches and no cache contention, whereas realistic workloads typically lead to significantly lower IPS values. Memory hierarchy also greatly affects processor performance, an issue barely considered in IPS calculations. Because of these problems, synthetic benchmarks such as Dhrystone are now generally used to estimate computer performance in commonly used applications, and raw IPS has fallen into disuse.

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. [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]

COMDEX former computer expo

COMDEX was a computer expo trade show held in the Las Vegas Valley of Nevada, USA, each November from 1979 to 2003. It was one of the largest computer trade shows in the world, usually second only to the German CeBIT, and one of the largest trade shows in any industry sector. COMDEX exhibitions were held in many other countries from 1982 to 2005, with 185 shows altogether. The first COMDEX was held in 1979 at the MGM Grand, with 167 exhibitors and 3904 attendees. In 1981, the first COMDEX/Spring was held in New York City.

Improvements

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
 FSFSegment
 GSGSegment
 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).

Intel 80386 family of 32-bit microprocessors introduced in 1985, including DX, SX and SL models

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.

In computer science, compare-and-swap (CAS) is an atomic instruction used in multithreading to achieve synchronization. It compares the contents of a memory location with a given value and, only if they are the same, modifies the contents of that memory location to a new given value. This is done as a single atomic operation. The atomicity guarantees that the new value is calculated based on up-to-date information; if the value had been updated by another thread in the meantime, the write would fail. The result of the operation must indicate whether it performed the substitution; this can be done either with a simple boolean response, or by returning the value read from the memory location.

In computer science, the fetch-and-add CPU instruction (FAA) atomically increments the contents of a memory location by a specified value.

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]

A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, which stores copies of the data from frequently used main memory locations. Most CPUs have different independent caches, including instruction and data caches, where the data cache is usually organized as a hierarchy of more cache levels.

The clock rate typically refers to the frequency at which the clock circuit of a processor can generate pulses, which are used to synchronize the operations of its components, and is used as an indicator of the processor's speed. It is measured in clock cycles per second or its equivalent, the SI unit hertz (Hz). The clock rate of the first generation of computers was measured in hertz or kilohertz (kHz), the first personal computers (PCs) to arrive throughout the 1970s and 1980s had clock rates measured in megahertz (MHz), and in the 21st century the speed of modern CPUs is commonly advertised in gigahertz (GHz). This metric is most useful when comparing processors within the same family, holding constant other features that may affect performance. Video card and CPU manufacturers commonly select their highest performing units from a manufacturing batch and set their maximum clock rate higher, fetching a higher price.

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).

Models

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 486DX2 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.

ModelCPU/bus
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 1994
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
UMC Green CPU U5SX KL UMC U5SX.jpg
UMC Green CPU U5SX

80486 compatible processors have been produced by other companies such as IBM, Texas Instruments, AMD, Cyrix, UMC, and 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 popularised 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.

Gaming

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.

Obsolescence

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.

80486-based machines 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 2000's.

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 have remained in use today, 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

Notes

  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.

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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 microprocessor

The Cyrix Cx486 or Cyrix 486 was an x86-Compatible microprocessor designed by Cyrix to primarily compete 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.

References

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  2. 486 32-bit CPU breaks new ground in chip density and operating performance. (Intel Corp.) (product announcement) EDN | May 11, 1989 | Pryce, Dave
  3. Lewis, Peter H. (October 22, 1989). "THE EXECUTIVE COMPUTER; The Race to Market a 80486 Machine". The New York Times. Retrieved May 5, 2010.
  4. Newnes 8086 Family Pocket Book – Ian Sinclair ( ISBN   0 4349 1872 5)
  5. "AMD-Intel Litigation History". yannalaw.com.
  6. "CISC: The Intel 80486 vs. The Motorola MC68040". July 1992. Retrieved May 20, 2013.
  7. 68040 Microprocessor Archived February 16, 2012, at the Wayback Machine
  8. "Minimum Hardware Requirements for a Windows 98 Installation". January 24, 2001. Archived from the original on December 5, 2004.
  9. "Windows NT 4.0 Workstation" (in German). WinHistory.de.
  10. "System Requirements". DOSBox.com.
  11. Tony Smith (May 18, 2006). "Intel cashes in ancient chips. i386, i486, i960 finally for the chop". HARDWARE. Retrieved May 20, 2012.

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.