Intel 80386
Encyclopedia
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 (CPU)
of many workstation
s and high-end personal computer
s of the time. As the original implementation of the 32-bit
extension of the 8086
architecture, the 80386 instruction set, programming model, and binary encodings are still the common denominator for all 32-bit x86 processors, this is termed x86, IA-32
, or i386-architecture, depending on context.
The 80386 could correctly execute most code intended for earlier 16-bit x86 processors such as the 8088 and 80286 that were ubiquitous in early PC
s. Following the same tradition, modern 64-bit x86 processors are able to run most programs written for older chips, all the way back to the original 16-bit 8086
of 1978. Over the years, successively newer implementations of the same architecture have become several hundreds of times faster than the original 80386 (and thousands of times faster than the 8086). A 33 MHz 80386 was reportedly measured to operate at about 11.4 MIPS.
The 80386 was launched in October 1985, but full-function chips were first delivered in the third quarter of 1986. Mainboards for 80386-based computer systems were cumbersome and expensive at first, but manufacturing was rationalized upon the 80386's mainstream adoption. The first personal computer
to make use of the 80386 was designed and manufactured by Compaq
and marked the first time a fundamental component in the IBM PC compatible
de facto-standard was updated by a company other than IBM
.
In May 2006, Intel announced that 80386 production would stop at the end of September 2007. Although it had long been obsolete as a personal computer
CPU, Intel and others had continued making the chip for embedded system
s. Such systems using an 80386 or one of many derivatives are common in aerospace
technology, among others. Some mobile phones also used the 80386 processor, such as BlackBerry 950
and Nokia 9000 Communicator.
The processor was a significant evolution in the x86 architecture, and the latest of a long line of processors that stretched back to the Intel 8008
. The predecessor of the 80386 was the Intel 80286
, a 16-bit
processor with a segment
-based memory management and protection system. The 80386 added a 32-bit
architecture and a paging
translation unit, which made it much easier to implement operating systems that used virtual memory
. It also had support for hardware
debugging.
The 80386 featured three operating modes: real mode, protected mode and virtual mode. The protected mode
which debuted in the 286 was extended to allow the 386 to address up to 4 GB
of memory. The all new virtual 8086 mode
(or VM86) made it possible to run one or more real mode
programs in a protected environment, although some programs were not compatible.
The 32-bit flat memory model
of the 386 would arguably be the most important feature change for the x86 processor family until AMD released x86-64
in 2003.
Chief architect in the development of the 80386 was John H. Crawford. He was responsible for extending the 80286 architecture and instruction set to 32-bit, and then led the microprogram development for the 80386 chip.
The 80486
and P5
Pentium line of processors were descendants of the 80386 design.
In 1988, Intel introduced the i386SX, a low cost version of the 80386 with a 16-bit data bus. The CPU remained fully 32-bit internally, but the 16-bit bus was intended to simplify circuit board layout and reduce total cost. The 16-bit bus simplified designs but hampered performance. Only 24 pins were connected to the address bus, therefore limiting addressing to 16 MB
, but this was not a critical constraint at the time. Performance differences were due not only to differing databus-widths, but also to performance-enhancing cache memories often employed on boards using the original chip.
The original 80386 was subsequently renamed i386DX to avoid confusion. However, Intel subsequently used the 'DX' suffix to refer to the floating-point capability of the i486DX. The i387SX was an i387 part that was compatible with the i386SX (i.e. with a 16-bit databus). The 386SX was packaged in a surface-mount QFP, and sometimes offered in a socket to allow for an upgrade.
), as well as different "sleep" modes to conserve battery
power. It also contained support for an external cache
of 16 to 64 kB
. The extra functions and circuit implementation techniques caused this variant to have over 3 times as many transistor
s as the i386DX. The i386SL was first available at 20 MHz clock speed, with the 25 MHz model later added.
. By extending the 16/24-bit IBM PC/AT standard into a natively 32-bit computing environment, Compaq became the first third party to implement a major technical hardware advance on the PC platform. IBM was offered use of the 80386, but had manufacturing rights for the earlier 80286. IBM therefore chose to rely on that processor for a couple of more years. The early success of the Compaq 386 PC played an important role in legitimizing the PC "clone" industry, and in de-emphasizing IBM's role within it.
Prior to the 386, the difficulty of manufacturing microchips and the uncertainty of reliable supply made it desirable that any mass-market semiconductor be multi-sourced, that is, made by two or more manufacturers, the second and subsequent companies manufacturing under license from the originating company. The 386 was for a time only available from Intel, since Andy Grove
, Intel's CEO at the time, made the decision not to encourage other manufacturers to produce the processor as second source
s. This decision was ultimately crucial to Intel's success in the market. The 386 was the first significant microprocessor to be single-sourced
. Single-sourcing the 386 allowed Intel greater control over its development and substantially greater profits in later years.
AMD
introduced its compatible Am386
processor in March 1991 after overcoming legal obstacles, thus ending Intel's monopoly on 386-compatible processors. IBM also later manufactured 386 chips under license.
Intel originally intended for the 80386 to debut at 16 MHz. However, due to poor yields, it was instead introduced at 12 MHz.
Early in production, Intel discovered a bug that could cause a system to unexpectedly halt when running 32-bit software. Not all of the processors already manufactured were affected, so Intel tested its inventory. Processors that were found to be bug-free were marked with a double-sigma
(ΣΣ), and affected processors were marked "16 BIT S/W ONLY". These latter processors were sold as good parts, since at the time 32 bit capability was not relevant for most users. Such chips are now extremely rare.
The i387 math coprocessor was not ready in time for the introduction of the 80386, and so many of the early 80386 motherboards instead provided a socket and hardware logic to make use of an 80287. In this configuration the FPU would operate asynchronously to the CPU, usually with a clock rate of 10 MHz. The original Compaq Deskpro 386 is an example of such design. However, this was an annoyance to those who depended on floating point performance, as the performance of the 287 was nowhere near that of the 387.
Intel later offered a modified version of its 80486DX in 80386 packaging, branded as the Intel RapidCAD
. This provided an upgrade path for users with 80386-compatible hardware. The upgrade was a pair of chips that replaced both the 80386 and 80387. Since the 80486DX design contained an FPU, the chip that replaced the 80386 contained the floating point functionality, and the chip that replaced the 80387 served very little purpose. However, the latter chip was necessary in order to provide the FERR signal to the mainboard and appear to function as a normal floating point unit. The CAD branding referred to the ease of upgrading existing OEM designs from 386 to 486 CPUs with rapid turn-around in the CAD room.
Third parties offered a wide range of upgrades, for both SX and DX systems. The most popular ones were based on the Cyrix 486DLC/SLC core, which typically offered a substantial speed improvement due to its more efficient instruction pipeline and internal L1 SRAM cache. The cache was usually 1 kB, or sometimes 8 kB in the TI variant. Some of these upgrade chips (such as the 486DRx2/SRx2) were marketed by Cyrix themselves, but they were more commonly found in kits offered by upgrade specialists such as Kingston, Evergreen and Improve-It Technologies. Some of the fastest CPU upgrade modules featured the IBM SLC/DLC family (notable for its 16 kB L1 cache), or even the Intel 486 itself. Many 386 upgrade kits were advertised as being simple drop-in replacements, but often required complicated software to control the cache and/or clock doubling. Part of the problem was that on most 386 motherboards, the A20 line
was controlled entirely by the motherboard with the CPU being unaware, which caused problems on CPUs with internal caches.
Overall it was very difficult to configure upgrades to produce the results advertised on the packaging, and upgrades were often less than 100% stable and/or less than 100% compatible.
(FPU) designed as pin-compatible replacements for an Intel 80386 processor and 80387 FPU.
and TTL compatible inputs (only 386SXSA). Usable with i387SX or i387SL FPUs.
. Usable with i387SX or i387SL FPUs.
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...
introduced by Intel in 1985. The first versions had 275,000 transistors and were used as the central processing unit (CPU)
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
of many workstation
Workstation
A workstation is a high-end microcomputer designed for technical or scientific applications. Intended primarily to be used by one person at a time, they are commonly connected to a local area network and run multi-user operating systems...
s and high-end 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...
s of the time. As the original implementation of the 32-bit
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
extension of the 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...
architecture, the 80386 instruction set, programming model, and binary encodings are still the common denominator for all 32-bit x86 processors, this is termed x86, IA-32
IA-32
IA-32 , also known as x86-32, i386 or x86, is the CISC instruction-set architecture of Intel's most commercially successful microprocessors, and was first implemented in the Intel 80386 as a 32-bit extension of x86 architecture...
, or i386-architecture, depending on context.
The 80386 could correctly execute most code intended for earlier 16-bit x86 processors such as the 8088 and 80286 that were ubiquitous in early PC
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...
s. Following the same tradition, modern 64-bit x86 processors are able to run most programs written for older chips, all the way back to the original 16-bit 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. Over the years, successively newer implementations of the same architecture have become several hundreds of times faster than the original 80386 (and thousands of times faster than the 8086). A 33 MHz 80386 was reportedly measured to operate at about 11.4 MIPS.
The 80386 was launched in October 1985, but full-function chips were first delivered in the third quarter of 1986. Mainboards for 80386-based computer systems were cumbersome and expensive at first, but manufacturing was rationalized upon the 80386's mainstream adoption. The first 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...
to make use of the 80386 was designed and manufactured by Compaq
Compaq
Compaq Computer Corporation is a personal computer company founded in 1982. Once the largest supplier of personal computing systems in the world, Compaq existed as an independent corporation until 2002, when it was acquired for US$25 billion by Hewlett-Packard....
and marked the first time a fundamental component in the IBM PC compatible
IBM PC compatible
IBM PC compatible computers are those generally similar to the original IBM PC, XT, and AT. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to...
de facto-standard was updated by a company other than IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
.
In May 2006, Intel announced that 80386 production would stop at the end of September 2007. Although it had long been obsolete as a 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...
CPU, Intel and others had continued making the chip for embedded system
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...
s. Such systems using an 80386 or one of many derivatives are common in aerospace
Aerospace
Aerospace comprises the atmosphere of Earth and surrounding space. Typically the term is used to refer to the industry that researches, designs, manufactures, operates, and maintains vehicles moving through air and space...
technology, among others. Some mobile phones also used the 80386 processor, such as BlackBerry 950
BlackBerry 950
BlackBerry 950 was an early BlackBerry model by Canadian smartphone manufacturer Research in Motion. There were two editions, the Exchange Edition and the Internet Edition, both identical in hardware and differing only in the provided software: the Exchange Edition could connect to corporate e-mail...
and Nokia 9000 Communicator.
Architecture
Intel 8008
The Intel 8008 was an early byte-oriented microprocessor designed and manufactured by Intel and introduced in April 1972. It was an 8-bit CPU with an external 14-bit address bus that could address 16KB of memory...
. The predecessor of the 80386 was the Intel 80286
Intel 80286
The Intel 80286 , introduced on 1 February 1982, was a 16-bit x86 microprocessor with 134,000 transistors. Like its contemporary simpler cousin, the 80186, it could correctly execute most software written for the earlier Intel 8086 and 8088...
, a 16-bit
16-bit
-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor. Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816. The Intel 8088 was program-compatible with the Intel 8086, and was 16-bit in that its registers were 16...
processor with a segment
Memory segment
x86 memory segmentation refers to the implementation of memory segmentation on the x86 architecture. Memory is divided into portions that may be addressed by a single index register without changing a 16-bit segment selector. In real mode or V86 mode, a segment is always 64 kilobytes in size . In...
-based memory management and protection system. The 80386 added a 32-bit
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
architecture and a paging
Paging
In computer operating systems, paging is one of the memory-management schemes by which a computer can store and retrieve data from secondary storage for use in main memory. In the paging memory-management scheme, the operating system retrieves data from secondary storage in same-size blocks called...
translation unit, which made it much easier to implement operating systems that used virtual memory
Virtual memory
In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...
. It also had support for hardware
Computer hardware
Personal computer hardware are component devices which are typically installed into or peripheral to a computer case to create a personal computer upon which system software is installed including a firmware interface such as a BIOS and an operating system which supports application software that...
debugging.
The 80386 featured three operating modes: real mode, protected mode and virtual mode. The protected mode
Protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...
which debuted in the 286 was extended to allow the 386 to address up to 4 GB
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...
of memory. The all new virtual 8086 mode
Virtual 8086 mode
In the 80386 microprocessor and later, virtual 8086 mode allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is running a protected mode operating system.VM86 mode uses a segmentation scheme identical to that of real mode In...
(or VM86) made it possible to run one or more real mode
Real mode
Real mode, also called real address mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space and unlimited direct software access to all memory, I/O addresses and peripheral hardware...
programs in a protected environment, although some programs were not compatible.
The 32-bit flat memory model
Flat memory model
Flat memory model or linear memory model refers to a memory addressing paradigm in low-level software design such that the CPU can directly address all of the available memory locations without having to resort to any sort of memory segmentation or paging schemes.Memory management and...
of the 386 would arguably be the most important feature change for the x86 processor family until AMD released x86-64
X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
in 2003.
Chief architect in the development of the 80386 was John H. Crawford. He was responsible for extending the 80286 architecture and instruction set to 32-bit, and then led the microprogram development for the 80386 chip.
The 80486
Intel 80486
The Intel 80486 microprocessor 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...
and 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 line of processors were descendants of the 80386 design.
Datatypes of 80386
The 80386 supports the following 17 data types , each of which is discussed here in brief.- Bit.
- Bit field-A group of at most 32 bits i.e, 4 bytes.
- Bit string- A string of continuous bits of maximum 4Gbytes length.
- Signed Byte- Signed byte data. Sign of the operand depends upon its most significant bit. If it is 0, then the number is positive. else it is negative. Range is from -128 to 127.
- Unsigned Byte-Unsigned byte data.Range from 0 to 255.
- Integer word- Signed 16-bit data. Range from -32,768 to 32,767.
- Long Integer-32-bit signed data that is represented in 2's complement form. Range is from -2.147*109 to 2.1479.
- Unsigned Integer Word- Unsigned 16-bit data.Range from 0 to 65,535.
- Unsigned Long Integer-Unsigned 32 bit data. Range from 0 to 4,294,967,295.
- Signed quad word- A signed 64-bit or four word data.
- Unsigned Quad Word-An unsigned 64-bit data.
- Offset-A 16 or 32- bit displacement that references a memory location using any of the addressing modes.
- Pointer-16-bit selector and 16 or 32 bit offset.
- Character-An ASCII equavelent to any of the alphanumeric or control characters .
- Strings-These are the sequences of bytes , words or double words . A string may contain upto maximum 4 Gbytes.
- BCD- Decimal digits from 0-9 represented by unpacked bytes.
- Packed BCD- This represents two packed BCD digits using a byte from 00 to99.
The i386SX variant
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...
, but this was not a critical constraint at the time. Performance differences were due not only to differing databus-widths, but also to performance-enhancing cache memories often employed on boards using the original chip.
The original 80386 was subsequently renamed i386DX to avoid confusion. However, Intel subsequently used the 'DX' suffix to refer to the floating-point capability of the i486DX. The i387SX was an i387 part that was compatible with the i386SX (i.e. with a 16-bit databus). The 386SX was packaged in a surface-mount QFP, and sometimes offered in a socket to allow for an upgrade.
The i386SL variant
The i386SL was introduced as a power efficient version for laptop computers. The processor offered several power management options (e.g. SMMSystem Management Mode
System Management Mode is an operating mode in which all normal execution is suspended, and special separate software is executed in high-privilege mode. It was first released with the Intel 386SL...
), as well as different "sleep" modes to conserve battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
power. It also contained support for an external cache
Cache
In 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...
of 16 to 64 kB
Kilobyte
The 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...
. The extra functions and circuit implementation techniques caused this variant to have over 3 times as many transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s as the i386DX. The i386SL was first available at 20 MHz clock speed, with the 25 MHz model later added.
Business importance
The first company to design and manufacture a PC based on the Intel 80386 was CompaqCompaq
Compaq Computer Corporation is a personal computer company founded in 1982. Once the largest supplier of personal computing systems in the world, Compaq existed as an independent corporation until 2002, when it was acquired for US$25 billion by Hewlett-Packard....
. By extending the 16/24-bit IBM PC/AT standard into a natively 32-bit computing environment, Compaq became the first third party to implement a major technical hardware advance on the PC platform. IBM was offered use of the 80386, but had manufacturing rights for the earlier 80286. IBM therefore chose to rely on that processor for a couple of more years. The early success of the Compaq 386 PC played an important role in legitimizing the PC "clone" industry, and in de-emphasizing IBM's role within it.
Prior to the 386, the difficulty of manufacturing microchips and the uncertainty of reliable supply made it desirable that any mass-market semiconductor be multi-sourced, that is, made by two or more manufacturers, the second and subsequent companies manufacturing under license from the originating company. The 386 was for a time only available from Intel, since Andy Grove
Andrew Grove
Andrew Stephen Grove , is a Hungarian-born Jewish-American Businessman/ Engineer, Author & a science pioneer in the semiconductor industry. He escaped from Communist-controlled Hungary at the age of 20 and moved to the U.S., where he finished his education...
, Intel's CEO at the time, made the decision not to encourage other manufacturers to produce the processor as second source
Second source
In the electronics industry, a second source is a company that is licensed to manufacture and sell components originally designed by another company ....
s. This decision was ultimately crucial to Intel's success in the market. The 386 was the first significant microprocessor to be single-sourced
Single source publishing
Single source publishing, also known as single sourcing, allows the same content to be used in different documents or in various formats. The labour-intensive and expensive work of editing need only be carried out once, on one document. Further transformations are carried out mechanistically, by...
. Single-sourcing the 386 allowed Intel greater control over its development and substantially greater profits in later years.
AMD
Advanced Micro Devices
Advanced Micro Devices, Inc. or AMD is an American multinational semiconductor company based in Sunnyvale, California, that develops computer processors and related technologies for commercial and consumer markets...
introduced its compatible Am386
Am386
The Am386 CPU was a 100%-compatible clone of the Intel 80386 design released by AMD in 1991. It sold millions of units, positioning AMD as a legitimate competitor to Intel, rather than being merely a second source for x86 CPUs ....
processor in March 1991 after overcoming legal obstacles, thus ending Intel's monopoly on 386-compatible processors. IBM also later manufactured 386 chips under license.
Compatibles
- The AMD Am386Am386The Am386 CPU was a 100%-compatible clone of the Intel 80386 design released by AMD in 1991. It sold millions of units, positioning AMD as a legitimate competitor to Intel, rather than being merely a second source for x86 CPUs ....
SX and Am386DX were almost exact clones of the 80386SX and 80386DX. Legal disputes caused production delays for several years, but AMD's 40 MHz part eventually became very popular with computer enthusiasts as a low cost and low power alternative to the 25 MHz 486SX. The power draw was further reduced in the "notebook models" (Am386 DXL/SXL/DXLV/SXLV) which could operate with 3.3V and were implemented in fully static CMOSCMOSComplementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
circuitry.
- Chips and TechnologiesChips and TechnologiesChips and Technologies was the first fabless semiconductor company, a model developed by its founder Gordon Campbell. Founded by Dado Banatao.Its first product was an EGA IBM compatible graphics chip...
Super386 38600SX and 38600DX were developed using reverse engineeringReverse engineeringReverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation...
. They sold poorly, due to some technical errors and incompatibilities, as well as their late appearance on the market. They were therefore short-lived products.
- CyrixCyrixCyrix 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...
Cx486SLC/Cx486DLC could be (simplistically) described as a kind of 386/486 hybrid chip that included a small amount of on-chip cache. It was popular among computer enthusiasts but did poorly with OEMOriginal Equipment ManufacturerAn original equipment manufacturer, or OEM, manufactures products or components that are purchased by a company and retailed under that purchasing company's brand name. OEM refers to the company that originally manufactured the product. When referring to automotive parts, OEM designates a...
s. The Cyrix Cx486SLC and Cyrix Cx486DLC processors were pin-compatible with 80386SX and 80386DX respectively. These processors were also manufactured and sold by Texas InstrumentsTexas InstrumentsTexas Instruments Inc. , widely known as TI, is an American company based in Dallas, Texas, United States, which develops and commercializes semiconductor and computer technology...
.
- IBMIBMInternational Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
386SLC386SLCThe 386SLC was an Intel-licensed version of the 386SX , developed and manufactured by IBM in 1991. It included power-management capabilities and an 8KB internal cache, which enabled it to yield comparable performance to 386DX processors of the same clock speed, which were considerably more expensive...
and 486SLC/DLC were variants of Intel's design which contained a large amount of on-chip cache (8 kB, and later 16 kB). The agreement with Intel limited their use to IBM's own line of computers and upgrade boards only, so they were not available on the open market.
Early problems
Early in production, Intel discovered a bug that could cause a system to unexpectedly halt when running 32-bit software. Not all of the processors already manufactured were affected, so Intel tested its inventory. Processors that were found to be bug-free were marked with a double-sigma
Sigma
Sigma is the eighteenth letter of the Greek alphabet, and carries the 'S' sound. In the system of Greek numerals it has a value of 200. When used at the end of a word, and the word is not all upper case, the final form is used, e.g...
(ΣΣ), and affected processors were marked "16 BIT S/W ONLY". These latter processors were sold as good parts, since at the time 32 bit capability was not relevant for most users. Such chips are now extremely rare.
The i387 math coprocessor was not ready in time for the introduction of the 80386, and so many of the early 80386 motherboards instead provided a socket and hardware logic to make use of an 80287. In this configuration the FPU would operate asynchronously to the CPU, usually with a clock rate of 10 MHz. The original Compaq Deskpro 386 is an example of such design. However, this was an annoyance to those who depended on floating point performance, as the performance of the 287 was nowhere near that of the 387.
Pin-compatible upgrades
RapidCAD
RapidCAD 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...
. This provided an upgrade path for users with 80386-compatible hardware. The upgrade was a pair of chips that replaced both the 80386 and 80387. Since the 80486DX design contained an FPU, the chip that replaced the 80386 contained the floating point functionality, and the chip that replaced the 80387 served very little purpose. However, the latter chip was necessary in order to provide the FERR signal to the mainboard and appear to function as a normal floating point unit. The CAD branding referred to the ease of upgrading existing OEM designs from 386 to 486 CPUs with rapid turn-around in the CAD room.
Third parties offered a wide range of upgrades, for both SX and DX systems. The most popular ones were based on the Cyrix 486DLC/SLC core, which typically offered a substantial speed improvement due to its more efficient instruction pipeline and internal L1 SRAM cache. The cache was usually 1 kB, or sometimes 8 kB in the TI variant. Some of these upgrade chips (such as the 486DRx2/SRx2) were marketed by Cyrix themselves, but they were more commonly found in kits offered by upgrade specialists such as Kingston, Evergreen and Improve-It Technologies. Some of the fastest CPU upgrade modules featured the IBM SLC/DLC family (notable for its 16 kB L1 cache), or even the Intel 486 itself. Many 386 upgrade kits were advertised as being simple drop-in replacements, but often required complicated software to control the cache and/or clock doubling. Part of the problem was that on most 386 motherboards, the A20 line
A20 line
The A20 or addressing line 20 is one of the plethora of electrical lines that make up the system bus of an x86-based computer system. The A20 line in particular is used to transmit the 21st bit on the address bus....
was controlled entirely by the motherboard with the CPU being unaware, which caused problems on CPUs with internal caches.
Overall it was very difficult to configure upgrades to produce the results advertised on the packaging, and upgrades were often less than 100% stable and/or less than 100% compatible.
i386DX
Original version, released in October 1985.- Capable of working with 16- or 32-bit external busses
- Cache: depends on mainboard
- Package: PGAPin grid arrayA pin grid array, often abbreviated PGA, is a type of integrated circuit packaging. In a PGA, the package is square or roughly square, and the pins are arranged in a regular array on the underside of the package...
-132 or PQFP-132 - Process: First types CHMOS III, 1.5 µm, later CHMOS IV, 1 µm
- Die size: 104 mm² (ca. 10 mm x 10 mm) in CHMOS III and 39 mm² (6 mm x 6.5 mm) in CHMOS IV.
- Transistor count: 275 000
- Specified max clock: 12 MHz (early models), later 16, 20, 25 and 33 MHz
RapidCAD
A specially packaged Intel 486DX and a dummy floating point unitFloating point unit
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, and square root...
(FPU) designed as pin-compatible replacements for an Intel 80386 processor and 80387 FPU.
i376
This was an embedded version of the i386SX which did not support real mode and paging in the MMU.i386EX, i386EXTB and i386EXTC
System and power management and built in peripheral and support functions: Two 82C59A interrupt controllers; Timer, Counter (3 channels); Asynchronous SIO (2 channels); Synchronous SIO (1 channel); Watchdog timer (Hardware/Software); PIO. Usable with i387SX or i387SL FPUs.- Data/address bus: 16 / 26 bits
- Package: PQFPPQFPPQFP, or plastic quad flat pack, is a type of IC packaging. PQFP is a special case of QFP, as is the thinner TQFP package.PQFP packages can vary in thickness from 2.0 mm to 3.8 mm.-References:*...
-132, SQFP-144 and PGA-168 - Process: CHMOS V, 0.8 µm
- Specified max clock:
- i386EX: 16 MHz @2.7~3.3 volt or 20 MHz @3.0~3.6 volt or 25 MHz @4.5~5.5 volt
- i386EXTB: 20 MHz @2.7~3.6 volt or 25 MHz @3.0~3.6 volt
- i386EXTC: 25 MHz @4.5~5.5 volt or 33 MHz @4.5~5.5 volt
i386CXSA and i386SXSA (or i386SXTA)
Transparent power management mode, integrated MMUMemory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
and TTL compatible inputs (only 386SXSA). Usable with i387SX or i387SL FPUs.
- Data/address bus: 16 / 26 bits (24 bits for i386SXSA)
- Package: PQFPPQFPPQFP, or plastic quad flat pack, is a type of IC packaging. PQFP is a special case of QFP, as is the thinner TQFP package.PQFP packages can vary in thickness from 2.0 mm to 3.8 mm.-References:*...
-100 - Voltage: 4.5~5.5 volt (25 and 33 MHz); 4.75~5.25 volt (40 MHz)
- Process: CHMOS V, 0.8 µm
- Specified max clock: 25, 33, 40 MHz
i386CXSB
Transparent power management mode and integrated MMUMemory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
. Usable with i387SX or i387SL FPUs.
- Data/address bus: 16 / 26 bits
- Package: PQFPPQFPPQFP, or plastic quad flat pack, is a type of IC packaging. PQFP is a special case of QFP, as is the thinner TQFP package.PQFP packages can vary in thickness from 2.0 mm to 3.8 mm.-References:*...
-100 - Voltage: 3.0 volt (16 MHz) or 3.3 volt (25 MHz)
- Process: CHMOS V, 0.8 µm
- Specified max clock: 16, 25 MHz