Intel 8086

Intel 8086

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The 8086 is 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...

 microprocessor
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...

 chip designed by Intel between early 1976 and mid-1978, when it was released. The 8086 gave rise to the x86 architecture
X86 architecture
The term x86 refers to a family of instruction set architectures based on the Intel 8086 CPU. The 8086 was launched in 1978 as a fully 16-bit extension of Intel's 8-bit based 8080 microprocessor and also introduced segmentation to overcome the 16-bit addressing barrier of such designs...

 of Intel's future processors. The Intel 8088
Intel 8088
The Intel 8088 microprocessor was a variant of the Intel 8086 and was introduced on July 1, 1979. It had an 8-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers and the one megabyte address range were unchanged, however...

, released in 1979, was a slightly modified chip with an external 8-bit data bus (allowing the use of cheaper and fewer supporting logic chips), and is notable as the processor used in the original IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...

.

Background


In 1972, Intel launched the 8008
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 first 8-bit microprocessor. It implemented an instruction set
Instruction 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...

 designed by Datapoint
Datapoint
Datapoint Corporation, originally known as Computer Terminal Corporation , was a computer company based in San Antonio, Texas, United States. Founded in 1967 by Phil Ray and Gus Roche, its first products were, as the company's initial name suggests, computer terminals...

 corporation with programmable CRT terminals
Computer terminal
A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system...

 in mind, that also proved to be fairly general purpose. The device needed several additional IC
Integrated circuit
An 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...

s to produce a functional computer, in part due to its small 18-pin "memory-package", which ruled out the use of a separate address bus (Intel was primarily a DRAM
Dram
Dram or DRAM may refer to:As a unit of measure:* Dram , an imperial unit of mass and volume* Armenian dram, a monetary unit* Dirham, a unit of currency in several Arab nationsOther uses:...

 manufacturer at the time).

Two years later, in 1974, Intel launched the 8080
Intel 8080
The Intel 8080 was the second 8-bit microprocessor designed and manufactured by Intel and was released in April 1974. It was an extended and enhanced variant of the earlier 8008 design, although without binary compatibility...

, employing the new 40-pin DIL package
Dual in-line package
In microelectronics, a dual in-line package is an electronic device package with a rectangular housing and two parallel rows of electrical connecting pins. The package may be through-hole mounted to a printed circuit board or inserted in a socket.A DIP is usually referred to as a DIPn, where n is...

s originally developed for calculator
Calculator
An electronic calculator is a small, portable, usually inexpensive electronic device used to perform the basic operations of arithmetic. Modern calculators are more portable than most computers, though most PDAs are comparable in size to handheld calculators.The first solid-state electronic...

 ICs to enable a separate address bus. It had an extended instruction set that was source- (not binary-) compatible with the 8008 and also included some 16-bit instructions to make programming easier. The 8080 device, often described as the first truly useful microprocessor, was eventually replaced by the depletion-load based 8085
Intel 8085
The Intel 8085 is an 8-bit microprocessor introduced by Intel in 1977. It was binary-compatible with the more-famous Intel 8080 but required less supporting hardware, thus allowing simpler and less expensive microcomputer systems to be built....

 (1977) which could cope with a single 5V power supply instead of the three different operating voltages of earlier chips. Other well known 8-bit microprocessors that emerged during these years were Motorola 6800
Motorola 6800
The 6800 was an 8-bit microprocessor designed and first manufactured by Motorola in 1974. The MC6800 microprocessor was part of the M6800 Microcomputer System that also included serial and parallel interface ICs, RAM, ROM and other support chips...

 (1974), General Instrument PIC16X
PIC microcontroller
PIC is a family of Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1650 originally developed by General Instrument's Microelectronics Division...

 (1975), MOS Technology 6502
MOS Technology 6502
The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...

 (1975), Zilog Z80
Zilog Z80
The Zilog Z80 is an 8-bit microprocessor designed by Zilog and sold from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes...

 (1976), and Motorola 6809
Motorola 6809
The Motorola 6809 is an 8-bit microprocessor CPU from Motorola, designed by Terry Ritter and Joel Boney and introduced 1978...

 (1978).

The first x86 design


The 8086 project started in May 1976 and was originally intended as a temporary substitute for the ambitious and delayed iAPX 432 project. It was an attempt to draw attention from the less-delayed 16 and 32-bit processors of other manufacturers (such as Motorola
Motorola
Motorola, 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...

, Zilog
Zilog
Zilog, Inc., previously known as ZiLOG , is a manufacturer of 8-bit and 24-bit microcontrollers, and is most famous for its Intel 8080-compatible Z80 series.-History:...

, and National Semiconductor
National Semiconductor
National Semiconductor was an American semiconductor manufacturer, that specialized in analog devices and subsystems,formerly headquartered in Santa Clara, California, USA. The products of National Semiconductor included power management circuits, display drivers, audio and operational amplifiers,...

) and at the same time to counter the threat from the Zilog Z80
Zilog Z80
The Zilog Z80 is an 8-bit microprocessor designed by Zilog and sold from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes...

 (designed by former Intel employees), which became very successful. Both the architecture and the physical chip were therefore developed rather quickly by a small group of people, and using the same basic microarchitecture
Microarchitecture
In computer engineering, microarchitecture , also called computer organization, is the way a given instruction set architecture is implemented on a processor. A given ISA may be implemented with different microarchitectures. Implementations might vary due to different goals of a given design or...

 elements and physical implementation techniques as employed for the slightly older 8085
Intel 8085
The Intel 8085 is an 8-bit microprocessor introduced by Intel in 1977. It was binary-compatible with the more-famous Intel 8080 but required less supporting hardware, thus allowing simpler and less expensive microcomputer systems to be built....

 (and for which the 8086 also would function as a continuation).

Marketed as source compatible, the 8086 was designed so that assembly language
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...

 for the 8008, 8080, or 8085 could be automatically converted into equivalent (sub-optimal) 8086 source code, with little or no hand-editing. The programming model and instruction set was (loosely) based on the 8080 in order to make this possible. However, the 8086 design was expanded to support full 16-bit processing, instead of the fairly basic 16-bit capabilities of the 8080/8085.

New kinds of instructions were added as well; full support for signed integers, base+offset addressing, and self-repeating operations were akin to the Z80 design but were all made slightly more general in the 8086. Instructions directly supporting nested
Nested function
In computer programming, a nested function is a function which is lexically encapsulated within another function. It can only be called by the enclosing function or by functions directly or indirectly nested within the same enclosing function. In other words, the scope of the nested function is...

 ALGOL
ALGOL
ALGOL is a family of imperative computer programming languages originally developed in the mid 1950s which greatly influenced many other languages and became the de facto way algorithms were described in textbooks and academic works for almost the next 30 years...

-family languages such as Pascal
Pascal (programming language)
Pascal is an influential imperative and procedural programming language, designed in 1968/9 and published in 1970 by Niklaus Wirth as a small and efficient language intended to encourage good programming practices using structured programming and data structuring.A derivative known as Object Pascal...

 and PL/M
PL/M
The PL/M programming languageis a high-level language developed byGary Kildall in 1972 for Intel for its microprocessors....

 were also added. According to principal architect Stephen P. Morse, this was a result of a more software centric approach than in the design of earlier Intel processors (the designers had experience working with compiler implementations). Other enhancements included microcode
Microcode
Microcode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed...

d multiply and divide instructions and a bus-structure better adapted to future co-processors (such as 8087
Intel 8087
The Intel 8087, announced in 1980, was the first floating-point coprocessor for the 8086 line of microprocessors. It had 45,000 transistors and was manufactured as a 3 μm depletion load HMOS circuit. The 8087 was built to be paired with the Intel 8088 or 8086 microprocessors...

 and 8089
Intel 8089
The Intel 8089 input/output coprocessor was available for use with the 8086/8088 central processor. It used the same programming technique as 8087 for input/output operations, such as transfer of data from memory to a peripheral device, and so reducing the load on the CPU.Because IBM didn't use it...

) and multiprocessor systems.

The first revision of the instruction set and high level architecture was ready after about three months, and as almost no CAD-tools were used, four engineers and 12 layout people were simultaneously working on the chip. The 8086 took a little more than two years from idea to working product, which was considered rather fast for a complex design in 1976–1978.

The 8086 was sequenced using a mixture of random logic and microcode
Microcode
Microcode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed...

 and was implemented using depletion-load nMOS circuitry with approximately 20,000 active 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 (29,000 counting all ROM and PLA
Programmable logic array
A programmable logic array is a kind of programmable logic device used to implement combinational logic circuits. The PLA has a set of programmable AND gate planes, which link to a set of programmable OR gate planes, which can then be conditionally complemented to produce an output...

 sites). It was soon moved to a new refined nMOS manufacturing process called HMOS
HMOS
Depletion-load nMOS/NMOS is a form of nMOS logic family which uses depletion-mode n-type MOSFETs as load transistors as a method to enable single voltage operation and achieve greater speed than possible with pure enhancement-load devices...

 (for High performance MOS) that Intel originally developed for manufacturing of fast static RAM products. This was followed by HMOS-II, HMOS-III versions, and, eventually, a fully static CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...

 version for battery-powered devices, manufactured using Intel's CHMOS processes. The original chip measured 33 mm² and minimum feature size was 3.2 μm.

The architecture was defined by Stephen P. Morse with some help and assistance by Bruce Ravenel (the architect of the 8087) in refining the final revisions. Logic designer Jim McKevitt and John Bayliss were the lead engineers of the hardware-level development team and William Pohlman the manager for the project. The legacy of the 8086 is enduring in the basic instruction set of today's personal computers and servers; the 8086 also lent its last two digits to later extended versions of the design, such as the Intel 286 and the Intel 386, all of which eventually became known as the x86 family. (Another reference is that the PCI Vendor ID
PCI Configuration Space
PCI configuration space is the underlying way that the Conventional PCI, PCI-X and PCI Express perform auto configuration of the cards inserted into their bus.- Technical information :...

 for Intel devices is 8086h.)

Details


EWLINE
Main registers
AH AL AX (primary accumulator)
BH BL BX (base, accumulator)
CH CL CX (counter, accumulator)
DH DL DX (accumulator, other functions)
Index registers
SI Source Index
DI Destination Index
BP Base Pointer
SP Stack Pointer
Status register
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 (bit position)
- - - - O
Overflow flag
In computer processors, the overflow flag is usually a single bit in a system status register used to indicate when an arithmetic overflow has occurred in an operation, indicating that the signed two's-complement result would not fit in the number of bits used for the operation...

D
Direction flag
The Direction Flag is a flag that controls the left-to-right or right-to-left direction of string processing stored in the FLAGS register on all x86 compatible CPUs. It is bit 10....

I
IF (x86 flag)
IF is a system flag bit in the x86 architecture's FLAGS register, which determines whether or not the CPU will handle maskable hardware interrupts....

T
Trap flag
A trap flag permits operation of a processor in single-step mode. If such a flag is available, debuggers can use it to step through the execution of a computer program.-Single-step interrupt:...

S Z
Zero flag
The Zero Flag is a single bit flag that is a central feature on most conventional CPU architectures . It is often stored in a dedicated register, typically called status register or flag register, along with other flags...

- A
Adjust flag
The Adjust flag is a flag stored in the FLAGS register on all x86 compatible CPUs. It is bit 4. It is used to indicate when an arithmetic carry or borrow has been generated out of the 4 least significant bits. It is primarily used in BCD arithmetics....

- P
Parity flag
In computer processors the parity flag indicates if the number of set bits is odd or even in the binary representation of the result of the last operation. It is normally a single bit in a processor status register....

- C
Carry flag
In computer processors the carry flag is a single bit in a system status register used to indicate when an arithmetic carry or borrow has been generated out of the most significant ALU bit position...

Flags
Segment register
CS Code Segment
DS Data Segment
ES ExtraSegment
SS Stack Segment
Instruction pointer
IP Instruction Pointer
The 8086 registers

Buses and operation


All internal registers as well as internal and external data buses were 16 bits wide, firmly establishing the "16-bit microprocessor" identity of the 8086. A 20-bit external address bus gave a 1 MB
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...

 physical address space (220 = 1,048,576). This address space was addressed by means of internal 'segmentation'. The data bus was multiplexed with the address bus in order to fit a standard 40-pin dual in-line package
Dual in-line package
In microelectronics, a dual in-line package is an electronic device package with a rectangular housing and two parallel rows of electrical connecting pins. The package may be through-hole mounted to a printed circuit board or inserted in a socket.A DIP is usually referred to as a DIPn, where n is...

. 16-bit I/O addresses meant 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...

 of separate I/O space (216 = 65,536). The maximum linear address space was limited to 64 KB, simply because internal registers were only 16 bits wide. Programming over 64 KB boundaries involved adjusting segment registers (see below) and was therefore fairly awkward (and remained so until the 80386).

Some of the control pins, which carry essential signals for all external operations, had more than one function depending upon whether the device was operated in min or max mode. The former was intended for small single processor systems whilst the latter was for medium or large systems, using more than one processor.

Registers and instructions


The 8086 had eight (more or less general) 16-bit registers
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...

 including the stack pointer
Stack-based memory allocation
Stacks in computing architectures are regions of memory where data is added or removed in a last-in-first-out manner.In most modern computer systems, each thread has a reserved region of memory referred to as its stack. When a function executes, it may add some of its state data to the top of the...

, but excluding the instruction pointer, flag register and segment registers. Four of them, AX, BX, CX, DX, could also be accessed as twice as many 8-bit registers (see figure) while the other four, BP, SI, DI, SP, were 16-bit only.

Due to a compact encoding inspired by 8-bit processors, most instructions were one-address or two-address operations which means that the result were stored in one of the operands. At most one of the operands could be in memory, but this memory operand could also be the destination, while the other operand, the source, could be either register or immediate. A single memory location could also often be used as both source and destination which, among other factors, further contributed to a code density comparable to (and often better than) most eight bit machines.

Although the degree of generality of most registers were much greater than in the 8080 or 8085, it was still fairly low compared to the typical contemporary minicomputer
Minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...

, and registers were also sometimes used implicitly by instructions. While perfectly sensible for the assembly programmer, this complicated register allocation for compilers compared to more regular 16- and 32-bit processors such as the PDP-11
PDP-11
The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corporation from 1970 into the 1990s, one of a succession of products in the PDP series. The PDP-11 replaced the PDP-8 in many real-time applications, although both product lines lived in parallel for more than 10 years...

, VAX, 68000, etc.; on the other hand, compared to semi-contemporary simple (but popular and ubiquitous) 8-bit microprocessors such as the 6502
MOS Technology 6502
The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...

, 6809
Motorola 6809
The Motorola 6809 is an 8-bit microprocessor CPU from Motorola, designed by Terry Ritter and Joel Boney and introduced 1978...

, or 8085
Intel 8085
The Intel 8085 is an 8-bit microprocessor introduced by Intel in 1977. It was binary-compatible with the more-famous Intel 8080 but required less supporting hardware, thus allowing simpler and less expensive microcomputer systems to be built....

, it was significantly easier to generate code for the 8086 design.

The 8086 also featured 64 KB of 8-bit (or alternatively 32 K-word of 16-bit) I/O
I/O
I/O may refer to:* Input/output, a system of communication for information processing systems* Input-output model, an economic model of flow prediction between sectors...

 space. A 64 KB (one segment) stack
Stack (data structure)
In computer science, a stack is a last in, first out abstract data type and linear data structure. A stack can have any abstract data type as an element, but is characterized by only three fundamental operations: push, pop and stack top. The push operation adds a new item to the top of the stack,...

 growing towards lower addresses is supported by hardware
Hardware
Hardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....

; 2-byte words are pushed to the stack and the stack top (bottom) is pointed out by SS:SP. There are 256 interrupt
Interrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....

s, which can be invoked by both hardware and software. The interrupts can cascade, using the stack to store the return address
Return address
In postal mail, a return address is an explicit inclusion of the address of the person sending the message. It provides the recipient with a means to determine how to respond to the sender of the message if needed....

es.

The processor had some new instructions (not present in the 8080 and 8085) to better support stack based high level programming languages such as Pascal and PL/M
PL/M
The PL/M programming languageis a high-level language developed byGary Kildall in 1972 for Intel for its microprocessors....

; some of the more useful ones were push mem-op, and ret size, supporting the "pascal calling convention
Calling convention
In computer science, a calling convention is a scheme for how subroutines receive parameters from their caller and how they return a result; calling conventions can differ in:...

" directly. (Several others, such as push immed and enter, would be added in the subsequent 80186, 80286, and 80386 designs.) It also had a stack-marker mechanism. There are three control flags IF(Intrrupt Flag)TF(Trap Flag)DF(Direction flag).

Flags


8086 has a 16-bit flag register. Out of these, 9 are active, and indicate the current state of the processor. These are — Carry flag
Carry flag
In computer processors the carry flag is a single bit in a system status register used to indicate when an arithmetic carry or borrow has been generated out of the most significant ALU bit position...

, Parity flag
Parity flag
In computer processors the parity flag indicates if the number of set bits is odd or even in the binary representation of the result of the last operation. It is normally a single bit in a processor status register....

, Auxiliary flag, Zero flag
Zero flag
The Zero Flag is a single bit flag that is a central feature on most conventional CPU architectures . It is often stored in a dedicated register, typically called status register or flag register, along with other flags...

, Sign flag, Trap flag
Trap flag
A trap flag permits operation of a processor in single-step mode. If such a flag is available, debuggers can use it to step through the execution of a computer program.-Single-step interrupt:...

, Interrupt flag
IF (x86 flag)
IF is a system flag bit in the x86 architecture's FLAGS register, which determines whether or not the CPU will handle maskable hardware interrupts....

, Direction flag
Direction flag
The Direction Flag is a flag that controls the left-to-right or right-to-left direction of string processing stored in the FLAGS register on all x86 compatible CPUs. It is bit 10....

 and Overflow flag
Overflow flag
In computer processors, the overflow flag is usually a single bit in a system status register used to indicate when an arithmetic overflow has occurred in an operation, indicating that the signed two's-complement result would not fit in the number of bits used for the operation...

.

Segmentation


There were also four 16-bit segment registers (see figure) that allowed the 8086 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...

 to access one megabyte
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...

 of memory in an unusual way. Rather than concatenating the segment register with the address register, as in most processors whose address space exceeded their register size, the 8086 shifted the 16-bit segment only four bits left before adding it to the 16-bit offset (16×segment + offset), therefore producing a 20-bit external (or effective or physical) address from the 32-bit segment:offset pair. As a result, each external address could be referred to by 212 = 4096 different segment:offset pairs. The 16-byte separation between segment bases (due to the 4-bit shift) was called a paragraph. Although considered complicated and cumbersome by many programmers, this scheme also had advantages; a small program (less than 64 KB) could be loaded starting at a fixed offset (such as 0) in its own segment, avoiding the need for relocation
Relocation (computer science)
"Relocation is the process of assigning load addresses to various parts of [a] program and adjusting the code and data in the program to reflect the assigned addresses."...

, with at most 15 bytes of alignment waste.

Compilers for the 8086-family commonly supported two types of pointer, near and far. Near pointers were 16-bit offsets implicitly associated with the program's code and/or data segment and so could be used only within parts of a program small enough to fit in one segment. Far pointers were 32-bit segment:offset pairs resolving to 20-bit external addresses. Some compilers also supported huge pointers, which were like far pointers except that pointer arithmetic on a huge pointer treated it as a linear 20-bit pointer, while pointer arithmetic on a far pointer wrapped around
Integer overflow
In computer programming, an integer overflow occurs when an arithmetic operation attempts to create a numeric value that is too large to be represented within the available storage space. For instance, adding 1 to the largest value that can be represented constitutes an integer overflow...

 within its 16-bit offset without touching the segment part of the address.

To avoid the need to specify near and far on numerous pointers, data structures, and functions, compilers also supported "memory models" which specified default pointer sizes. The tiny (max 64K), small (max 128K), compact (data > 64K), medium (code > 64K), large (code,data > 64K), and huge (individual arrays > 64K) models covered practical combinations of near, far, and huge pointers for code and data. The tiny model meant that code and data was shared in a single segment, just as in most 8-bit based processors, and could be used to build .com
COM file
In many computer operating systems, a COM file is a type of executable file; the name is derived from the file name extension .COM. Originally, the term stood for "Command file", a text file containing commands to be issued to the operating system , on many of the Digital Equipment Corporation mini...

-files for instance. Precompiled libraries often came in several versions compiled for different memory models.

According to Morse et al., the designers actually contemplated using an 8-bit shift (instead of 4-bit), in order to create a 16 MB physical address space. However, as this would have forced segments to begin on 256-byte boundaries, and 1 MB was considered very large for a microprocessor around 1976, the idea was dismissed. Also, there were not enough pins available on a low-cost 40-pin package.

In principle, the address space of the x86 series could have been extended in later processors by increasing the shift value, as long as applications obtained their segments from the operating system and did not make assumptions about the equivalence of different segment:offset pairs. In practice the use of "huge" pointers and similar mechanisms was widespread and the flat 32-bit addressing made possible with the 32-bit offset registers in the 80386 eventually extended the limited addressing range in a more general way (see below).

Porting older software


Small programs could ignore the segmentation and just use plain 16-bit addressing. This allowed 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...

 software to be quite easily ported to the 8086. The authors of MS-DOS took advantage of this by providing an Application Programming Interface
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...

 very similar to CP/M
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...

 as well as including the simple .com executable file format, identical to CP/M. This was important when the 8086 and MS-DOS was new, because it allowed many existing CP/M (and other) applications to be quickly made available, greatly easing acceptance of the new platform.

Performance


Although partly shadowed by other design choices in this particular chip, the multiplexed bus limited performance slightly; transfers of 16-bit or 8-bit quantities were done in a four-clock memory access cycle (which was faster on 16-bit, although slower on 8-bit quantities, compared to typical contemporary "8-bit" CPUs). As instructions varied from one to six bytes, fetch and execution were made concurrent
Concurrency (computer science)
In computer science, concurrency is a property of systems in which several computations are executing simultaneously, and potentially interacting with each other...

 (as it remains in today's x86 processors): The bus interface unit fed the instruction stream to the execution unit through a 6-byte prefetch queue (a form of loosely coupled pipelining), speeding up operations on register
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...

s and immediate
Operand
In mathematics, an operand is the object of a mathematical operation, a quantity on which an operation is performed.-Example :The following arithmetic expression shows an example of operators and operands:3 + 6 = 9\;...

s, while memory operations unfortunately became slower; four years later, this performance problem was fixed with the 80186 and 80286). However, the full (instead of partial) 16-bit architecture with a full width ALU
Arithmetic logic unit
In computing, an arithmetic logic unit is a digital circuit that performs arithmetic and logical operations.The ALU is a fundamental building block of the central processing unit of a computer, and even the simplest microprocessors contain one for purposes such as maintaining timers...

 meant that 16-bit arithmetic instructions could now be performed with a single ALU cycle (instead of two, via carry), speeding up such instructions considerably. Combined with orthogonalizations of operations versus operand
Operand
In mathematics, an operand is the object of a mathematical operation, a quantity on which an operation is performed.-Example :The following arithmetic expression shows an example of operators and operands:3 + 6 = 9\;...

-types and addressing mode
Addressing mode
Addressing modes are an aspect of the instruction set architecture in most central processing unit designs. The various addressing modes that are defined in a given instruction set architecture define how machine language instructions in that architecture identify the operand of each instruction...

s, as well as other enhancements, this made the performance gain over the 8080 or 8085 fairly significant, despite cases where the older chips may be faster (see below).
Execution times for typical instructions
(in clock cycles)
instruction register-register register immediate register-memory memory-register memory-immediate
mov 2 4 8+EA 9+EA 10+EA
ALU 3 4 9+EA, 16+EA, 17+EA
jump register => 11 ; label => 15 ; condition,label => 16
integer multiply 70~160 (depending on operand data as well as size) plus EA
signed integer divide 80~190 (depending on operand data as well as size) plus EA
  • EA = time to compute effective address, ranging from 5 to 12 cycles.
  • Timings are best case, depending on prefetch status, instruction alignment, and other factors.

As can be seen from these tables, operations on registers and immediates were fast (between 2 and 4 cycles), while memory-operand instructions and jumps were quite slow; jumps took more cycles than on the simple 8080
Intel 8080
The Intel 8080 was the second 8-bit microprocessor designed and manufactured by Intel and was released in April 1974. It was an extended and enhanced variant of the earlier 8008 design, although without binary compatibility...

 and 8085
Intel 8085
The Intel 8085 is an 8-bit microprocessor introduced by Intel in 1977. It was binary-compatible with the more-famous Intel 8080 but required less supporting hardware, thus allowing simpler and less expensive microcomputer systems to be built....

, and the 8088 (used in the IBM PC) was additionally hampered by its narrower bus. The reasons why most memory related instructions were slow were threefold:
  • Loosely-coupled fetch and execution units are efficient for instruction prefetch, but not for jumps and random data access (without special measures).
  • No dedicated address calculation adder was afforded; the microcode routines had to use the main ALU for this (although there was a dedicated segment + offset adder).
  • The address and data buses were multiplex
    Multiplexing
    The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...

    ed, forcing a slightly longer (33~50%) bus cycle than in typical contemporary 8-bit processors.


However, memory access performance was drastically enhanced with Intel's next generation chips. The 80186
Intel 80186
The 80188 is a version with an 8-bit external data bus, instead of 16-bit. This makes it less expensive to connect to peripherals. The 80188 is otherwise very similar to the 80186. It has a throughput of 1 million instructions per second....

 and 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...

 both had dedicated address calculation hardware, saving many cycles, and the 80286 also had separate (non-multiplexed) address and data buses.

Floating point


The 8086/8088 could be connected to a mathematical coprocessor to add hardware/microcode-based 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...

 performance. The Intel 8087
Intel 8087
The Intel 8087, announced in 1980, was the first floating-point coprocessor for the 8086 line of microprocessors. It had 45,000 transistors and was manufactured as a 3 μm depletion load HMOS circuit. The 8087 was built to be paired with the Intel 8088 or 8086 microprocessors...

 was the standard math coprocessor for the 8086 and 8088, operating on 80-bit numbers. Manufacturers like 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...

 (8087-compatible) and Weitek
Weitek
Weitek Corporation was a chip-design company that originally concentrated on floating point units for a number of commercial CPU designs. During the early to mid-1980s, Weitek designs could be found powering a number of high-end designs and parallel processing supercomputers...

 (non 8087-compatible) eventually came up with high performance floating point co-processors that competed with the 8087 as well as with the subsequent, higher performing Intel 80387.

Chip versions


The clock frequency was originally limited to 5 MHz (IBM PC used 4.77 MHz, 4/3 the standard NTSC color burst frequency), but the last versions in HMOS
HMOS
Depletion-load nMOS/NMOS is a form of nMOS logic family which uses depletion-mode n-type MOSFETs as load transistors as a method to enable single voltage operation and achieve greater speed than possible with pure enhancement-load devices...

 were specified for 10 MHz. HMOS-III and CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...

 versions were manufactured for a long time (at least a while into the 1990s) 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, although its successor, the 80186
Intel 80186
The 80188 is a version with an 8-bit external data bus, instead of 16-bit. This makes it less expensive to connect to peripherals. The 80188 is otherwise very similar to the 80186. It has a throughput of 1 million instructions per second....

/80188 (which includes some on-chip peripherals), has been more popular for embedded use.

Derivatives and clones



Compatible—and, in many cases, enhanced—versions were manufactured by Fujitsu
Fujitsu
is a Japanese multinational information technology equipment and services company headquartered in Tokyo, Japan. It is the world's third-largest IT services provider measured by revenues....

, Harris
Harris Corporation
Harris Corporation is a Florida-based international communications equipment company that produces wireless equipment, electronic systems, and both terrestrial and spaceborne antennas for use in the government, defense, and commercial sectors. It is also the largest private-sector employer in...

/Intersil
Intersil
Intersil Corporation is an American company that specializes in the design, development and manufacturing of high-performance analog semiconductors for four high-growth markets — Communications, Computing, High End Consumer and Industrial.-Company history:...

, OKI
Oki Electric Industry
, commonly referred to as OKI, OKI Electric or the OKI Group, is a Japanese company manufacturing and selling info-telecom and printer products. Headquartered in Tokyo, Japan, OKI operates in over 120 countries around the world....

, Siemens AG
Siemens AG
Siemens AG is a German multinational conglomerate company headquartered in Munich, Germany. It is the largest Europe-based electronics and electrical engineering company....

, Texas Instruments
Texas 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...

, NEC
NEC
, a Japanese multinational IT company, has its headquarters in Minato, Tokyo, Japan. NEC, part of the Sumitomo Group, provides information technology and network solutions to business enterprises, communications services providers and government....

, Mitsubishi
Mitsubishi
The Mitsubishi Group , Mitsubishi Group of Companies, or Mitsubishi Companies is a Japanese multinational conglomerate company that consists of a range of autonomous businesses which share the Mitsubishi brand, trademark and legacy...

, and AMD. For example, the NEC V20
NEC V20
The NEC V20 was a processor made by NEC that was a reverse-engineered, pin-compatible version of the Intel 8088 with an instruction set compatible with the Intel 80186...

 and NEC V30 pair were hardware compatible with the 8088 and 8086, respectively, but incorporated the instruction set of the 80186 along with some (but not all) of the 80186 speed enhancements, providing a drop-in capability to upgrade both instruction set and processing speed without manufacturers having to modify their designs. Such relatively simple and low-power 8086-compatible processors in CMOS are still used in embedded systems.

The electronics industry of the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 was able to replicate the 8086 through both industrial espionage
Industrial espionage
Industrial espionage, economic espionage or corporate espionage is a form of espionage conducted for commercial purposes instead of purely national security purposes...

 and reverse engineering. The resulting chip, K1810BM86
K1810BM86
The K1810BM86 was a Soviet-made clone of the Intel 8086 microprocessor....

, was binary and pin-compatible with the 8086, but was not mechanically compatible because it used metric measurements.

The 8088 and 8086 were the respective cores of the Soviet-made PC-compatible ES1840 and ES1841 desktops. However, these computers had significant hardware differences from their authentic prototypes, and the data/address bus circuitry was designed independently of Intel products. ES1841 was the first PC compatible computer with dynamic bus sizing (US Pat. No 4,831,514). Later some of the ES1841 principles were adopted in PS/2 (US Pat. No 5,548,786) and some other machines (UK Patent Application, Publication No. GB-A-2211325, Published June. 28, 1989).

Microcomputers using the 8086

  • One of the most influential microcomputers of all, the IBM PC
    IBM PC
    The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...

    , used the Intel 8088
    Intel 8088
    The Intel 8088 microprocessor was a variant of the Intel 8086 and was introduced on July 1, 1979. It had an 8-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers and the one megabyte address range were unchanged, however...

    , a version of the 8086 with an eight-bit data bus (as mentioned above).
  • The first commercial microcomputer built on the basis of the 8086 was the Mycron
    Mycron
    Mycron was a pioneer manufacturer of microcomputers, located in Oslo, Norway. Originally named Norsk Data Industri, the company was founded in 1975 by Lars Monrad Krohn, who was also one of the founding fathers of Norsk Data....

     2000.
  • The IBM Displaywriter word processing machine and the Wang Professional Computer, manufactured by Wang Laboratories
    Wang Laboratories
    Wang Laboratories was a computer company founded in 1951 by Dr. An Wang and Dr. G. Y. Chu. The company was successively headquartered in Cambridge , Tewksbury , and finally in Lowell, Massachusetts . At its peak in the 1980s, Wang Laboratories had annual revenues of $3 billion and employed over...

    , also used the 8086. Also, this chip could be found in the AT&T 6300 PC
    Olivetti M24
    The Olivetti M24 was a computer sold by Olivetti in 1983 using the Intel 8086 CPU.It was also sold as AT&T 6300.Contrary to other PC clones of that era, the M24 was highly compatible with IBM PC. One of its characteristics was a more powerful CPU than the Intel 8088 used in IBM's own PC...

     (built by Olivetti
    Olivetti
    Olivetti S.p.A. is an Italian manufacturer of computers, printers and other business machines.- Founding :The company was founded as a typewriter manufacturer in 1908 in Ivrea, near Turin, by Camillo Olivetti. The firm was mainly developed by his son Adriano Olivetti...

    ).
  • The first Compaq Deskpro
    Compaq Deskpro
    The Compaq Deskpro was a line of business-oriented personal computers manufactured by Compaq, discontinued after the merger with Hewlett-Packard...

     used an 8086 running at 7.14 MHz, but was capable of running add-in cards designed for the 4.77 MHz IBM PC XT.
  • The FLT86 is a well established training system for the 8086 CPU still being manufactured by Flite Electronics International Limited in Southampton, England.
  • The IBM PS/2
    IBM Personal System/2
    The Personal System/2 or PS/2 was IBM's third generation of personal computers. The PS/2 line, released to the public in 1987, was created by IBM in an attempt to recapture control of the PC market by introducing an advanced proprietary architecture...

     models 25 and 30 were built with an 8 MHz 8086.
  • The Tandy 1000
    Tandy 1000
    The Tandy 1000 was the first in a line of more-or-less IBM PC compatible home computer systems produced by the Tandy Corporation for sale in its Radio Shack chain of stores.-Overview:...

     SL-series machines used 8086 CPUs.
  • The Amstrad PC1512
    PC-1512
    The Amstrad PC1512 was Amstrad's mostly IBM PC-compatible computer system, first manufactured in 1986. It was later succeeded by the PC1640.It launched for £499 and sold very well, as it was one of the first cheap PCs in Europe...

    , PC1640, PC2086, PC3086 and PC5086 all used 8086 CPUs at 8 MHz.
  • NASA
    NASA
    The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

     used original 8086 CPUs on equipment for ground-based maintenance of the Space Shuttle Discovery
    Space Shuttle Discovery
    Space Shuttle Discovery is one of the retired orbiters of the Space Shuttle program of NASA, the space agency of the United States, and was operational from its maiden flight, STS-41-D on August 30, 1984, until its final landing during STS-133 on March 9, 2011...

     until the end of the space shuttle program in 2011. This decision was made to prevent software regression
    Software regression
    A software regression is a software bug which makes a feature stop functioning as intended after a certain event...

    that might result from upgrading or from switching to imperfect clones.

External links