8086 Architecture

[Liisa Komara]

The8086 microprocessor is an 8-bit/16-bit microprocessor designed by Intel in the late 1970s. It is the first member of the x86 family of microprocessors, which includes many popular CPUs used in personal computers.

The architecture of the 8086 microprocessor is based on a complex instruction set computer (CISC) architecture, which means that it supports a wide range of instructions, many of which can perform multiple operations in a single instruction. The 8086 microprocessor has a 20-bit address bus, which can address up to 1 MB of memory, and a 16-bit data bus, which can transfer data between the microprocessor and memory or I/O devices.

The 8086 microprocessor has a segmented memory architecture, which means that memory is divided into segments that are addressed using both a segment register and an offset. The segment register points to the start of a segment, while the offset specifies the location of a specific byte within the segment. This allows the 8086 microprocessor to access large amounts of memory, while still using a 16-bit data bus.

The 8086 microprocessor has two main execution units: the execution unit (EU) and the bus interface unit (BIU). The BIU is responsible for fetching instructions from memory and decoding them, while the EU executes the instructions. The BIU also manages data transfer between the microprocessor and memory or I/O devices.

The 8086 microprocessor has a rich set of registers, including general-purpose registers, segment registers, and special registers. The general-purpose registers can be used to store data and perform arithmetic and logical operations, while the segment registers are used to address memory segments. The special registers include the flags register, which stores status information about the result of the previous operation, and the instruction pointer (IP), which points to the next instruction to be executed.

The size of the internal registers(present within the chip) indicates how much information the processor can operate on at a time (in this case 16-bit registers) and how it moves data around internally within the chip, sometimes also referred to as the internal data bus.

The Prefetch Unit in the 8086 microprocessor is a component responsible for fetching instructions from memory and storing them in a queue. The prefetch unit allows the 8086 to perform multiple instruction fetches in parallel, improving the overall performance of the microprocessor.

The prefetch unit consists of a buffer and a program counter that are used to fetch instructions from memory. The buffer stores the instructions that have been fetched and the program counter keeps track of the memory location of the next instruction to be fetched. The prefetch unit fetches several instructions ahead of the current instruction, allowing the 8086 to execute instructions from the buffer rather than from memory.

This parallel processing of instruction fetches helps to reduce the wait time for memory access, as the 8086 can continue to execute instructions from the buffer while it waits for memory access to complete. This results in improved overall performance, as the 8086 is able to execute more instructions in a given amount of time.

The prefetch unit is an important component of the 8086 microprocessor, as it allows the microprocessor to work more efficiently and perform more instructions in a given amount of time. This improved performance helps to ensure that the 8086 remains competitive in its performance and capabilities, even as technology continues to advance.

The Decode Unit works in parallel with the Prefetch Unit, which fetches instructions from memory and stores them in a queue. The Decode Unit reads the instructions from the queue and translates them into micro-operations that can be executed by the microprocessor.

The Decode Unit is an important component of the 8086 microprocessor, as it allows the microprocessor to execute instructions efficiently and accurately. The decode unit ensures that the microprocessor can execute complex instructions, such as jump instructions and loop instructions, by translating them into a series of simple micro-operations.

The Decode Unit is responsible for decoding instructions, performing register-to-register operations, and performing memory-to-register operations. It also decodes conditional jumps, calls, and returns, and performs data transfers between memory and registers.

The Decode Unit helps to improve the performance of the 8086 microprocessor by allowing it to execute instructions quickly and accurately. This improved performance helps to ensure that the 8086 remains competitive in its performance and capabilities, even as technology continues to advance.

The Control Unit in the 8086 microprocessor is a component that manages the overall operation of the microprocessor. The control unit is responsible for controlling the flow of instructions through the microprocessor and coordinating the activities of the other components, including the Decode Unit, Execution Unit, and Prefetch Unit.

The Control Unit acts as the central coordinator for the microprocessor, directing the flow of data and instructions and ensuring that the microprocessor operates correctly. It also monitors the state of the microprocessor, ensuring that the correct sequence of operations is followed.

The Control Unit is an essential component of the 8086 microprocessor, as it allows the microprocessor to operate efficiently and accurately. The control unit ensures that the microprocessor can execute complex instructions, such as jump instructions and loop instructions, by coordinating the activities of the other components.

The Control Unit helps to improve the performance of the 8086 microprocessor by managing the flow of instructions and data through the microprocessor, ensuring that the microprocessor operates correctly and efficiently. This improved performance helps to ensure that the 8086 remains competitive in its performance and capabilities, even as technology continues to advance.

1.Address Bus: The address bus is used to send the memory address of the instruction or data being read or written. The address bus is 16 bits wide, allowing the 8086 to address up to 64 kilobytes of memory.

3.Control Bus: The control bus is used to transfer control signals between the microprocessor and other components in the computer system. The control bus is used to send signals such as read, write, and interrupt requests, and to transfer status information between the microprocessor and other components.

The 8086[3] (also called iAPX 86)[4] is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979,[5] is a slightly modified chip with an external 8-bit data bus (allowing the use of cheaper and fewer supporting ICs),[note 1] and is notable as the processor used in the original IBM PC design.

The 8086 gave rise to the x86 architecture, which eventually became Intel's most successful line of processors. On June 5, 2018, Intel released a limited-edition CPU celebrating the 40th anniversary of the Intel 8086, called the Intel Core i7-8086K.[5]

In 1972, Intel launched the 8008, Intel's first 8-bit microprocessor.[note 2] It implemented an instruction set designed by Datapoint Corporation with programmable CRT terminals in mind, which also proved to be fairly general-purpose. The device needed several additional ICs to produce a functional computer, in part due to it being packaged in a small 18-pin "memory package", which ruled out the use of a separate address bus (Intel was primarily a DRAM manufacturer at the time).

Two years later, Intel launched the 8080,[note 3] employing the new 40-pin DIL packages originally developed for calculator ICs to enable a separate address bus. It has an extended instruction set that is source-compatible (not binary compatible) with the 8008[6] and also includes some 16-bit instructions to make programming easier. The 8080 device was eventually replaced by the depletion-load-based 8085 (1977), which sufficed with a single +5 V power supply instead of the three different operating voltages of earlier chips.[note 4] Other well known 8-bit microprocessors that emerged during these years are Motorola 6800 (1974), General Instrument PIC16X (1975), MOS Technology 6502 (1975), Zilog Z80 (1976), and Motorola 6809 (1978).

The first revision of the instruction set and high level architecture was ready after about three months,[note 5] and as almost no CAD tools were used, four engineers and 12 layout people were simultaneously working on the chip.[note 6] The 8086 took a little more than two years from idea to working product, which was considered rather fast for a complex design in the 1970s

The 8086 was sequenced[note 7] using a mixture of random logic[8] and microcode and was implemented using depletion-load nMOS circuitry with approximately 20,000 active transistors (29,000 counting all ROM and PLA sites). It was soon moved to a new refined nMOS manufacturing process called HMOS (for High performance MOS) that Intel originally developed for manufacturing of fast static RAM products.[note 8] This was followed by HMOS-II, HMOS-III versions, and, eventually, a fully static CMOS version for battery powered devices, manufactured using Intel's CHMOS processes.[note 9] The original chip measured 33 mm and minimum feature size was 3.2 μm. The MUL and DIV instructions were very slow due to being microcoded so x86 programmers usually just used the bit shift instructions for multiplying and dividing instead.[citation needed]

The 8086 was die-shrunk to 2 μm in 1981; this version also corrected a stack register bug in the original 3.5 μm chips. Later 1.5 μm and CMOS variants were outsourced to other manufacturers and not developed in-house.

The architecture was defined by Stephen P. Morse with some help from 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[note 10] and Bill 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 for Intel devices is 8086h.)

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