8086 Processor Overview[2017 Latest]


8086 Microprocessor Overview

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8086 Microprocessor is an enhanced version of 8085 microprocessor launched by Intel in 1976. 8086 is 16bit Microprocessor external 8-bit data bus and is notable as the processor used in the original IBM PC design, including the widespread version called IBM PC XT.

8086 intern gave rise to modern day X-86 Architecture which is used in every single consumer grade processor.8086 Microprocessor has 16-bit data bus along with 20-bit address bus.It can access up to 1MB data.

It supports two modes of operation maximum mode and minimum mode.Minimum mode operation is suitable for systems that have a single microprocessor whereas maximum mode is for multiprocessor systems.

The member of the 8086microprocessor family includes 80186,80188,80286,80386,80486 etc.the 8086 Microprocessor is upward compatible.

The 8086 Microprocessor provides an instruction queue capable enough to store a six instruction from memory.The next instruction is fetched while the present instruction is being executed.

Comparision between 8085 and 8086

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Architecture Of 8086 Microprocessor

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8086 Microprocessor is divided into two functional units, i.e., EU (Execution Unit) and BIU (Bus Interface Unit).

Execution Unit(E.U.)

The main function of EU is decoding and execution of the instruction.In order to carry out its tasks it has the following units:

  1. Arithmetic logic unit
  2. Flag Register
  3. General Purpose Register
  4. Control Unit
  5. Decoder
  6. pointer and Index register

    1.Arithmetic logic unit

  • The ALU in EU is 16-bit unit i.e. it can perform 16-bit operation simultaneously.
  • It is capable of performing a variety of arithmetic and logical operation such as add, subtract, AND, OR, NOt, EX-Or etc

    2.Flag Register

  • Flag Register is a flip-flop.It indicates some condition produced by the execution of an instruction.For example, the zero flag will set if the result of execution of an instruction is zero.
  • A flag is a flip flop.It indicates some condition produced by the execution of an instruction.For example, the zero flag(ZF) will set if the result of execution of an instruction is zero.
  • A flag can control certain operation of EU.
  • The remaining seven flags marked “U” are undefined flags.

       Active flags

  • There are nine active flags out of 16, in the 8086 flags register.The remaining are undefined flags.

      Control Flags

  • Out of the nine active flags, six are conditioned flags and the remaining three are called as the control flags because they are used to control certain operation processor.
  • The three control flags are:
  1. The trap flag
  2. The interrupt flag
  3. The direction flag

     :>The trap flag(TF)

  1. Setting TF puts the processor into single step mode for debugging.In single stepping, the microprocessor executes an instruction and enters into single step ISR.
  2. After that user can check register or memory contents, if found ok, he/she will proceed further, else necessary action will be taken.This utility is to interrupt after each instruction, allowing a program to be inspected as it executes instruction by instruction.

     :>The interrupt flag

  1. If the user sets IF flag, the CPU will recognize external interrupt requests.
  2. Clearing IF disable this instruction.

     :>The directional flag

  1. This bit is specifically for string instruction.In string instruction, we use SI and DI register as offset register to point source area and destination area respectively.DF flag controls the direction of SI and DI pointers.
  2. If DF=1 the string instruction will automatically decrement the pointer.

   Conditional Flags

It represents the result of the last arithmetic or logical instruction executed. Following is the list of conditional flags −

  • Carry flag − This flag indicates an overflow condition for arithmetic operations.
  • Auxiliary flag − When an operation is performed at ALU, it results in a carry/borrow from lower nibble (i.e. D0 – D3) to upper nibble (i.e. D4 – D7), then this flag is set, i.e. carry given by D3 bit to D4 is AF flag. The processor uses this flag to perform binary to BCD conversion.
  • Parity flag − This flag is used to indicate the parity of the result, i.e. when the lower order 8-bits of the result contains an even number of 1’s, then the Parity Flag is set. For an odd number of 1’s, the Parity Flag is reset.
  • Zero flag − This flag is set to 1 when the result of an arithmetic or logical operation is zero else it is set to 0.
  • Sign flag − This flag holds the sign of the result, i.e. when the result of the operation is negative, then the sign flag is set to 1 else set to 0.
  • Overflow flag − This flag represents the result when the system capacity is exceeded.


   3.General Purpose Registers

There are 8 general purpose registers, i.e., AH, AL, BH, BL, CH, CL, DH, and DL. These registers can be used individually to store 8-bit data and can be used in pairs to store 16bit data. The valid register pairs are AH and AL, BH and BL, CH and CL, and DH and DL. It is referred to the AX, BX, CX, and DX respectively.

  • AX register − It is also known as accumulator register. It is used to store operands for arithmetic operations.
  • BX register − It is used as a base register. It is used to store the starting base address of the memory area within the data segment.
  • CX register − It is referred to as counter. It is used in loop instruction to store the loop counter.
  • DX register − This register is used to hold I/O port address for I/O instruction.

   4.Control Circuitry

The control circuit is a part of EU.It is used for directing the internal operations.

   5. A Decoder

  •    The process of translation from instruction into action is known as decoding
  • A decoder in execution unit is used for translating the instruction fetched from the memory into a series of action.
  • The EU will actually carry out these actions.

  6.Pointer and Index register

The execution unit also contains the following 16bit registers.

  1. Base Pointer Register: -This is a 16bit register in EU.It holds the 16bit offset relative to the stack segments register
  2. Stack Pointer Register: -This is also 16-bit register in EU.It holds the 16 offset address relative to stack segments register.It is used for sequential access of stack segments.
  3. Source Index Register: -This is register is used for holding the offset of the data word in the data segments.The physical address of the location in the data segment can be generated by adding hardwired zero to the segment base held by the data segments register and then by adding the offset in the SI register to it.
  4. Destination Index Register: -This register is used for holding the 16bit offset of the data word in extra segments.The source index register and destination index register are used for the string related instruction.

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The Bus Interfacing Unit(BIU)

BIU takes care of all data and addresses transfers on the buses for the EU like sending addresses, fetching instructions from the memory, reading data from the ports and the memory as well as writing data to the ports and the memory. EU has no direct connection with System Buses so this is possible with the BIU. EU and BIU are connected with the Internal Bus.

It has the following functional parts −

  • Instruction queue − BIU contains the instruction queue. BIU gets up to 6 bytes of next instructions and stores them in the instruction queue. When EU executes instructions and is ready for its next instruction, then it simply reads the instruction from this instruction queue resulting in increased execution speed.
  • Fetching the next instruction while the current instruction executes is called pipelining.
  • Segment register − BIU has 4 segment buses, i.e. CS, DS, SS& ES. It holds the addresses of instructions and data in memory, which are used by the processor to access memory locations. It also contains 1 pointer register IP, which holds the address of the next instruction to executed by the EU.
    • CS − It stands for Code Segment. It is used for addressing a memory location in the code segment of the memory, where the executable program is stored.
    • DS − It stands for Data Segment. It consists of data used by the program and is accessed in the data segment by an offset address or the content of another register that holds the offset address.
    • SS − It stands for Stack Segment. It handles memory to store data and addresses during execution.
    • ES − It stands for Extra Segment. ES is additional data segment, which is used by the string to hold the extra destination data.
  • Instruction pointer − It is a 16-bit register used to hold the address of the next instruction to be executed.


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