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8086 Microprocessor Overview

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8086 Microprocessor Overview essay
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Microprocessor – the computer whose entire CPU is contained on one integrated circuits. The important characteristics of a microprocessor are the widths of its internal and external address bus and data bus (and instruction), its clock rate and its instruction set.

Major Features

8086 Microprocessor is a 16-bit microprocessor. It is the highest data carrying capacity of 8086. However, it can handle 8-bit data as well.

There are 20 address lines for 8086. Address lines define how much memory the processor can access. In this case, it is 220=1 Mb memory.

Operating frequency= 5 MHz

It has 14 16-bit registers.


Basic architecture of 8086 Microprocessor

The internal architecture of 8086 Microprocessor is essentially divided into 2 parts viz.

  1. Bus Interface Unit (BIU)
  2. Execution Unit (EU)

Bus Interface Unit

It handles all data and addresses on the buses for the Execution Unit i.e. it sends out addresses, fetches instructions from memory, reads data from ports and memory and writes data to ports and memory. BIU also generates the 20-bit physical address.

The BIU consists of a number of functional groups or parts such as:

  • Instruction Queue. It prefetches 6 instruction bytes from memory to the queue of processor. It is a FIFO kind of buffer.
  • The flow of operation is such: Memory -> Queue -> EU

    It simply reads the instruction from the instruction queue when the EU is ready for the next instruction. It is much faster than sending address to system memory and then fetching the instruction. Queue follows the concept of pipelining.

  • Segmentation. Memory being divided into smaller segments is segmentation.

BIU contains four 16-bit registers, namely:

  1. Code Segment (CS): Used for addressing a memory location where executable program is stored.
  2. Data Segment (DS): Contains most data used by the program. It holds the offset address.
  3. Stack Segment (SS): A section of memory to store addresses and data while a subprogram executes.
  4. Extra Segment (ES): Additional data segment used to hold extra destination data. Each register can hold 64kb data.

Instruction Pointer: It is present below the segment register. The IP holds the 16-bit address of the next code byte within the code segment.

Execution Unit

It basically does the arithmetic and logical calculations. It tells the BIU where to fetch instructions from; it decodes the instructions and executes them.

The EU, like BIU, also consists of a number of components, such as:

  • Arithmetic & Logic Unit: Does all arithmetic and logical calculations.
  • Control Unit: Performs various internal operations.

The EU also consists of registers which are:

  • General Purpose Register: Used for temporary storage of 8-bit data ii.
  • Flag Register: A flip-flop indicating occurrence of any condition during execution of an instruction.

There are 2 types: a) Control Flags, b) Conditional Flags

General Purpose Registers

8 general purpose registers:

  1. AX Register: Accumulator which stores operands for arithmetic calculations.
  2. BX Register: Base register holding starting address of memory region.
  3. CX Register: Used to store loop counter in loop instruction.
  4. DX Register: Contains i/o port address for i/o instructions.

Stack Pointer Register

The stack pointer (SP) register contains the 16-bit offset from the start of the segment to the memory location where a word was most recently stored on the stack. Most recently used address is stored on top of the stack.

Other registers such as the SI, DI, BP are also used for temporary storage purposes.

Flag Register:

Conditional Flags: Indicate occurrence of mathematical operations. They are:

  • Carry Flag (CF): Indicates if carry is generated in MSB position.
  • Auxiliary Flag (AF): Performs Binary to BCD conversion.
  • Parity Flag (PF): Indicates parity of the result.
  • Zero Flag (ZF): It is high if the result of operation is zero.
  • Sign Flag (SF): Indicates the sign magnitude of the result.
  • Overflow Flag (OF): Indicates the exceeding of capacity of machine.

Control Flags: They are set or reset to control certain operations of the processor with the help of specific user-input instructions in the program. They are:

  • Trap Flag (TF): Allows user to execute one instruction at a time
  • Interrupt Flag (IF): Allows/prohibits interruption of program
  • Direction Flag (DF): Used in string operations.


The 8086 microprocessor supports 8 types of instructions. They are:

Data Transfer Instructions. These instructions are used to transfer the data from the source operand to the destination operand. Following are the list of instructions under this group.

  • MOV – Used to copy the byte or word from the provided source to the provided destination.
  • PPUSH – Used to put a word at the top of the stack.
  • POP – Used to get a word from the top of the stack to the provided location.
  • PUSHA – Used to put all the registers into the stack.

Arithmetic Instructions. These instructions are used to perform arithmetic operations like addition, subtraction, multiplication, division, etc.

  • ADD – Used to add the provided byte to byte/word to word.
  • SUB – Used to subtract the byte from byte/word from word.
  • MUL – Used to multiply unsigned byte by byte/word by word.
  • DIV – Used to divide the unsigned word by byte or unsigned double word by word.

Bit Manipulation Instructions. These instructions are used to perform operations where data bits are involved, i.e. operations like logical, shift, etc.

  • NOT – Used to invert each bit of a byte or word.
  • AND – Used for adding each bit in a byte/word with the corresponding bit in another byte/word.
  • OR – Used to multiply each bit in a byte/word with the corresponding bit in another byte/word.
  • XOR – Used to perform Exclusive-OR operation over each bit in a byte/word with the corresponding bit in another byte/word.

String Instructions. String is a group of bytes/words and their memory is always allocated in a sequential order.

  • REP – Used to repeat the given instruction till CX ? 0.
  • REPE/REPZ – Used to repeat the given instruction until CX = 0 or zero flag ZF = 1.
  • REPNE/REPNZ – Used to repeat the given instruction until CX = 0 or zero flag ZF = 1.
  • MOVS/MOVSB/MOVSW – Used to move the byte/word from one string to another.
  • COMS/COMPSB/COMPSW – Used to compare two string bytes/words.

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