MPMC Microprocessor

ARCHITECTURE OF

8086

MICROPROCESSOR

By

A.S.Krishna

What is a Microprocessor ?

A microprocessor incorporates the functions of a computer's central processing unit (CPU) on a single integrated circuit (IC), or at most a few integrated circuits.

The microprocessor is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and provides results as output. It has internal memory. Microprocessors operate on numbers and symbols represented in the binary numeral system.

What is 8086 Microprocessor ?

The 8086 is also called iAPX 86. It is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released.

It is notable as the processor used in the original IBM PC.

Architecture of an 8086 Microprocessor

The architecture of 8086 provides a number of improvements over 8085 architecture.

It supports a 16-bit ALU, a set of 16-bit registers and provides segmented memory addressing capability, a rich instruction set, powerful interrupt structure, fetched instruction queue for overlapped fetching and execution etc.,

Architecture of 8086

BIU(Bus Interface

Unit)

EU(Execution Unit)

Bus Interface Unit (BIU)

In the BIU we have some collection components. They are:-

1. Address Adder.

2. Segment Registers and Program Counter.

3. Instruction Queue

1. Address Adder

In the address adder the adder adds the addresses and transfer them to the memory though the internal data bus path.

2. Segment Registers and Program Counter

Segment Registers Code segment

Data segment

Stack segment

Extra segment

Segment Registers CS

DS

SS

ES

Program Counter IP

Program Counter Instruction Pointer

3. Instruction Queue

Instruction Queue 6

5

4

3

2

1

This a Queue which can store the given instructions one-by-one.

Execution Unit(EU)

In the EU we have some collection components. They are:- 1. General Registers.

2. Execution Unit Control System.

3. ALU

4. Operands and Flags.

1. General Registers

In this we have two types of registers.

They are:- 1. Main Registers.

2. Index Registers.

Main Registers

AH AL AX(Primary accumulator)

BH BL BX(Base accumulator)

CH CL CX(Counter accumulator)

DH DL DX(accumulator other funtions)

Index Registers

SP Stack Pointer

BP Base Pointer

SI Source Index

DI Destination Index

2. Execution Unit Control System

In this unit the Execution is controlled and is transferred to the all other components through the internal data bus.

3. ALU(Arithmetic Logical Unit)

Here all the ALU operation are performed and transferred the data through the internal data bus.

4.Flags Registers

This flag is identical to 8085 flag register, with an additional overflow flag, which is not present in 8085.

The 8086 has a 16-bit flag register which is divided into two parts, viz.(a) condition code (or) status flags and (b) machine control flags.

The condition code flag register is the lower byte of the 16-bit flag register along with the overflow flag.

The machine control flag register is the higher byte of the flag register of 8086 which contains three flags, viz. direction flag(D), interrupt flag(I) and trap flag(T).

The bit configuration of 8086 flag register.

15

14

13

12

11

10

9 8 7 6 5 4 3 2 1 0

X X X X O D I T S Z X Ac

X P X CyO – Overflow flag

D – Direction flagI – Interrupt flagT – Trap flagS – Sign flagZ – Zero flagAc – Auxillary carry flagP – Parity flagCy – Carry flagX – Not used (or) Future use

Memory Address and data bus interface

Address/Status

A16/S3

Address/Data

AD0/AD15

Instruction Queue (6 bytes)

Decoding circuit

Timing and control circuit

Clock and control signals

Address Adder

CS DS SS ES

IP

ALU (16)

FLAGS (16) IP IP IP

IP

AH ALBH BLCH CLDH DL

15 0

AX

BX

CX

DX

Register Bank

B

I

U

E

U

Pin Diagram of 8086 Microprocessor

Any

Quires?....