8086 Internal Block Diagram Enotes

8086 Internal Block diagram (Intel Corp.) eNotes

By

Prof. S. Jagannathan, HOD – Department of Electronics and Communication Engineering, R.V. College of Engineering, Bangalore

Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore

Session - III ADVANCED MICROPROCESSORS Contents •

Block Diagram of 8086

•

segment registers

•

8086 flag register format

8086 Internal Block diagram (Intel Corp.)

The block diagram of 8086 is as shown. This can be subdivided into two parts, namely the Bus Interface Unit and Execution Unit. The Bus Interface Unit consists of segment registers, adder to generate 20 bit address and instruction prefetch queue. Once this address is sent out of BIU, the instruction and data bytes are fetched from memory and they fill a First In First Out 6 byte queue.

Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore

Execution Unit: The execution unit consists of scratch pad registers such as 16-bit AX, BX, CX and DX and pointers like SP (Stack Pointer), BP (Base Pointer) and finally index registers such as source index and destination index registers. The 16-bit scratch pad registers can be split into two 8-bit registers. For example, AX can be split into AH and AL registers. The segment registers and their default offsets are given below. Segment Register

Default Offset

CS

IP (Instruction Pointer)

DS

SI, DI

SS

SP, BP

ES

DI

The Arithmetic and Logic Unit adjacent to these registers perform all the operations. The results of these operations can affect the condition flags. Different registers and their operations are listed below: Register

Operations

AX

Word multiply, Word divide, word I/O

AL

Byte Multiply, Byte Divide, Byte I/O, translate, Decimal Arithmetic

AH

Byte Multiply, Byte Divide

BX

Translate

CX

String Operations, Loops

CL

Variable Shift and Rotate

DX

Word Multiply, word Divide, Indirect I/O

Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore

8086/8088 MPU IP

Instruction Pointer

CS

Code Segment Register

DS

Data Segment Register

SS

Stack Segment Register

ES

Extra Segment Register

AX

AH

AL

BX

BE

BL

CX

CE

CL

DX

DH

DL

SP

Stack Pointer Register

BP

Break Pointer Register

SI

Source Index Register

DI

Destination Index Register

SR

Status Register

MEMORY 00000016

Code Segment (64Kb)

Data Segment (64Kb)

Stack Segment (64Kb)

Extra Segment (64Kb)

Generation of 20-bit Physical Address:

Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore

FFFFF16

LOGICAL ADDRESS

SEGMENT REGISTER

0000

ADDER

20 BIT PHYSICAL MEMORY ADDRESS

Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore

8086 flag register format BIT

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

U

U

U

U

0F

DF

IF

TF

SF

ZF

U

AF

U

PF

U

CF

U= UNDEFINED

(a) (b) (c) (d) (e) (f) (g) (h) (i)

(a) (b) (c) (d) (e) (f) (g) (h) (i)

: CARRY FLAG – SET BY CARRY OUT OF MSB : PARITY FLAG – SET IF RESULT HAS EVEN PARITY : AUXILIARY CARRY FLAG FOR BCD : ZERO FLAG – SET IF RESULT = 0 : SIGN FLAG = MSB OF RESULT : SINGLE STEP TRAP FLAG : INTERRUPT ENABLE FLAG : STRING DIRECTION FLAG : OVERFLOW FLAG

There are three internal buses, namely A bus, B bus and C bus, which interconnect the various blocks inside 8086. The execution of instruction in 8086 is as follows: The microprocessor unit (MPU) sends out a 20-bit physical address to the memory and fetches the first instruction of a program from the memory. Subsequent addresses are sent Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore

out and the queue is filled upto 6 bytes. The instructions are decoded and further data (if necessary) are fetched from memory. After the execution of the instruction, the results may go back to memory or to the output peripheral devices as the case may be.

Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore