MICROPROCESSOR SYSTEMS AND

INTERFACING (EEE342)

Dr. Omer Chughtai

Assistant Professor, CUI, Wah Campus

Email: omer.chughtai@outlook.com
8086 Microprocessor
Architecture

Execution Unit (EU) Bus Interface Unit (BIU)

EU executes instructions that have BIU fetches instructions, reads data

already been fetched by the BIU. from memory and I/O ports, writes
data to memory and I/ O ports.
BIU and EU functions separately.
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8086 Microprocessor
Architecture Bus Interface Unit (BIU)

Dedicated Adder to
generate 20 bit address

Four 16-bit segment

registers

Code Segment (CS)

Data Segment (DS)
Stack Segment (SS)
Extra Segment (ES)

Segment Registers >> 3

8086 Microprocessor
Architecture Bus Interface Unit (BIU)

Segment Code Segment Register

Registers
16-bit

CS contains the base or start of the current code segment;

IP contains the distance or offset from this address to the
next instruction byte to be fetched.

BIU computes the 20-bit physical address by logically

shifting the contents of CS 4-bits to the left and then
adding the 16-bit contents of IP.

That is, all instructions of a program are relative to the

contents of the CS register multiplied by 16 and then
offset is added provided by the IP.

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8086 Microprocessor
Architecture Bus Interface Unit (BIU)

Segment Data Segment Register

Registers
16-bit

Points to the current data segment; operands for most

instructions are fetched from this segment.

The 16-bit contents of the Source Index (SI) or

Destination Index (DI) or a 16-bit displacement are used
as offset for computing the 20-bit physical address.

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8086 Microprocessor
Architecture Bus Interface Unit (BIU)

Segment Stack Segment Register

Registers
16-bit

Points to the current stack.

The 20-bit physical stack address is calculated from the

Stack Segment (SS) and the Stack Pointer (SP) for stack
instructions such as PUSH and POP.

In based addressing mode, the 20-bit physical stack

address is calculated from the Stack segment (SS) and the
Base Pointer (BP).

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8086 Microprocessor
Architecture Bus Interface Unit (BIU)

Segment Extra Segment Register

Registers
16-bit

Points to the extra segment in which data (in excess of

64K pointed to by the DS) is stored.

String instructions use the ES and DI to determine the 20-

bit physical address for the destination.

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8086 Microprocessor
Architecture Bus Interface Unit (BIU)

Segment Instruction Pointer

Registers
16-bit

Always points to the next instruction to be executed

within the currently executing code segment.

So, this register contains the 16-bit offset address pointing

to the next instruction code within the 64Kb of the code
segment area.

Its content is automatically incremented as the execution

of the next instruction takes place.

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8086 Microprocessor
Architecture Bus Interface Unit (BIU)

Instruction queue

A group of First-In-First-
Out (FIFO) in which up to
6 bytes of instruction
code are pre fetched
from the memory ahead
of time.

This is done in order to

speed up the execution
by overlapping
instruction fetch with
execution.

This mechanism is known

as pipelining.

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 8086 Microprocessor
Architecture Execution Unit (EU)

EU decodes and
executes instructions.

A decoder in the EU
control system
translates instructions.

16-bit ALU for

performing arithmetic
and logic operation

Four general purpose

registers(AX, BX, CX, DX);

Pointer registers (Stack

Pointer, Base Pointer);

and
Some of the 16 bit registers can be
Index registers (Source used as two 8 bit registers as :
Index, Destination Index)
each of 16-bits AX can be used as AH and AL
BX can be used as BH and BL
CX can be used as CH and CL 11
DX can be used as DH and DL
8086 Microprocessor
Architecture Execution Unit (EU)

EU Accumulator Register (AX)

Registers
Consists of two 8-bit registers AL and AH, which can be
combined together and used as a 16-bit register AX.

AL in this case contains the low order byte of the word,

and AH contains the high-order byte.

The I/O instructions use the AX or AL for inputting /

outputting 16 or 8 bit data to or from an I/O port.

Multiplication and Division instructions also use the AX or

AL.

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8086 Microprocessor
Architecture Execution Unit (EU)

EU Base Register (BX)

Registers
Consists of two 8-bit registers BL and BH, which can be
combined together and used as a 16-bit register BX.

BL in this case contains the low-order byte of the word,

and BH contains the high-order byte.

This is the only general purpose register whose contents

can be used for addressing the 8086 memory.

All memory references utilizing this register content for

addressing use DS as the default segment register.

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8086 Microprocessor
Architecture Execution Unit (EU)

EU Counter Register (CX)

Registers
Consists of two 8-bit registers CL and CH, which can be
combined together and used as a 16-bit register CX.

When combined, CL register contains the low order byte of

the word, and CH contains the high-order byte.

Instructions such as SHIFT, ROTATE and LOOP use the

contents of CX as a counter.

Example:

The instruction LOOP START automatically decrements

CX by 1 without affecting flags and will check if [CX] =
0.

If it is zero, 8086 executes the next instruction;

otherwise the 8086 branches to the label START.

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8086 Microprocessor
Architecture Execution Unit (EU)

EU  Data Register (DX)

Registers
Consists of two 8-bit registers DL and DH, which can be
combined together and used as a 16-bit register DX.

When combined, DL register contains the low order byte of

the word, and DH contains the high-order byte.

Used to hold the high 16-bit result (data) in 16 X 16

multiplication or the high 16-bit dividend (data) before a
32 16 division and the 16-bit reminder after division.

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8086 Microprocessor
Architecture Execution Unit (EU)

EU Stack Pointer (SP) and Base Pointer (BP)

Registers
SP and BP are used to access data in the stack segment.

SP is used as an offset from the current SS during

execution of instructions that involve the stack segment in
the external memory.

SP contents are automatically updated (incremented/

decremented) due to execution of a POP or PUSH
instruction.

BP contains an offset address in the current SS, which is

used by instructions utilizing the based addressing mode.

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8086 Microprocessor
Architecture Execution Unit (EU)

EU Source Index (SI) and Destination Index (DI)

Registers
Used in indexed addressing.

Instructions that process data strings use the SI and DI

registers together with DS and ES respectively in order to
distinguish between the source and destination addresses.

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8086 Microprocessor
Architecture Execution Unit (EU)

EU Source Index (SI) and Destination Index (DI)

Registers
Used in indexed addressing.

Instructions that process data strings use the SI and DI

registers together with DS and ES respectively in order to
distinguish between the source and destination addresses.

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 8086 Microprocessor
Architecture Execution Unit (EU)
Auxiliary Carry Flag
Carry Flag
Flag Register This is set, if there is a carry from the
This flag is set, when there is
lowest nibble, i.e, bit three during
addition, or borrow for the lowest a carry out of MSB in case of
nibble, i.e, bit three, during addition or a borrow in case
subtraction. of subtraction.

Sign Flag Zero Flag Parity Flag

This flag is set, when the This flag is set, if the result of This flag is set to 1, if the lower
result of any computation the computation or comparison byte of the result contains even
is negative performed by an instruction is number of 1’s ; for odd number
zero of 1’s set to zero.

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

OF DF IF TF SF ZF AF PF CF

Tarp Flag
Over flow Flag If this flag is set, the processor
This flag is set, if an overflow occurs, i.e, if the result of a signed enters the single step execution
operation is large enough to accommodate in a destination
mode by generating internal
register. The result is of more than 7-bits in size in case of 8-bit
signed operation and more than 15-bits in size in case of 16-bit interrupts after the execution of
sign operations, then the overflow will be set. each instruction

Direction Flag Interrupt Flag

This is used by string manipulation instructions. If this flag bit
is ‘0’, the string is processed beginning from the lowest Causes the 8086 to recognize
address to the highest address, i.e., auto incrementing mode. external mask interrupts; clearing IF
Otherwise, the string is processed from the highest address disables these interrupts.
towards the lowest address, i.e., auto incrementing mode. 19
8086 Microprocessor
Architecture

8086 registers
categorized 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

into 4 groups OF DF IF TF SF ZF AF PF CF

Sl.No. Type Register width Name of register

1 General purpose register 16 bit AX, BX, CX, DX

8 bit AL, AH, BL, BH, CL, CH, DL, DH

2 Pointer register 16 bit SP, BP

3 Index register 16 bit SI, DI

4 Instruction Pointer 16 bit IP

5 Segment register 16 bit CS, DS, SS, ES

6 Flag (PSW) 16 bit Flag register

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8086 Microprocessor
Architecture Registers and Special Functions

Register Name of the Register Special Function

AX 16-bit Accumulator Stores the 16-bit results of arithmetic and logic

operations

AL 8-bit Accumulator Stores the 8-bit results of arithmetic and logic

operations

BX Base register Used to hold base value in base addressing mode

to access memory data

CX Count Register Used to hold the count value in SHIFT, ROTATE

and LOOP instructions

DX Data Register Used to hold data for multiplication and division

operations

SP Stack Pointer Used to hold the offset address of top stack

memory

BP Base Pointer Used to hold the base value in base addressing

using SS register to access data from stack
memory

SI Source Index Used to hold index value of source operand (data)

for string instructions

DI Data Index Used to hold the index value of destination

operand (data) for string operations 21