8086 Internal

Architecture

Prof. Sonal Shroff

Computer Engg Dept
TSEC
 8086 Microprocessor – Internal 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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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 >>

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 Bus Interface Unit (BIU)

Segment
Registers

8086’s 1-megabyte The 8086 can directly address Programs obtain access to
memory is divided into four segments (256 K bytes code and data in the
segments of up to 64K within the 1 M byte of memory) segments by changing the
bytes each. at a particular time. segment register content to
point to the desired
segments.

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 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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 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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 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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 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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 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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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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 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
DX can be used as DH and DL 11
 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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 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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 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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 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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 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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 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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 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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 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
 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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 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
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