help you to learn about 8086

1. BY: AKASH UGALE
8086 Microprocessor

2. Important Features Of 8086
Buses:-Address ,Data ,Control
8086 supports Pipelining.
8086 has 2 Operating Modes.
8086 provides Memory Banks.
8086 supports Memory Segmentation.
8086 supports 256 interrupts.
8086 has 16-bit IO address.

3. Architecture Of 8086

4. Bus Interface Unit(BIU)
Provide interface of 8086 to external memory &
IO devices.
Operates w.r.t Bus cycle
It generates 20-bit physical address.
Fetches Instruction from memory.
Transfer data to or from the memory and IO.
Supports Pipelining using the 6-byte instruction
queue.

5. Segment Registers
CS Register
DS Register
SS Register
ES Register

6. Instruction Pointer
It is a 16-bit register.
It holds offset of the next instruction in the
code segment.
Address of the next instruction is calculated as
CS * 10H+IP.
IP is incremented after every instruction byte
is fetched.
IP gets a new value whenever a branch occurs.

7. Address Generation Circuit
BIU has Physical Address Generation Circuit.
It generates 20-bit Physical address using the
formula:
PA=Segment Address*10H + Offset AddressPA=Segment Address*10H + Offset Address
Eg:-
Segment Address:-1234H(0001 0010 0011 0100)
Offset Address:-0005H(0000 0000 000 0101)
PA=1234*10H+0005=12345H
=0001 0010 0011 0100 0000 +0001 0010 0011 0101
= 0001 0010 0011 0100 0101 =12345H

8. 6-Byte Pre-Fetch Queue
It is a 6-byte FIFO RAM use to implement
Pipelining.
BIU Fetches the next “six instruction-bytes”
from CS and store it in Queue.
EU removes instruction from the queue and execute
them.
Queue is refilled atleast 2 byte are empty.
As soon as 8086 detects a branch operation
,it clears/discard the entire queue.

9. Execution Unit(EU)
It fetches instruction from the queue in
BIU,decodes and execute them.
It performs Arithmatic,logic and internal data
transfer operation within the mup.
It sends Signal to the BIU to access the external
module.
It operates w.r.t.T-States.

10. Execution Unit Contd…….
General Purpose Registers:-AX,BX,CX,DX
Special Purpose Registers:-SP,BP,SI,DI
ALU
Operand Register
Instruction Register and Instruction Decoder

11. Flag Register(16-Bits)
It has 9 flags.
These flags of two types : 6-satus (condition) flags and
3-control flags.
Status Flags:-OF,SF,ZF,AF,PF,CF
Control Flags:-DF,IF,TF
X X X X OF DF IF TF SF ZF X AF X PF X CF

12. Memory Segmentation
Dividing the memory into logically different
parts called segments.
20-bit Address bus hence total 1MB memory.
4 Segments:-CS,DS,SS,ES
Max size of segment 64KB.
8086 can access only 4 segment at time.
Four 16-bit register to hold base address.
Four 16-bit register to hold offset address.

13. Memory Segmentation Contd…..
Advantages of Segmentation:
 It permits the programmer to access 1mb using
only 16-bit address.
 Its divides the memory logically to store
instruction,data and stack separately.
 Segmentation is very useful for multiuser
environment
Disadvantages of Segmentation:
 Although the total memory is 16*64KB ,at time only
4*64KB memory can be accessed.

14. Memory Banking
Odd bank Even bank
Also called as “higher bank” Also called as “lower bank”
Address range: Address range:
0001H 0000H
0003H 0002H
0005H 0006H
. .
. .
FFFFH FFFEH
Selected when BHE=0 Selected when A0=0
512KB 512KB
1MB

15. Memory Banking Contd….
BHE A0 Operation
0 0 R/W 16-bit from both banks
0 1 R/W 8-bit from higher bank
1 0 R/W 8-bit from lower bank
1 1 No Operation(Idle)