The 8086 is a 16-bit microprocessor chip designed by Intel in the late 1970s. It had a 16-bit internal data bus, 20-bit address bus, and was separated into a Bus Interface Unit and Execution Unit to improve performance via instruction pipelining. The BIU fetched instructions and operands from memory while the EU executed instructions. The 8086 used general purpose registers, pointer/index registers, segment registers, and an instruction pointer to support its programming model. It was a significant advancement that helped drive the personal computing revolution.

1. Microprocessor
A microprocessor is an electronic device which computes on the given
input similar to CPU of a computer. It is made by fabricating millions(or
billions) of transistors on a single chip.
History of Microprocessor
Microprocessor journey started with a 4-bit processor called 4004, it
was made by Intel corporation in 1971. It was 1st single chip
processor. Then the idea was extended to 8-bit processors like 8008,
8080 and then 8085 (all are Intel products). 8085 was a very successful
one among the 8-bit processors, however its application is very limited
because of its slower computing speed and other quality factors.
Some years later Intel came up with its 1st 16-bit processors 8086.
Intel 8086:
The 8086 is a 16-bit microprocessor chip designed by Intel corporation
in between early 1976 and mid-1978. The release of Intel's 8086
microprocessor in 1978 was a watershed moment for personal
computing.

2. 2
8086 Architecture
Elements of the 8086 Microprocessor Architecture
The 80x86 has:
• 16-bit internal data bus
• 20-bit address bus: 220 = 1,048,576 = 1 megabyte
• Control bus
• Execution Unit
• Bus Interface Unit
Among the on-chip peripherals are:
• 2 direct memory access controllers (DMA)
• Three 16-bit programmable timers
• Clock generator
• Chip select unit
• Programmable Control Registers
The 8086 Processor Model
The simplified block diagram of the 80x86 processor model is organized
as two separate processors :
1) Bus Interface Unit (BIU)
2) Execution Unit (EU).

3. 3
8086 Architecture
Bus Interface Unit (BIU)
The BIU provides hardware functions. Including generation of the
memory and I/0 addresses for the transfer of data between itself and
the outside world.
Following functions are supported by BIU.
 It provides a full 16 bit bidirectional data bus and 20 bit address
bus.
 It sends address of memory or I/O.
 It fetches instruction from memory.
 It reads data from port/memory.
 It writes data into port/memory.
 It supports instruction queuing .
 It makes 8086’s interface to the outside world.

4. 4
8086 Architecture
 The BIU uses a mechanism known as an instruction stream queue
to implement a pipeline architecture.
 If the BIU is already in the process of fetching an instruction when
the EU request it to read or write operands from memory or I/O,
the BIU first completes the instruction fetch bus cycle before
initiating the operand read / write cycle.
 The BIU also contains a dedicated adder which is used to generate
the 20bit physical address.
Execution Unit (EU)
The EU receives program instruction codes and data from the BIU,
executes these instructions, and stores the results in the general
registers.
Following functions are supported by BIU.
 The Execution unit is responsible for decoding and executing all
instructions.
 The EU extracts instructions from the top of the queue in the BIU.
 During the execution of the instruction, the EU tests the status
and control flags and updates them based on the results of
executing the instruction.
 If the queue is empty, the EU waits for the next instruction byte to
be fetched and shifted to top of the queue.
 The EU accesses the queue from the output end. It reads one
instruction byte after the other from the output of the queue.
 It tells BIU from where to fetch instructions or data, decodes
instructions & execute instructions.

5. 5
8086 Architecture
The main linkage between the two functional blocks is the instruction
queue, with the BIU looking ahead of the current instruction being
executed in order to keep the queue filled with instructions for the EU
to decode and operate on.
Instruction Queue
• It is of 6 Bytes.
• To increase the execution speed, BIU fetches as many as six
instruction bytes ahead to time from memory.
• It operates on the principle first in first out (FIFO).
• Then all bytes are given to EU one by one.
• This pre-fetching operation of BIU may be in parallel with
execution operation of EU.
• It improves the execution speed of the instruction.
The Fetch and Execute Cycle
The organization of the processor into a separate BIU and EU allows the
fetch and execute cycles to overlap. To see this, consider what happens
when the 8086 is first started. Figure 3.2.
1. The BIU outputs the contents of the instruction pointer register (IP)
onto the address bus, causing the selected byte or word in memory to
be read into the BIU.
2. Register IP is incremented by one to prepare for the next instruction
fetch.

6. 6
8086 Architecture
3. Once inside the BIU, the instruction is passed to the queue: a first-
in/first-out storage register sometimes likened to a pipeline.
4. Assuming that the queue is initially empty, the EU immediately draws
this instruction from the queue and begins execution.
5. While the EU is executing this instruction, the BIU proceeds to fetch a
new instruction. Depending on the execution time of the first
instruction, the BIU may fill the queue with several new instructions
before the EU is ready to draw its next instruction.
6. The cycle continues, with the BIU filling the queue with instructions
and the EU fetching and executing these instructions. The BIU is
programmed to fetch a new instruction whenever the queue has room
for two additional bytes. The advantage to this pipelined architecture is
that the EU can execute instructions (almost) continually instead of
having to wait for the BIU to fetch a new instruction. This is shown
schematically in the following Figure

7. 7
8086 Architecture
The “Wait” mode
There are three conditions that will cause the EU to enter a "wait"
mode.
I. When an instruction requires access to a memory location. The
BIU must suspend fetching instructions and output the address of
this memory location. After waiting for the memory access, the
EU can resume executing instruction codes from the queue, and
the BIU can resume filling the queue.
II. When the instruction to be executed is a jump instruction. In this
case, control is to be transferred to a new address. The EU must
wait while the instruction at the jump address is fetched. Any
bytes presently in the queue must be discarded (they are
overwritten).
III. During the execution of slow-executing instructions.
The 8086 Programming Model
The programming model for a microprocessor shows the various
internal registers that are accessible to the programmer. The Following
Figure is a model for the 8086. In general, each register has a special
function.

8. 8
8086 Architecture
Registers of 8086
• General Purpose Registers
• Pointer and Index Registers
• Segment Registers
• Instruction Pointer
• Status Flags

9. 9
8086 Architecture
General Purpose Registers
There are four 16-bit general purpose registers:
 AX
 BX
 CX
 DX
Each of these 16-bit registers are further subdivided into two 8-bit
registers.
AX
BX
CX
DX
1. AX Register: AX register is also known as accumulator register that
stores operands for arithmetic operation like divided, rotate.
2.BX Register: This register is mainly used as a base register. It holds
the starting base location of a memory region within a data segment.
3.CX Register: It is defined as a counter. It is primarily used in loop
instruction to store loop counter.
4.DX Register: DX register is used to contain I/O port address for I/O
instruction.
AH AL
BH BL
CH CL
BH DL

10. 10
8086 Architecture
Pointers and Index Registers
Following four registers are under this category:
1. Stack Pointer (SP),
2. Base Pointer (BP),
3. Source Index (SI),
4. Destination Index (DI).
Following Registers can also be used as a general Purpose Registers.
1. Stack Pointer (SP) is a 16-bit register pointing to program Stack,
also contains 16-Bit offset address.
2. Base Pointer (BP) is a 16-bit register pointing to data in stack
segment. BP register is usually used for based indexed or register
indirect addressing.
3. Source Index (SI) is a 16-bit register. SI is used for indexed, based
indexed and register indirect addressing, as well as a source data
address in string manipulation Instructions
4. Destination Index (DI) is a 16-bit register. DI is used for indexed,
based indexed and register indirect addressing, as well as a
destination data address in string manipulation instructions.
Segment Register
There are four segment registers in Intel 8086:
1. Code Segment Register (CS),
2. Data Segment Register (DS),
3. Stack Segment Register (SS),
4. Extra Segment Register (ES).

11. 11
8086 Architecture
A segment register points to the starting address of a memory segment.
For Example: The code segment register points to the starting address
of the code segment.
The data segment register points to the starting address of the data
segment, and so on.
The maximum capacity of a segment may be up to 64 KB.
Code segment (CS):-
It is a 16-bit register containing address of 64 KB segment with
processor instructions. The processor uses CS segment for all accesses
to instructions referenced by instruction pointer (IP) register. CS
register cannot be changed directly. The CS register is automatically
updated during far jump, far call and far return instructions .
Stack segment (SS):-
It is a 16-bit register containing address of 64KB segment with program
stack. By default, the processor assumes that all data referenced by the
stack pointer (SP) and base pointer (BP) registers is located in the stack
segment. SS register can be changed directly using POP instruction.
Data segment (DS):-
It is a 16-bit register containing address of 64KB segment with program
data. By default, the processor assumes that all data referenced by
general registers (AX, BX, CX, DX) and index register (SI, DI) is located in
the data segment.

12. 12
8086 Architecture
Extra segment (ES):-
It is a 16-bit register containing address of 64KB segment, usually with
program data. By default, the processor assumes that the DI register
references the ES segment in string manipulation instructions. It is
possible to change default segments used by general and index registers by
prefixing instructions with a CS, SS,DS or ES prefix.
Concept of Segmented Memory
• It allows the memory addressing capacity to be 1 Mbytes.
• It allows instruction code, data stack and portion of program to be
more than 64KB long.
• It facilitates use of separate memory areas for program, data and
stack.
• It permits a program or its data to be put in different areas of
memory
• In this program can be relocated which is very useful in
multiprogramming i.e.multitasking becomes easy.

13. 13
8086 Architecture
Concept of Segmented Memory
22
Bottom Of Data Segment
Top Of Data Segment
Top Of Code Segment
Code Segment Base CS=348AH
Stack Segment Base SS=5000H
Top Of Stack Segment
Extra Segment Base ES=7000H
Top Of Extra Segment
Highest Address
20000H
2FFFFH
348A0H
4489FH
50000H
5FFFFH
7FFFFH
FFFFFH
64KB
64KB
64KB
64KB
Instruction Pointer
The Instruction Pointer (IP) in 8086 acts as a Program Counter. It points
to the address of the next instruction to be executed. Its content is
automatically incremented when the execution of a program proceeds
further. The contents of the IP and Code Segment Register are used to
compute the memory address of the instruction code to be fetched.
This is done during the Fetch Cycle.
Status Flags
Status Flags determines the current state of the accumulator. They are
modified automatically by CPU after mathematical operations. This
allows to determine the type of the result. 8086 has 16-bit status

14. 14
8086 Architecture
register. It is also called Flag Register or Program Status Word (PSW).
There are nine status flags and seven bit positions remain unused.
OF DF IF TF SF ZF AF PF CF
25
070809101112131415 04 03 02 01 0006 05
Undefined
CarryFlag
Parity
Flag
Auxiliary CarryFlag
Zero Flag
SignFlag
Trap Flag
InterruptFlag
DirectionFlag
OverflowFlag
The following Figure shows the bit definitions for the 16-bit flag
register.
• Six of the flags are status indicators reflecting properties of the result
of the last arithmetic or logical instruction
• 8086 flag word. DF, IF, and TF can be set or reset to control the
operation of the processor.
• The remaining flags are status indicators. Bits marked X are
undefined.