Introduction: operations with the help of the


8086 Microprocessor is an upgraded form of the 8085
Microprocessor. This 16-bit microprocessor provides a storage capacity of 1MB
and comprises 20 address lines and 16 data lines.

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It can operate in two
modes; Maximum and Minimum modes respectively. Maximum mode is apt when the
system has multiple processors and Minimum mode is for a single processor

Microprocessor is divide into two:


Execution Unit                                                                                               Bus
Interface Unit




The EU passes
instructions to the BIU on where to fetch data from, how to decode and further
execute those instructions. It controls all operations with the help of the
instruction decoder and the ALU (Arithmetic and Logic Unit). It is connected to
the system only through the BIU.

The ALU performs
arithmetic and logical operations like addition, subtraction, OR, Not, etc.

1 Flag Register: A
16-bit register that relies on the accumulator result for its status. It
contains 9 flags further divided into Conditional and Control flags.

Conditional flags:

1.      Carry
flag: Set to 1 when there is an overflow in arithmetic operations.

2.      Auxiliary
flag: Set to 1 when a carry/borrow is generated in the lower nibble and passed
on to the higher nibble.

3.      Parity
flag: Set to 1 when the lower order 8-bit result contains even number of 1’s.
Set to 0 when there are odd number of 1’s.

4.      Zero
flag: Set to 1 when an arithmetic or logical operation result is zero.

5.      Sign
flag: Set to 1 when an operation results in a negative.

6.      Overflow
flag: Set to 1 when system capacity is exceeded.

Control flags:

1.      Direction
flag: Set to 1 when it is required access the higher memory address in string

2.      Interrupt
flag: Set to 1 when interrupt conditions needs enabling which means that the
interruption of a program has to be allowed.

3.      Trap
flag: Set to 1 when the program is to be run in single step mode which means
one instruction is executed at a time to help in debugging.

2 General Purpose
Register: AH, AL, BH, BL, CH, CL, DH and DL are the 8 general purpose
registers. They can either be used individually (8-bit) or in pairs (16-bit).

AH and AL = AX

BH and BL = BX

CH and CL = CX

DH and DL = DX

AX register: Accumulator register;
stores operands for arithmetic operations

BX register: Base register; stores the
initial base address of memory within data segment

CX register: Counter register; stores
loop counter in loop instructions

DX register: stores I/O port address for
I/O instructions

3 Stack Pointer
Register: It stored the starting address of the segment top the memory location
in 16-bit form of where the most recent word was stored on the stack.

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