UNIT – II

Programmers Model for Intel 8086

 Introduction
• 8086 has 20 address lines, ie 1 MB of memory
• The memory has further divided in to 4
segments
• Each has 64 KB memory segment
• There are four different 64 KB segments for
instructions
• The functions of segment registers are to store
the starting address of the segments in the
memory.
Introduction

Code segment (CS)

Stack segment (SS)
Data segment (DS)
Extra segment(ES)
Introduction
 Introduction
• All general registers of the 8086
microprocessor be used for arithmetic and
logic Operations

• The l6 bit general registers are:

• Accumulator register (AX)
• Base register (BX)
• Count register (CX)
• Data register (DX)
 Introduction
• It consists of two 8-bit registers AL and AH,
which can be combined together and used
as a l6-bit register AX.

• Similar for BX, CX and DX

Introduction
 Accumulator Register:

• Accumulator can be used for I/O operations

and string manipulation
 Base register:

• BX register usually contains a data pointer

used for based, based indexed or
Register in direct addressing.
 Count register

• Count register can be used as a count in

string manipulation and shift/rotate
instructions.
 Data register

• Data register can be used as a port number in I/O operations

In integer 32-bit multiply and divide instruction

• DX register contains high-order word of the initial or resulting

number.
 Segment Registers
Code segment (CS)

• It is an l6-bit register containing address of 64 KB

segment with processor instructions.

• The processors uses CS register 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 RET instructions.
 Segment Registers
Stack segment (SS)

• It is an l6-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 are located in the stack segment

• SS register can be changed directly using POP

instruction.
 Segment Registers
 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

• DS register can be changed directly using POP and

LDS instructions.
 Segment Registers
 Extra segment (ES)

• It is a 16-bit register containing address of 64KB

segment, usually with program data.

• the processor assumes that the DI register references the

ES segment in string Manipulation instructions ES
register can b e changed directly using POP and LES
instructions.

• It is possible to change defaults segments used by

general and index registers by prefixing instructions with
a CS, SS, DS or ES prefix.
 Pointer Registers
Stack Pointer (SP)

• It is an l6-bit register pointing to program stack.

Base Pointer (BP)

• It is an l6-bit register pointing to data in the stack

segment
• BP register is usually used for based, based indexed
or register indirect addressing.
 Index Registers
• Source Index (SI)

• It is an l6-bit register.

• SI is used for indexed, based indexed and register

indirect addressing,

• Also a sourced data addressing string

manipulation instructions.
 Index Registers
• Destination Index (DI)

• It is a l6-bit register

• DI is also used for indexed, based indexed and

register indirect addressing

• Also a destination data addressing string

manipulation instructions.
 Instruction Pointer (IP)
• It is a 16-bit register

• The operation is same as the Program Counter

• The IP register is updated by the BIU to point to the

address of the next instruction.

• Programs do not have direct access to the IP

• During execution of a program the IP can be modified or

saved and restored from the stack.
 Flag register
• 6 status flags and 3 Control flags

• It is a l6-bit register containing nine l-bit flags

• Six status or condition flags (OF, SF, ZF, AF, PF, CF)

• Three control flags ( TF, DF, IF)

 Flag register
• Overflow Flag (OF) - set if the result is too large positive
number or is too small negative number to fit into destination
operand.

• Sign Flag (SF) - set if the most significant bit of the result is set.
• Zero Flag (ZF) - set if the result is zero.
• Auxiliary carry Flag (AF)- set if there was a carry from or
borrow to bits 0-3 in the AL register.
• Parity Flag (PF) - set if parity (the number of "1" bits) in the
low-order byte of the result is even.
• Carry Flag (CF) - set if there was a carry from or borrow to the
most significant bit during last result calculation
 Flag register

• Trap or Single-step Flag(TF)- if set then single-step

interrupt will occur after the next instruction.

• Direction Flag (DF) - if set then string manipulation

instructions will auto decrement index registers. If cleared then the
index registers will be auto incremented.

• Interrupt-enable Flag (IF) - setting this bit enables maskable

interrupts.
 Memory Write operation:

• To execute Memory Write Operation,

• the CPU places the value zero on the data bus, the
address(say 1250H ) on the address bus,
• Asserts the w rite line (since the CPU is writing data to
memory).
Memory Write operation:
 Memory Read operation:

• To execute Memory Read Operation,

• the CPU places the address (say1 250H) on the

address bus, assert the read line

• Then reads the resulting data from the data bus.

Memory Read operation:
Memory Organization:
 Instruction Set:

• Intel 8086 has approximately 117 instructions.

• These instructions are used to transfer data between

registers, register to memory, memory to register or
register to I/O ports and other instructions are used for
data manipulation
 Instruction Set:
• 8086 instruction set consists of

• Data moving instructions.

• Arithmetic instructions- add, subtraction, increment

decrement convert byte/word and compare.

• Logic instructions- AND, OR, exclusive OR,

shift/rotate and test.

• String manipulation instructions- load, store, move,

compare and scan for byte/word.
 Instruction Set:

• Control transfer instructions- conditional,

unconditional, call subroutine and return from
subroutine.

• Input/Output instructions.

• Other instructions- setting/clearing flag bits stack

operations, software interrupts, etc.
 Addressing modes:
• An addressing mode is the way the 8086 identifies the
operands for the instruction.
• All instructions that access the data use one or more of
the addressing modes.
• The memory address of an operand consists of two
components:
» Starting address of the memory segment
» Offset
• The starting address of the memory segment resides in
the corresponding segment register.
• When an operand is stored in a memory location, the
operand's memory location is within a memory segment
from the starting address of the segment, is called Offset
or Effective Address( EA).
 Addressing modes Conti….:
• An offset is determined by adding any combination of
three address elements:
» Displacement, Base or Index.

• The combination depends on the addressing mode of

the instruction.
Displacement:
» It is an 8-bit or l6-bit immediate value given in the
instruction
Base:
» It is the content of the base register, BX or BP
Index:
» It is the content of the index register, SI or DI
 Addressing modes Conti….:

• Memory address = starting address of the

memory segment + offset
 Addressing modes Conti….:
The 8086 has the following addressing modes:
» Register Addressing Mode

» Immediate Addressing Mode

» Direct Addressing Mode

» Register Indirect Addressing Mode

» Base Addressing Mode

» Indexed Addressing Mode

» Based Indexed Addressing Mode

» String Addressing Mode

» I/O Port Addressing Mode

» Relative Addressing Mode
» Implied Addressing Mode
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Data Transfer Instructions :
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