ch03 addressing modes

8/7/2019 Ch03 Addressing Modes

1/27

1Dr. Bassel SoudanProcessors & Assembly Language

Chapter 38088/8086 Assembly Language Programming

The Addressing Modes


Assembly Language

The native language for all microprocessors ismachine language.

However, machine language is very difficult todeal with.

Therefore, a matching symbolic language wascreated Assembly Language.

The assembly language instructions are identical tothose of the processors machine language except insymbolic form.

Each machine language instruction has a single uniquematching assembly language instruction (and viceversa).


2/27


Assembly Language Instruction

Every assembly language instruction is made up of 2parts:

Mnemonic (Opcode) Symbolic name for the operation the

instruction performs. Operands Specifiers for where the data for the instruction

is coming from (sources) and where are the results going togo (destination).

Two optional parts can be added to the instruction forreadability:

Label Symbolic name for the address of the instruction.

Comment textual description of what is the instructionbeing used for.


Example Instructions

ADD AX, BX ; Add variable X to Y

The Opcode for this instruction is ADD.

The sources for the data are the AX and BX registers.

The destination for the result is AX.

The meaning of the instruction is:

Add the value in register BX to the value in register AX.

The comment indicates that register BX has the value ofvariable X and it is being added to variable Y which is in AX.

Opcode Operands Comment


3/27


Example Instructions

START: MOV AX, BX ; Keep a copy of X

An instruction may be preceded by a label. A label is a symbolic name for the address of this

instruction.

It allows other parts of the program to refer to this pointwithout having to know the exact address.

Extremely useful for loopsThis instruction copies the contents of BX to AX.

The comment indicates that this is being done for safekeeping.

Label

Opcode

Operands Comment


The 8088/8086 Instruction Set

The 8088 and 8086 can implement 117 differentinstructions.

These instructions are grouped into six major

categories:Data Transfer Instructions

Arithmetic Instructions

Logic Instructions

String Operation Instructions

Control Transfer Instructions

Processor Control Instructions.


4/27


The 8088/8086 Addressing Modes

Each instruction may operate on data in differentlocations.

For example, an instruction may operate on data that exists

in a register or in a memory location. The method of specifying the location of the data is calledthe addressing mode of the instruction.

The 8088 and 8086 instructions can handle three types ofdata:

Data stored in registers Register Operands

Data given as part of the instruction

Immediate Operands

Data stored in memory

Memory Operands


The MOV Instruction

To assist in explaining the different addressingmodes, we will use the MOV instruction.

The MOV instruction allows copying data from thesource to a destination.

A very versatile instruction that allows operatingon many data types and on data in basically allpossible locations.

None

Flags affected

D SMOV D, SCopyMOV

operationFormatMeaningMnemonic


5/27


Register Operand Addressing

Most of the operations are done on values storedin the internal registers of the microprocessor.

Therefore, almost every assembly languageinstruction will need a way to access values inregisters.

Values in registers are accessed by specifyingthe registers name in the operands section of aninstruction.

Some instructions operate only on data in

registers while others combine data in registerswith other types of data.

Instructions that operate only on data in registers arecalled register addressing mode instruction.


Register Addressing Mode

All sources of the instruction are registers. The destination of the result is also a register.

All user accessible registers can be specified assource or destination.

Only the Data Registers can be accessed as byte(8 bits) or word (16 bits).

All other registers can only be accessed as words(16 bits).

Operand sizes must match.


6/27


Register Mode Example

xx02001

xx02000

Next Instruction01102

CA01101

MOV DX, CS8C01100

InstructionMemory

Content

Address

Main Memory0100 IP

0100 CS0200 DS

ES

SS

AX

BX

CX

xxxx DX

SP

BP

SI

DI

MPU


Register Mode Example

xx02001

xx02000


CA01101

MOV DX, CS8C01100

InstructionMemory

Content

Address

Main MemoryIP

CS

DS

ESSS

AX

BX

CX

DX

SP

BP

SI

DI

MPU


7/27


Register Mode Exception

Some instruction use registers implicitly.For example, multiplication results always go to

registers AX and DX. Can not change thiscombination.

Reason: To save machine languagecombinations.

Disadvantage: programmer/compiler restricted.

Some of these were removed in later versions ofthe x86 architecture.


Immediate Addressing Mode

MOV AX, 15H

One of the operands is available immediately as

part of the instruction.The immediate operand is constant data.

The data is fixed at the time the program is written andwill not change from execution to execution.

Used to implement high level language initializationstatements like: a = 21;

Immediate should match size of other operand.

Opcode Immediate operand


8/27


Immediate Mode Example


1501001

MOV AH, 15HB001000

InstructionMemory

Content

Address

Main Memory0000 IP

0100 CS

DS

ES

SS

AX

BX

CX

DX

SP

BP

SI

DI

MPU

xx




1501001

MOV AH, 15HB001000

InstructionMemory

Content

Address

Main MemoryIP

CS

DS

ESSS

AX

BX

CX

DX

SP

BP

SI

DI

MPU


9/27



Write a set of instructions to store the value1245H into register CX.


Addressing Operands in Memory

There are five modes for addressing operandsstored in memory:

1. Direct Addressing

2. Register Indirect Addressing3. Based Addressing

4. Indexed Addressing

5. Based-indexed Addressing


10/27


Addressing Operands in Memory

To reference an operand in memory, the8088/8086 must calculate the physical address

(PA) for the operand and then initiate a read orwrite operations of this storage location

PA is computed from a Segment Base Address(SBA) and an offset.

The different memory addressing modes differ inhow you specify the SBA and the offset.


Direct Addressing Mode

MOV AX, [1234H]

The offset is a constant determined at program

time and is given explicitly in the instruction. The brackets around the number tell the assembler that

this is a direct mode instruction and not an immediatemode.

The offset is taken to be from the start of the DataSegment.

Therefore, the PA addressed in this instruction isDS:1234H


11/27


Direct Mode Example

xx02001

FE03235

xx01004

ED03234

xx02000

1201003

3401002

0E01001MOV CX, [1234H]8B01000

InstructionMemory

Content

Address

Main Memory

0000 IP

0100 CS0200 DS

ES

SS

AX

BX

xxxx CX

DX

SP

BP

SI

DI

MPU


Direct Mode Example

02001

03235

xx01004

03234

02000

1201003

3401002

0E01001

MOV CX, [1234H]8B01000

InstructionMemory

Content

Address

Main Memory

IP

CS

DS

ESSS

AX

BX

CX

DX

SP

BP

SI

DI

MPU


12/27


Register Indirect Addressing Mode

MOV AX, [SI]

The offset is taken from a register.The offset is no longer fixed. It is now dynamic.

Only a specific set of registers can be used to holdthe offset:

BX, BP, SI, and DI.

The offset is taken from the start of the Stack Segment ifBP is used. Otherwise its the Data Segment.

Useful for implementing pointer operations in highlevel languages:

a = *b;


Register Indirect Mode Example

MOV AX, [SI]8B01000

0401001

xx01002

xx01003

xx02001

FE03235

xx01004

ED03234

xx02000

InstructionMemory

Content

Address

Main Memory

0000 IP

0100 CS0200 DS

ESSS

xxxx AX

BX

CX

DX

SP

BP

1234H SI

DI

MPU


13/27


Register Indirect Mode Example

MOV AX, [SI]8B010000401001

xx01002

xx01003

xx02001

FE03235

xx01004

ED03234

xx02000

InstructionMemory

Content

Address

Main Memory

IP

0100 CS0200 DS

ES

SS

AX

BX

CX

DX

SP

BP

SI

DI

MPU


Based Addressing Mode

The offset is generated by adding either a director indirect displacement to a base register.

Allows for accessing elements of an array. To implement a[6] in a high level language, a base

register is used to hold the address of a[0] and a directoffset (6) is used to reach the actual element.

To implement a[i], a base register is used to hold theaddress of a[0] and an indirect displacement is used tospecify the i.

To access the next element in the array, the registerholding the indirect displacement is simply incremented.


14/27


Based Addressing Mode

MOV [BX]+1234H, AL

The offset is generated by adding either a director indirect displacement to a base register.

Only the base register BX or base pointer BP canbe used as the base.

The displacement can be:

A 16 bit direct displacement.

An indirect displacement from one of the 4 registers BX,

BP, SI, and DI. The offset is taken from the start of the Stack Segment if

BP is used. Otherwise its the Data Segment.


Based Addressing Mode Example 1

MOV [BX]+1234H, AL8801000

8701001

3401002

1201003

xx02001

xx03235

xx01004

xx03234

xx02000

InstructionMemory

Content

Address

Main Memory

0000 IP

0100 CS0200 DS

ESSS

BE AX

1000 BX

CX

DX

SP

BP

SI

DI

MPU

ED


15/27



MOV [BX]+1234H, AL88010008701001

3401002

1201003

02001

03235

xx01004

03234

02000

InstructionMemory

Content

Address

Main Memory

IP

0100 CS0200 DS

ES

SS

AX

BX

CX

DX

SP

BP

SI

DI

MPU



MOV [BX]+[SI], AX8801000

8701001

3401002

1201003

xx02001

xx03235

xx01004

xx03234

xx02000

InstructionMemory

Content

Address

Main Memory

0000 IP

0100 CS0200 DS

ESSS

BEED AX

1000 BX

CX

DX

SP

BP

1234 SI

DI

MPU


16/27



MOV [BX]+[SI], AX88010008701001

3401002

1201003

02001

03235

xx01004

03234

02000

InstructionMemory

Content

Address

Main Memory

0000 IP

0100 CS0200 DS

ES

SS

AX

BX

CX

DX

SP

BP

SI

DI

MPU


Indexed Addressing Mode

MOV AL, [SI]+1234H

The offset is generated by either adding a direct

or indirect displacement to an index register.Only the Source Index (SI) or Destination Index

(DI) registers can be used.

The displacement can be:

A 16 bit direct displacement.

An indirect displacement from one of BX, BP, SI, and DI.

The offset is taken from the start of the Stack Segment ifBP is used. Otherwise its the Data Segment.

Another method for accessing array elements.


17/27


Indexed Addressing Mode Example 1

MOV AL, [SI]+1234H8A010008401001

3401002

1201003

xx02001

xx05235

xx01004

BE05234

xx02000

InstructionMemory

Content

Address

Main Memory

0000 IP

0100 CS0200 DS

ES

SS

xx AX

BX

CX

DX

SP

BP

2000 SI

DI

MPU

xx


Indexed Addressing Mode Example 1

MOV AL, [SI]+1234H8A01000

8401001

3401002

1201003

02001

05235

xx01004

05234

02000

InstructionMemory

Content

Address

Main Memory

IP

0100 CS0200 DS

ESSS

AX

BX

CX

DX

SP

BP

SI

DI

MPU


18/27


Based-Indexed Addressing Mode

MOV AH, [BX][SI]+1234H

The offset is generated by adding Base registerand Indexed register and an optional immediatedisplacement.

Useful for high level language array operationslike a[i+5].

The base register would be set to the address of a[0].

The index register would be used for i.

The immediate displacement would be the constantvalue 5.

The offset is taken from the start of the StackSegment if BP is used. Otherwise its the DataSegment.


Based-Indexed Addressing Mode Example 1

MOV AH, [BX][SI]+1234H8A01000

A001001

3401002

1201003

xx02001

ED06235

xx01004

BE06234

xx02000

InstructionMemory

Content

Address

Main Memory

0000 IP

0100 CS0200 DS

ESSS

xx AX

1000 BX

CX

DX

SP

BP

2000 SI

DI

MPU

xx


19/27


Based-Indexed Addressing Mode Example 1

MOV AH, [BX][SI]+1234H8A01000A001001

3401002

1201003

02001

06235

xx01004

06234

02000

InstructionMemory

Content

Address

Main Memory

0000 IP

0100 CS0200 DS

ES

SS

AX

BX

CX

DX

SP

BP

SI

DI

MPU


Based-Indexed Addressing Examples

Assume

CS = 0000H

BX = 0040H

SI = 0002H

ORG 0040HTBL: DB 9, 8, 7 ,6 ,5

MOV AX, CS

MOV DS, AX DS = 0000H

MOV AL, [BX]+1

PA = 00000 + 0040 + 1 = 00041 AL = 08

MOV AL, [SI]+TBL

PA = 00000 + 0002 + 0040 = 00042 AL = 07

MOV AL, [BX+SI]+1PA = 00000 + 0040 + 0002 + 1 = 00043 AL = 06

0500044

0600043

0700042

0800041

0900040


20/27


Memory Addressing Summary

Direct Addressing

{ }Direct:DSPA =



Register Indirect Addressing

=

DI

SI

BP

BX

:DS

SSPA


21/27



Based Addressing

+

=ntdisplacemebit16

ntdisplacemebit8

BP

BX:

DS

SSPA



Indexed Addressing

+

=ntdisplacemebit16

ntdisplacemebit8

DI

SI:DSPA


22/27



Based Indexed Addressing

+

+

=ntdisplacemebit16

ntdisplacemebit8

DI

SI

BP

BX:

DS

SSPA


The very versatile MOV instruction

Seg-regMem16

Seg-regReg16

Mem16Seg-reg

Reg16Seg-reg

ImmediateMemory

ImmediateRegister

RegisterMemory

MemoryRegister

RegisterRegister

MemoryAccumulator

AccumulatorMemory

SourceDestination

None

Flags affected

D SMOV D, SCopyMOV

operationFormatMeaningMnemonic


23/27


Development of an Assembly Language Program

An assembly language program passes throughseveral stages.

Edit Time Writing of the source programSymbolic Assembly Language

Output is prog.asm

The program will contain symbolic names such aslabels, named constants, initialized memoryarrays, reserved memory areas, etc.

Will use symbolic names for instructions.

What appears to be the same instruction may translate todifferent machine codes when addressing mode info isincorporated.


Development of an Assembly Language Program

Assembly Time Translation of the symbolic assemblylanguage to the matching machine language.

Labels, symbolic constants, etc. are replaced with theirequivalent values.

Two output files: prog.lst Listing file

Text file that shows the actual translation of the assemblylanguage to the matching machine language.

prog.obj Object file

Binary program file that will be executed by the processor.

Link Time (Optional) combine several files together tocreate one executable program.

Output file prog.exe

Run Time Actual execution of the machine language onthe microprocessor.


24/27


Assembler Directives

Directives are commands given to the assembler to tell itwhat to do.

They are not part of the program.

The ORG directiveORG 1000H

This directive tells the Assembler where in memory the firstinstruction should go.

The location of all other instructions is determined based onthis.

Every program needs to start with an ORG.

It is possible to have multiple ORG directives.

This will place different sections of the program in differentlocations in memory.



EQU directiveLEN EQU 32

Used for defining named constants.

Everywhere the assembler sees the sequenceLEN in the following program, it will replace it with32.

This is done before instructions are translated to machinecode.

Allows for quickly changing sizes of arrays or lists.


25/27



DB DirectiveMSG DB hello

Define Byte reserve a sequence of bytes and set them tothe values indicated.

The first byte will be reserved wherever this directiveappears relative to the ORG.

Example:ORG 1000H

MSG: DB helloSTART: MOV AX, MSG

This program segment will reserve 5 memory locationsstarting at offset 1000H and fill them up with the ASCII codefor the characters h e l l o respectively.

The instruction at the label START sets the AX register tothe address associated with the label MSG.

The address of the first of the 5 bytes.



DW DirectiveMSG DW 4455H

Define Word reserve a sequence of words and set them tothe values indicated.

The first byte will be reserved wherever this directiveappears relative to the ORG.

Example:ORG 1000H

CONST: DW 4455H

START: MOV AX, MSG

This program segment will reserve the memory location atoffset 1000H and fill it with the value 55H and location at

offset 1001H and fill it with the value 44H.


26/27



RESB DirectiveINPUT RESB 5

Reserve a number of byte sized memorylocations.

This directive simply reserves the locations. Itdoes not set their contents.

Useful for reserving storage space for values thatwill be generated during program execution.

Example usage:ORG 1000H

INPUT: RESB 5START: MOV AX, INPUT

MOV [AX], BL ; Save 8 bit value in reserved word



RESW DirectiveINPUT RESW 5

Reserve a number of word sized memorylocations.

This directive simply reserves the locations. Itdoes not set their contents.

Useful for reserving storage space for values thatwill be generated during program execution.

Example usage:ORG 1000H

INPUT: RESW 5

START: MOV AX, INPUT

MOV [AX], BX ; Save 16 bit value in reserved word


27/27


Operand Specification

The assembler is smart enough to do some translationsand/or calculations.

An operand may be specified as one of the following: A decimal value (-390 is an acceptable value)

An ASCII code (a, 0)

A symbolic name (LEN)

A simple operation (LEN+2)

The assembler will replace LEN with its value, add 2 to thatvalue and use the result in coding the equivalent machine

instruction.

LEN + 2 is not calculated during assembly not execution.

The assembler will automatically translate all of these toHexadecimal before coding the machine instructions.

ch03 addressing modes

Documents