4-The Art of Assembly Language Programming

Microprocessor and Programming

The Art of Assembly Language Programming

4. The art of Assembly Language Programming 08 Marks Syllabus: Program development steps defining problem, writing algorithms, flowchart, initialization 4.1 checklist, choosing instructions, converting algorithms to assembly language programs. Assembly Language Programming Tools Editors, Assembler, Linker, Debugger. 4.2 Assembler directives and operators 4.3 Program development steps:

In order to develop programs in any language, certain steps are followed. Following these steps is helpful to efficiently develop the programs. 1. Defining the problem:

The first step in writing a program is to think carefully the problem that is to be solved with the help of program. If a program works fine, but it does not solve s olve the problem according accord ing to the requirement, it is of no use. In this step program is not written but it is known „what the program must do‟. 2. Representing program operations: Algorithm:

The formula or sequence of operations used to solve a programming problem is called the algorithm. Flowchart:

The flowchart is graphical representation of sequence of operations to solve a problem. In flow chart different operations are represented by certain symbols. Operation

Symbol

Start and stop

………………………

Processing

………………………

Input/Output/

………………………

Read/Write/ Load/Store Subroutine/

………………………

Procedure

Decision making

………………………

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Connector

………………………

Direction of control flow

………………………


3. Initialization checklist:

The program may be using variables and constants. It may be using segments, flags, I/O ports. Before using these entities it may be required to initialize them. It is better to prepare a list of items to be initialized. Once the list is available, all parameters in the list can be checked whether these are initialized properly or not. 4. Choosing instructions: instructions:

Next step is to determine the instructions i nstructions required for f or different parts of program. pro gram. For this, you must know all the instructions available in the instruction set of microprocessor and their working. For each instruction its general form, number and type of operands which will be used with instruction should be known. It must be cleared that which flags are affected b y different instructions. instructions . 5. Converting algorithms to assembly language programs:

Once instructions have been selected for different operations, these are arranged in sequence as per algorithm. If different segment registers are used, such as data segment, stack segment, extra segment, they must be initialized properly. pro perly. If variables are used, they th ey must be declared first and initialized. Jump J ump and loop labels must be used carefully at proper locations. Procedures/subroutines, if used, must be defined carefully. In the last, program must be terminated properly.

Assembly language programming tools: Editor:

An editor is a program which allows you to create a file containing the assembly language statements or instructions of your program. The main function of an editor is to help you to construct assembly language program in right format. The file created by the editor is in ASCII codes and called as source file. The source file is saved with .ASM extension. This source file is used by assembler to create an object file which has machine language instructions of your program. Any editor can be used in order to create source file e.g. DOS based text editor EDIT or Windows based text editor Note Pad etc. Computer Department, Jamia Polytechnic, Akkalkuwa

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Assembler:

An assembler is a program which is used to translate the assembly language program to machine language program. The assembler reads the source file from disk where it is stored. The translation may be done in one pass or two passes. In first pass, the assembler creates a symbol table in which displacement of named data items and offset of the labels used in the programs is stored. After successful assembly, it generates two file. The first file is called object file and has an extension “.OBJ”. This file contains the machine code for the assembly language instructions. The second file generated by assembler is a list file and has an extension “.LST”. This file has some extra information

as compared to object file such as assembly language statements, binary or machine code for each instruction and offset of each instruction. TASM (Turbo Assembler) is assembler from Borland Software Corporation and MASM is Microsoft Macro Assembler from Microsoft Corporation.

Source file

Assembler

Object file

fileName.ASM

TASM / MASM

FileName.OBJ

Linker:

A linker is a program used to link (or combine/join) several object files into one executable program. When the programs are large, they are usually divided into smaller modules. Each module is individually written, tested and debugged. After this, all modules can be linked together to form a large program.

Object file

Linker

FileName.OBJ

Executable file FileName.EXE

The linker produces relocatable executable files. It also produces a link map file which contains the address information about the linked files. TLINK is a linker from Borland Software Corporation and LINK is a linker from Microsoft Corporation.

Debugger :

Debugger is a program which allows you to load machine language program in memory and execute it. Using debugger, contents of registers and memory locations can be examined. These contents can Computer Department, Jamia Polytechnic, Akkalkuwa

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be changed and you can c an re-execute re-execu te your program. pro gram. Using debugger debug ger programs can be executed execu ted in single step mode. A debugger allows setting break points in program. The program will be executed up to the break point and stop. Registers and an d memory locations l ocations can be examined ex amined to see if the results are correct at that point. If the results are correct, the break point can be removed and can be set to a new location. If the results are not correct, the program can be checked to fix the problem. TD is a debugger from Borland Software Corporation and Code View CV is a debugger from Microsoft Corporation.

Assembler Directives:

Assembler provides a number of reserved words. These reserved words are known as assembler directives. Assembler directives are helpful in writing programs. These direct or tell the assembler to perform a particular task. Since these are not the part of 8086 instruction set, machine code is not generated for assembler directives. ASSUME:

This directive is used to inform the assembler the name of logical segment. This name is used to refer to a particular segment. In assembly language programming each segment is given a name e.g. code segment may be given a name as CODE, data segment as DATA and so on. General form: ASSUME

segment_register: segment_name, segment_register: segment_ name … where segment_register may be CS, DS, ES, SS and segment_name is the name given to the segment.

For example, ASSUME CS: CODE, CODE, DS: DS: DATA

DB: Define Byte

This directive is used to define a byte type variable. It can be used to define a single-byte variable or a multi-byte variable. General form: varable_name DB value or values separated by comma For example, NUM1 DB 10H

; Hexadecimal value

NUM2 DB 15

; Decimal value

BLOCK1 DB 10H, 20H, 30H, 40H, 50H RESULT DB ?

; Array of hexadecimal values

; value undefined

SNAME DB „BATLIWALA MALIK‟

; String of bytes


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DW: Define Word

This directive is used to define a word type (i.e. 2 byte) variable. It can be used to define a single-word variable or a multiple-word variable. General form: varable_name DW value or values separated by comma For example, NUM1 DW 1230H NUM2 DW 15H BLOCK1 DW 100H, 2000H, 3045H, 7409H, 5100H RESULT DW ?

; Array of hexadecimal values

; value undefined

DD: Define Double Word

This directive is used to define a double word type (i.e. 4 byte) variable. It can be used to define a single or a multiple double word variable. General form: varable_name DD value or values separated by comma For example, NUM1 DD 12345678H

DQ: Define Quad Word

This directive is used to define a quad word type (i.e. 8 byte) variable. It can be used to define a single or a multiple quad word variable. General form: varable_name DQ value or values separated by comma

DT: Define Ten Bytes

This directive is used to define a ten byte type variable. It can be used to define a single or a multiple ten-byte variable. General form: variable_name DT value or values separated by comma

EQU: Equate

This directive is used to give a name to some value or symbol. Each time, while assembling, the assembler finds the given name in the program, it will replace the name with the value or symbol. General form: Computer Department, Jamia Polytechnic, Akkalkuwa

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symbol_name EQU value or symbol For example, COUNTER EQU 10 ADDITION EQU ADD ASCII_ADJUST EQU AAA Once a name is given to a value or symbol, this name can be used in place of that value or symbol. For example, MOV CX, COUNTER



MOV CX, 10

ADDITION AL, BL



ADD AL, BL

END: End program

This directive is used after the last statement of a program to tell the assembler that this is the end of the program. The assembler will ignore any statements after and END directive. Some assemblers require a label after END directive. This label is same as the label used for first instruction in code segment. General form: END Label For example, END START END MAIN

ENDP: End procedure

This directive along with procedure name is used to indicate the end of a procedure. General form: procedure_name ENDP For example, FACTORIAL ENDP

ENDS: End Segment

This directive is used with the name of a segment to indicate the end of logical segment. General form: segment_name ENDS For example, CODE ENDS DATA ENDS Computer Department, Jamia Polytechnic, Akkalkuwa

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In the above statements, CODE and DATA are the segment names.

SEGMENT: Start segment

This directive is used to indicate the start of a logical segment. Preceding the SEGMENT directive is the name of segment. General form: segment_name SEGMENT For example, CODE SEGMENT DATA SEGMENT

PROC: Procedure

This directive is used to identify the start of a procedure. This directive is used after the name of procedure. After PROC directive, the terms NEAR or FAR is used to give the type of procedure. General form: procedure_name PROC NEAR/FAR For example, FACTORIAL PROC FAR At the end of procedure ENDP directive is used to indicate the end of procedure.

MACRO:

This directive informs the assembler the beginning of a macro. Before this directive, name of macro is used. After this directive, macro arguments can be given. At the end of macro ENDM directive is used to indicate end of the macro. General form: macro_name MACRO argument1, argument2…

For example, DISPLAY MACRO message In the above statement, DISPLAY is macro name and message is an argument.

ENDM: End Macro

This directive informs the assembler the end of the macro. This directive is used as last statement of a macro. General form: ENDM Computer Department, Jamia Polytechnic, Akkalkuwa

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EVEN: Align on even memory address

When assembler starts assembly process, it initializes a location counter and goes on incrementing it as assembly proceeds. It uses location counter value to assign addresses to variables, constants, instructions etc. the EVEN directive tells the assembler to increment the location counter to next even number if it is not already at even address. General form: EVEN For example, DATA SEGMENT AVGS DB 9 DUP(?)

; Location counter has a value 0009H after this declaration

EVEN

; Increment the location counter to next even number i.e. 000AH

SALES DW 100 DUP(0) ; …….

DATA ENDS

ALIGN: Align according to the number

This directive is used to force the assembler to align the next data item or instruction according to given value. General form: ALIGN NUMBER The NUMBER must be a power of 2 such as 2, 4, 8, 16, 32….

For example , ALIGN 4 In the above statement the assembler advances the location counter which is divisible by 4. In data segment, assembler fills unused bytes with 0s. In code segment, assembler fills NOP instructions for unused locations.

ORG: Originate

As assembler assembles the program, it uses a location counter to keep track of the allotted addresses. If the ORG statement is not used, the location counter is initialized to 0000H. If an „ORG

value

’

statement is present at the start of a logical segment then first data item or first instruction will be placed at an offset „value . For example, if ORG 0200H statement is present at the start of code ’

segment, then the code will be placed from offset 0200H in code segment. In other words, the location counter will be initialized with a value 0200H instead of 0000H. $ symbol can be used to refer current value of location counter. Computer Department, Jamia Polytechnic, Akkalkuwa

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LENGTH:

This directive is an operator which tells the assembler to determine the number of elements in some named data item, such as a string or an array. When a statement like

is used, the assembler will determine the no.

of elements in string1 and transfer it to CX.

PTR: Pointer This is used to declare the type of a label, variable or memory operand. This is used with BYTE or WORD. If used with BYTE, then the particular label, variable or memory operand is treated as an 8 bit quantity. quanti ty. If used u sed with WORD, then the particular partic ular label, variable or memory operand oper and is treated as a 16 bit quantity. For Example,

MOV

AL,

BYTE PTR[SI]

INC

BYTE PTR[BX]

MOV

BX,WORD PTR[2000H]

INC

WORD PTR[3000H]

OFFSET:

This directive tells the assembler to determine the offset or displacement of particular label from the start of the segment which contains it. Consider the fig. below: (See next page)

The offset of NUM1 can be determined and transferred into BX as follows:

MOV

BX, OFFSET NUM1

;BX=2000H


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SEG: Find segment address of a label

This directive tells the assembler to find the segment address of a label, variable or procedure e.g. let my_array is a variable defined in data segment that has DS=7000H.

MOV

AX, SEG my_array

In the above statement, SEG my_array will return my_array‟s segment address which is 7000H and

MOV instruction will transfer that into AX. So after execution of this instruction AX=7000H.

EXTERN: External and PUBLIC:

This directive is used to tell the assembler that the names, procedures and labels declared after this directive have already been defined in some other assembly module. While in the other module where the names, procedures and labels actually appear, they must be declared PUBLIC using PUBLIC directive. Suppose if one wants to call a procedure Factorial defined in module1 from module2, in module1, it must be declared PUBLIC and in module2, it must be declared EXTERN. Following code fragment explains this:

module1 PUBLIC

SEGMENT FACTORIAL

module1

ENDS

module2

SEGMENT

EXTERN

FACTORIAL:

Module2

FAR

FAR

ENDS

GLOBAL:

The GLOBAL directive can be used in place of PUBLIC or EXTERN directive. This directive makes a name or symbol available to other modules. For Example,

GLOBAL

DIVISOR makes DIVISOR public.

GROUP:

This directive is used to tell assembler to group the logical segment names which appear after the directive into one logical group segment. This allows the contents of all segments to be accessed from same group segment base. For Example,

SMALL_SYSTEM ASSUME

GROUP

CODE,

DATA,

STACK_SEG

CS: SMALL_SYSTEM, DS: SMALL_SYSTEM, SS: SMALL_SYSTEM


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INCLUDE: Include source code from file

This directive tells the assembler to insert a block of source code from the named file into current source module.


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4-The Art of Assembly Language Programming

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