introduction to assembly language data definition reserve words labels instruction mnemonic

Sahar Mosleh California State University San Marcos

Introduction to Assembly language

Data DefinitionReserve words

LabelsInstruction Mnemonic

Hello World example


Data Definitions

• We have covered some essential basic of computer hardware as well as specific knowledge of the IA-32 architecture

• Assembly language programmer absolutely must know their data backwards and forwards before writing executable code.

• In this chapter you will learn about how to define and declare variables and constants, using Microsoft Assembler (MASM) syntax.


Constant

• An integer constant is made up of an optional leading sign, one or more digits, and an optional suffix character indicating the number’s base:

• The radix may be one of the following (uppercase or lowercase):

h hexadecimald decimalq/o octal b binary

• Example:

26 decimal42o octal26d decimal0A3h hexadecimal11010011b binary42q Octal


Integer expression

• An integer expression is a mathematical expression involving integer values and arithmetic operators.

• The arithmetic operator are listed in below according to their precedence order from highest(1) to lowest(4).

Operator Name Precedence level

( ) Parentheses 1

+, - Unary plus, minus 2

*, / Multiply, divide 3

Mod Modulus 3

+, - Add, subtract 4


• Example:

4 + 5 * 2 multiply, add12 – 1 MOD 5 modulus, subtract-5 + 2 unary minus, add( 4+2) * 6 add, multiply

• Example:

Expression Value----------------- ---------16 / 5 ?-(3 +4) * (6-1) ?-3 + 4 * 6 – 1 ?25 mod 3 ?


Character and string constants

• Character constant is a single character enclosed in either single or double quotes. the assembler converts it to binary ASCII code matching the character.

• Example: ‘A’‘d’

• String constant is a string of characters enclosed in ether single or double quotes:

Example: ‘ABC’ ‘This is a test’


Reserved Words

• Assembly language has a list of words called reserved words. These have special meaning and can be used in their correct context. Reserved words can be any of the following:

• Instruction mnemonics, such as MOV, ADD, or MUL, which correspond to built-in operations preformed by Intel processor,

• Directives, which tell MAZM how to assemble programs.

• Attributes, which provide size and usage information for variables and operands. Examples are BYTE and WORD.

• Operators, used in constant expressions.

• A complete list of MAZM reserved words will be found in Appendix D.


Identifiers

• An identifier is a programmer chosen name. it might identify a variable, a constant, a procedure, or a code label. Keep the following in mind when creating identifiers:

• They may contain between 1 and 247 characters.• They are not case sensitive.• The first character must be either a letter (A…Z, a…z ),

underscore(_), $. Subsequent character may also be digits.• An identifier can not be the same as an assembler reserved word.

• Example of identifiers

• Var1 count $first Max open


Directive

• A directive is a command that is recognized and acted upon by the assembler as the program's source code is being assembled.

• Directives are being used for defining logical segments, choosing a model, defining variables, creating procedures, and so on.

• Different capitalization of the same directive are assumed to be equivalent. For example the assembler does not recognize any

difference between .data, .DATA, and .Data.

• Examples of directives are:

• .Data ;identifies the area of a program that contains variables;• .code ; identifies the area of a program that contains instructions;• A-name proc ;identifies the beginning of procedures;

• It would take a very long time to learn all the directives in MAZM, so we concentrate on the few that are most essential.


Instructions

• An instruction is a statement that is executed by the processor at runtime after the program has been loaded into memory and started.

• An instruction contain four basic parts:

• Label (option)• Instruction mnemonic (required)• Operand(s) (usually required)• Comments (optional)• The following diagram shows the standard format for

instructions.

Label: Mnemonic Operand(s) ;comment


label

• A lable is an identifier that acts as a place marker for either instructions (code) or data

• Example of code label:

target:mov eax, ebx…..jmp target

• Example of data label

first byte 10

Remember there is no colon after first


Instruction mnemonic

• An instruction mnemonic ( in English dictionary described as a device that assist memory) is a short word that identifies the operation carried out by an instruction.

• Examples:

• Add add two values• Sub subtract two values• Mul multiply two values• Jmp jump to new location( label)• Call call a procedure• Mov move (assign) one value to another


Operands

• Assembly language instruction can have between zero and three operands, each of which can be a register, memory operand, constant expression, or I/O port.

• We will discuss the different type of operand later.

Example Operand Type----------- ------------------96 constant (immediate value)

2+4 constant expressionEax registerCount memory


Comments

• Comments as you probably know, are an important way for the writer of a program to communicate information about how the program works to the person reading the source code.

• Comments can be specified by semicolon.

• Example:

• ;This line is comment;

• For Block comments you can use COMMENT directive and a user specified symbol.

• Example:

COMMENT &This line is commentthis line is also comment

&


Program Template

TITEL Program Template ; Program Description: ; Author: ; Creation Date: ; Revisions: ; Date: Modified by:

INCLUDE Irvine32.inc .data

; (insert variables here) .code main PROC ; (insert executable instructions here)

exit main ENDP

; (insert additional procedure here) END main


Irvine32 Library

• Irvine32 library contains useful procedures that have been written in assembly and by including them to your program, you can call them.

• The table presented in next slide references some of these procedures.


Procedure

Description

Clrscr

Crlf

Delay

DumpMem

DumpRegs

ReadChar

ReadHex

ReadInt

ReadStrng

WaitMsg

WritChar

WriteDec

WritInt

WriteString

Clear the console and locates at the upper left corner

Writes an end of the line sequence to standard output

Pauses the program execution for a specified n millisecond interval

Writes a block of memory to standard output in hexadecimal.

Displays the EAX, EBX, ECX, EDX, ESI, EDI, EBP, ESP, EFLAGS, and EIP registers in hex. Also display the Carry, Sign, Zero, and Overflow flags.

Read a single character from standard input

Reads a 32-bit hexadecimal integer from input, terminated by the Enter Key

Read a 32-bit signed decimal integer from standard input, terminated by the Enter key.

Reads a string from standard input, terminated by Enter Key.

Displays a message and waits for the enter Key to be pressed.

Writes a single character to standard output.

Writes an unsigned 32-bit integer to standard output in hex format

Writes a signed 32-bit integer to standard output in decimal format.

Writes a null terminated string to standard output.


detailed description of the procedures

• WriteString procedure writes a null terminated string to standard output. When calling it, place the string’s offset in EDX.

• Clrscr procedure clears the screen. This is typically done at the beginning and the ending of a program. If you call it at other times during a program’s execution, remember to pause the program (by calling WaitMsg ) before calling Clscr. This the will allow user to view the information already on the screen before it is erased.

• Crlf procedure advance the courser to the beginning of the next line of standard output.

• The example on the next slides puts all these information together to produce “Hello World” Message on the screen.


Example: TITLE Hello Program ; Program Description: This program will display Hello World message on the ; screen ; Author: Sahar Mosleh ; Creation Date: August 30, 2005 INCLUDE Irvine32.inc .data

; (insert variables here) prompt1 byte “Hello World”,0 ; Store the prompt1 in

; memory .code main PROC

; (insert executable instructions here)

call clrscr ; clear the screenmov edx, offset prompt1 ; move the address of prompt to

edxcall writestring ; display the string that is stored at

;the address pointed by edx

call crlf ; move the courser to next lineexit

main ENDP; (insert additional procedure here)

END main


More on library procedures

• WriteInt procedure write a 32 bit signed integer to standard output in decimal format with a leading sign and no leading Zeros. The integer needs to be placed in EAX first and then call WriteInt procedure.

• WaitMsg displays the message “press [Enter] to continue..” and

waits for the user to press the Enter key. This is useful when you want to pause the screen display before data scrolls off and disappears.

• Example on next slide


Example: TITLE Print Integer ; Program Description: This program will display integer number 216543 on the ; screen ; Author: Sahar Mosleh ; Creation Date: August 30, 2005 INCLUDE Irvine32.inc .data

; (insert variables here)

.code main PROC

; (insert executable instructions here)

call clrscr ; clear the screenmov eax, 2156543 ; move the integer to eaxcall writeInt ; display the INT that is stored at

eax call crlf ; move the courser to next line

exit main ENDP

; (insert additional procedure here) END main


Defining Data

• MASM defines various data types, each of which describes a set of values that can be assigned to variables and expressions of the given type.

• The Various integer data type is been defined in the fallowing table.

Type UsageBYTE 8-bit unsigned integer

SBYTE 8-bit signed integer

WORD 16-bit unsigned integer

SWORD 16-bit signed integer

DWORD 32-bit unsigned integer

SDWORD 32-bit signed integer

FWORD 48-bit integer

QWORD 64-bit integer

TBYTE 80-bit (10 byte) integer


Defining Byte and SBYTE Data

• The BYTE ( define byte) and SBYTE ( signed byte ) directives, used in data definition statements, allocate storage for one or more 8 bits signed or unsigned values.

• Each initializer must be an 8-bit integer or character constant.

• Example:

• Value1byte ‘A’ ; character constant

• Value2byte 0 ; Smallest unsigned byte

• Value3byte 255 ; Largest unsigned byte

• Value4sbyte -128 ; Smallest signed byte

• Value5sbyte +127 ; Largest signed byte


Variables

• A variable can be left un initialized by using a question mark for the initializer.

• This implies that the variable will be assigned a value at runtime by executable instructions.

• Example:

• Value6byte ?

• Variable name is a label that marks the offset of variable from the beginning of it’s enclosing segment.

• .data• Value1 Byte 10h• Value2 Byte 20h


Defining WORD and SWORD Data

• The WORD ( define word) and SWORD ( signed word ) directives, used in data definition statements, allocate storage for one or more 16- bit unsigned or signed integer.

.

• Example:

• word1 sword -32768 ; Smallest signed value

• word2 word 65535 ; Largest unsigned value

• word3 word ? ; un initialized, unsigned


Defining DWORD and SDWORD Data

• The DWORD ( define double word) and SDWORD ( signed double word ) directives, used in data definition statements, allocate storage for one or more 32- bit unsigned or signed integer.

.

• Example:

• value1 sdword -2147483648 ; Smallest signed

• value2 dword 12345678h ; unsigned value

• value3 dword ? ; un initialized, unsigned


ADD Instruction

• The ADD instruction adds a source operand to a destination operand of the same size. The syntax is:

•ADD dest, source

• Source is unchanged by the operation and the some is stored in the destination operand.


Example: TITLE Addition ; Program Description: This program will add two 32-bit number together and output the

result ; Author: Sahar Mosleh ; Creation Date: August 30, 2005 INCLUDE Irvine32.inc .data

var1 dword 10000var2 dword 20000Result byte “ 10000 + 20000 = “,0.code

main PROC

call clrscr ; clear the screenmov edx, offset Result ; move the address of prompt to edxcall writestringmov eax, var1 ; eax=10000add eax, var2 ; eax=10000+20000call writeint ; display the content of eax (30000)call crlf ; move the courser to next lineexit


END main


SubtractionExample: TITLE Subtraction ; Program Description: This program will subtract two 32-bit number together and output

the result ; Author: Sahar Mosleh ; Creation Date: August 30, 2005 INCLUDE Irvine32.inc .data

var1 dword 10000var2 dword 20000Result byte “ 10000 - 20000 = “,0.code

main PROC

call clrscr ; clear the screenmov edx, offset Result ; move the address of prompt to edxcall writestringmov eax, var1 ; eax=10000sub eax, var2 ; eax=10000-20000call writeint ; display the content of eax (-10000)call crlf ; move the courser to next lineexit


END main


NegateExample: TITLE Negate ; Program Description: This program will negate the number 27 ; Author: Sahar Mosleh ; Creation Date: August 30, 2005 INCLUDE Irvine32.inc .data

var1 dword 27Result byte “ The negate of 27 is “,0.code

main PROC

call clrscr ; clear the screenmov edx, offset Result ; move the address of prompt to edxcall writestringmov eax, var1 ; eax=27neg eax ; eax=-27call writeint ; display the content of eax (-27)call crlf ; move the courser to next lineexit


END main


INC and DECExample: TITLE Subtraction ; Program Description: This program will increment and decrement value of 10000. ; Author: Sahar Mosleh ; Creation Date: August 30, 2005 INCLUDE Irvine32.inc .data

var1 dword 10000INC1 byte “ increment of 10000 is“,0DEC1 byte “ Decrement of 10000 is“,0

.code main PROC

call clrscr ; clear the screenmov edx, offset INC1 ; move the address of INC1 to edxcall writestringmov eax, var1 ; eax=10000inc eax ; eax=10001call writeint ; display the content of eax (10001)call crlf ; move the courser to next linemov edx, offset DEC1 ; move the address of DEC1 to edxcall writestringmov eax, var1 ; eax=10000dec eax ; eax=9999call writeint ; display the content of eax (9999)exit


END main


Read Integer procedure

• ReadInt procedure reads a 32bit signed integer from standard input and return the value in EAX. The user can type an optional leading plus or minus singe, and the rest of the integer number can only consist of the digits.

• ReadInt will set the over flow flag and display an error message if the value entered can not be represented as a 32-bit signed integer

• See example on next slide


Example: TITLE Input Integer ; Program Description: This program will read integer from user and out put ;it on the screen ; Author: Sahar Mosleh ; Creation Date: August 30, 2005 INCLUDE Irvine32.inc .data

; (insert variables here) prompt1 byte “Please input an intege”,0 ; Store the prompt1 in

; memory .code main PROC

mov edx, offset prompt1 ; move the address of prompt to edxcall writestring ; display the string that is stored at

;the address pointed by edxcall crlf ; move the courser to next linecall Readint ;read the int from user and put it in

eaxcall writeint ;display the content of eaxexit


END main

introduction to assembly language data definition reserve words labels instruction mnemonic

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