Assembly Language for Intel

Assembly Language for Intel

Assembly Language for Intel - - Based Based Computers, 4

Computers, 4 ^{th th} Edition Edition

Chapter 3: Assembly Language Fundamentals

Kip R. Irvine

Chapter Overview Chapter Overview

• Basic Elements of Assembly Language

• Example: Adding and Subtracting Integers

• Assembling, Linking, and Running Programs

• Defining Data

• Symbolic Constants

• Real-Address Mode Programming

Basic Elements of Assembly Language Basic Elements of Assembly Language

• Integer constants

• Integer expressions

• Real-number constants

• Character and string constants

• Reserved words and identifiers

• Directives

• Instructions

• Labels

• Mnemonics

• Operands

• Comments

Integer Constants Integer Constants

• Optional leading + or – sign

• binary, decimal, hexadecimal, or octal digits

• Common radix characters:

• h – hexadecimal

• d – decimal

• b – binary

• r- encoded real

Examples: 30d, 6Ah, 42, 1101b

Hexadecimal beginning with letter: 0A5h

Integer Expressions Integer Expressions

• Operators and precedence levels:

• Examples:

Real Number Constants Real Number Constants

• Decimal Real

• [sign] integer.[integer [exponent]]

• E.g., 2., +3.0, -44.2E+05, 26.E5

• Encoded Real

• Specify a real constant in hexadecimal as an encoded real if you know the exact binary representation of the number

• E.g., 3F800000r -> +1.0

Character and String Constants Character and String Constants

• Enclose character in single or double quotes

• 'A', "x"

• ASCII character = 1 byte

• Enclose strings in single or double quotes

• "ABC"

• 'xyz'

• Each character occupies a single byte

• Embedded quotes:

• 'Say "Goodnight," Gracie'

Reserved Words and Identifiers Reserved Words and Identifiers

• Reserved words (Appendix D) cannot be used as identifiers

• Instruction mnemonics (ADD, MOV …), directives (tell MASM how to assemble programs), type attributes (BYTE, WORD …), operators (+, - …), predefined symbols (@data …)

• Identifiers

• 1-247 characters, including digits

• case insensitive (by default)

• first character must be a letter, _, @, or $

Directives Directives

• Directives

• not part of Intel instruction set

• part of the assembler’s syntax

• case insensitive

• .data, .code, proc

• Commands that are recognized and acted upon by the assembler as the program’s source code is being assembled

• Used to declare code, data areas, select memory model, declare procedures, etc.

• Different assemblers have different directives.

Instructions Instructions

• Assembled into machine code by assembler

• Executed at runtime by the CPU

• Member of the Intel IA-32 instruction set

• Parts

• Label

• Mnemonic

• Operand

• Comment

Labels Labels

• Act as place markers

• marks the address (offset) of code and data

• Follow identifier rules

• Data label

• example: first BYTE 10

• Code label

• target of jump and loop instructions

• example: L1:

target: mov ax, bx

…

jmp target

Mnemonics and Operands Mnemonics and Operands

• Instruction Mnemonics

• "reminder"

• examples: MOV, ADD, SUB, MUL, INC, DEC

• Operands

• constant (immediate value)

• constant expression

• register

• memory (data label)

Comments Comments

• Comments are good!

• explain the program's purpose

• when it was written, and by whom

• revision information

• tricky coding techniques

• application-specific explanations

• Single-line comments

• begin with semicolon (;)

• Multi-line comments

• begin with COMMENT directive and a programmer- chosen character

• end with the same programmer-chosen character

COMMENT ! comment.

also comment.

!

Instruction Format Examples Instruction Format Examples

• No operands

• stc ; set Carry flag

• One operand

• inc eax ; register

• inc myByte ; memory

• Two operands

• add ebx,ecx ; register, register

• sub myByte,25 ; memory, constant

• add eax,36 * 25 ; register, expression

Example: Adding and Subtracting Integers Example: Adding and Subtracting Integers

TITLE Add and Subtract (AddSub.asm)

; This program adds and subtracts 32-bit integers.

INCLUDE Irvine32.inc .code

main PROC

mov eax,10000h ; EAX = 10000h add eax,40000h ; EAX = 50000h sub eax,20000h ; EAX = 30000h

call DumpRegs ; display registers

exit ; call a predefined MS-Windows

; function that halts the program

; in Irvine32.inc

main ENDP

Example Output Example Output

Program output, showing registers and flags:

EAX=00030000 EBX=7FFDF000 ECX=00000101 EDX=FFFFFFFF

ESI=00000000 EDI=00000000 EBP=0012FFF0 ESP=0012FFC4

EIP=00401024 EFL=00000206 CF=0 SF=0 ZF=0 OF=0

Suggested Coding Standards [1/2]

Suggested Coding Standards [1/2]

• Some approaches to capitalization

• capitalize nothing

• capitalize everything

• capitalize all reserved words, including instruction mnemonics and register names

• capitalize only directives and operators

• Other suggestions

• descriptive identifier names

• spaces surrounding arithmetic operators

• blank lines between procedures

Suggested Coding Standards [2/2]

Suggested Coding Standards [2/2]

• Indentation and spacing

• code and data labels – no indentation

• executable instructions – indent 4-5 spaces

• comments: begin at column 40-45, aligned vertically

• 1-3 spaces between instruction and its operands

• ex: mov ax,bx

• 1-2 blank lines between procedures

Alternative Version of

Alternative Version of AddSub AddSub

TITLE Add and Subtract (AddSubAlt.asm)

; This program adds and subtracts 32-bit integers.

.386

.MODEL flat,stdcall .STACK 4096

ExitProcess PROTO, dwExitCode:DWORD DumpRegs PROTO

.code

main PROC

mov eax,10000h ; EAX = 10000h add eax,40000h ; EAX = 50000h sub eax,20000h ; EAX = 30000h call DumpRegs

INVOKE ExitProcess,0

main ENDP

Program Template Program Template

TITLE Program Template (Template.asm)

; Program Description:

; Author:

; Creation Date:

; Revisions:

; Date: Modified by:

INCLUDE Irvine32.inc .data

; (insert variables here) .code

main PROC

; (insert executable instructions here)

exit

Assembling, Linking, and Running Programs Assembling, Linking, and Running Programs

• Assemble-Link-Execute Cycle

• make32.bat

• Listing File

• Map File

Assemble

Assemble - - Link Execute Cycle Link Execute Cycle

• The following diagram describes the steps from creating a source program through executing the compiled program.

• If the source code is modified, Steps 2 through 4 must be repeated.

Source File

Object File Listing

File Link Library

Executable File Map

File

Output

Step 1: text editor

Step 2:

assembler

Step 3:

linker

Step 4:

OS loader

make32.bat make32.bat

• Called a batch file

• Run it to assemble and link programs

• Contains a command that executes ML.EXE (the Microsoft Assembler)

• Contains a command that executes LINK32.EXE (the 32-bit Microsoft Linker)

• Command-Line syntax:

make32 progName

(progName includes the .asm extension)

(use make16.bat to assemble and link Real-mode programs)

Listing and Map Files Listing and Map Files

• Listing File

• Use it to see how your program is compiled

• Contains

• source code

• offset addresses

• object code (machine language)

• segment names

• symbols (variables, procedures, and constants)

• Map File

• Information about each program segment:

• starting address

Data Types [1/2]

Data Types [1/2]

• BYTE, SBYTE

• 8-bit unsigned integer; 8-bit signed integer

• WORD, SWORD

• 16-bit unsigned & signed integer

• DWORD, SDWORD

• 32-bit unsigned & signed integer

• QWORD

• 64-bit integer

• TBYTE

• 80-bit integer

Data Types [2/2]

Data Types [2/2]

• REAL4

• 4-byte IEEE short real

• REAL8

• 8-byte IEEE long real

• REAL10

• 10-byte IEEE extended real

Data Definition Statement Data Definition Statement

• A data definition statement sets aside storage in memory for a variable.

• May optionally assign a name (label) to the data

• Syntax:

[name] directive initializer [,initializer] . . .

Defining BYTE and SBYTE Data Defining BYTE and SBYTE Data

value1 BYTE 'A' ; character constant

value2 BYTE 0 ; smallest unsigned byte value3 BYTE 255 ; largest unsigned byte value4 SBYTE -128 ; smallest signed byte value5 SBYTE +127 ; largest signed byte value6 BYTE ? ; uninitialized byte

Each of the following defines a single byte of storage:

A variable name is a data label that implies an offset (an address).

Defining Bytes Defining Bytes

list1 BYTE 10,20,30,40 list2 BYTE 10,20,30,40 BYTE 50,60,70,80 BYTE 81,82,83,84

list3 BYTE ?,32,41h,00100010b list4 BYTE 0Ah,20h,‘A’,22h

Examples that use multiple initializers:

Defining Strings [1/2]

Defining Strings [1/2]

• A string is implemented as an array of characters

• For convenience, it is usually enclosed in quotation marks

• It usually has a null byte at the end

• Examples:

str1 BYTE "Enter your name",0

str2 BYTE 'Error: halting program',0 str3 BYTE 'A','E','I','O','U'

greeting1 BYTE "Welcome to the Encryption Demo program "

BYTE "created by Kip Irvine.",0

greeting2 \

Defining Strings [2/2]

Defining Strings [2/2]

• End-of-line character sequence:

• 0Dh = carriage return

• 0Ah = line feed

str1 BYTE "Enter your name: ",0Dh,0Ah BYTE "Enter your address: ",0

newLine BYTE 0Dh,0Ah,0

Using the DUP Operator Using the DUP Operator

• Use DUP to allocate (create space for) an array or string.

• Counter must be constants or constant expressions

var1 BYTE 20 DUP(0) ; 20 bytes, all equal to zero var2 BYTE 20 DUP(?) ; 20 bytes, uninitialized

var3 BYTE 4 DUP("STACK") ; 20 bytes: "STACKSTACKSTACKSTACK"

var4 BYTE 10,3 DUP(0),20

Defining WORD and SWORD Data Defining WORD and SWORD Data

• Define storage for 16-bit integers

• or double characters

• single value or multiple values

word1 WORD 65535 ; largest unsigned value word2 SWORD –32768 ; smallest signed value word3 WORD ? ; uninitialized, unsigned word4 WORD "AB" ; double characters

myList WORD 1,2,3,4,5 ; array of words

Value Offset

0000: 1

0002: 2

0004: 3

0006: 4

0008: 5

Defining DWORD and SDWORD Data Defining DWORD and SDWORD Data

val1 DWORD 12345678h ; unsigned val2 SDWORD –2147483648 ; signed

val3 DWORD 20 DUP(?) ; unsigned array val4 SDWORD –3,–2,–1,0,1 ; signed array

Storage definitions for signed and unsigned 32-bit

integers:

Defining QWORD, TBYTE, Real Data Defining QWORD, TBYTE, Real Data

quad1 QWORD 1234567812345678h

val1 TBYTE 1000000000123456789Ah rVal1 REAL4 -2.1

rVal2 REAL8 3.2E-260 rVal3 REAL10 4.6E+4096

ShortArray REAL4 20 DUP(0.0)

Storage definitions for quadwords, tenbyte values,

and real numbers:

Little

Little Endian Endian Order Order

• All data types larger than a byte store their individual bytes in reverse order. The least significant byte occurs at the first (lowest) memory address.

• Example:

val1 DWORD 12345678h

Adding Variables to

Adding Variables to AddSub AddSub

TITLE Add and Subtract, Version 2 (AddSub2.asm)

; This program adds and subtracts 32-bit unsigned

; integers and stores the sum in a variable.

INCLUDE Irvine32.inc .data

val1 DWORD 10000h val2 DWORD 40000h val3 DWORD 20000h finalVal DWORD ? .code

main PROC

mov eax,val1 ; start with 10000h

add eax,val2 ; add 40000h

sub eax,val3 ; subtract 20000h

mov finalVal,eax ; store the result (30000h) call DumpRegs ; display the registers

exit

main ENDP

END main

Mixing Code and Data Mixing Code and Data

.code

mov eax, ebx .data

temp DWORD ? .code

mov temp, eax

…

Symbolic Constants Symbolic Constants

• Associating an identifier (a symbol) and either an integer expression or some text

• A symbolic constant does not use any storage (can not change at runtime)

• Equal-Sign Directive

• Calculating the Sizes of Arrays and Strings

• EQU Directive

• TEXTEQU Directive

Equal

Equal - - Sign Directive Sign Directive

• name = expression

• expression is a 32-bit integer (expression or constant)

• may be redefined

• name is called a symbolic constant

• good programming style to use symbols

COUNT = 500 .

.

Calculating the Size of a Byte Array Calculating the Size of a Byte Array

• current location counter: $

• subtract address of list

• difference is the number of bytes

list BYTE 10,20,30,40

ListSize = ($ - list)

Calculating the Size of a Word Array Calculating the Size of a Word Array

• current location counter: $

• subtract address of list

• difference is the number of bytes

• divide by 2 (the size of a word)

list WORD 1000h,2000h,3000h,4000h

ListSize = ($ - list) / 2

Calculating the Size of a

Calculating the Size of a Doubleword Doubleword Array Array

• current location counter: $

• subtract address of list

• difference is the number of bytes

• divide by 4 (the size of a doubleword)

list DWORD 1,2,3,4

ListSize = ($ - list) / 4

EQU Directive EQU Directive

• Define a symbol as either an integer or text expression

• Cannot be redefined

PI EQU <3.1416>

pressKey EQU <"Press any key to continue...",0>

.data

prompt BYTE pressKey

matrix1 EQU 10*10

matrix2 EQU <10*10>

TEXTEQU Directive TEXTEQU Directive

• Define a symbol as either an integer or text expression.

• Called a text macro

• Can be redefined

continueMsg TEXTEQU <"Do you wish to continue (Y/N)?">

rowSize = 5 .data

prompt1 BYTE continueMsg

count TEXTEQU %(rowSize * 2) ; evaluates the expression move TEXTEQU <mov>

setupAL TEXTEQU <move al,count>

Real Real - - Address Mode Programming Address Mode Programming

• Generate 16-bit MS-DOS Programs

• Advantages

• enables calling of MS-DOS

• no memory access restrictions

• Disadvantages

• must be aware of both segments and offsets

• cannot call Win32 functions (Windows 95 onward)

• limited to 640K program memory

• Requirements

• INCLUDE Irvine16.inc

Add and Subtract, 16

Add and Subtract, 16 - - Bit Version Bit Version

TITLE Add and Subtract, Version 2 (AddSub2.asm) INCLUDE Irvine16.inc

.data

val1 DWORD 10000h val2 DWORD 40000h val3 DWORD 20000h finalVal DWORD ? .code

main PROC

mov ax,@data ; initialize DS mov ds,ax

mov eax,val1 ; get first value add eax,val2 ; add second value

sub eax,val3 ; subtract third value mov finalVal,eax ; store the result

call DumpRegs ; display registers exit

main ENDP

END main