• Basic Elements of Assembly Language

(1)

Assembly Fundamentals

Computer Organization and Assembly Languages Yung-Yu Chuang

2006/10/30

with slides by Kip Irvine

Chapter Overview

• Basic Elements of Assembly Language

• Example: Adding and Subtracting Integers

• Assembling, Linking, and Running Programs

• Defining Data

• Symbolic Constants

Basic elements of assembly language

• Integer constants

• Integer expressions

• Character and string constants

• Reserved words and identifiers

• Directives and instructions

• Labels

• Mnemonics and Operands

• Comments

• Examples

Integer constants

• [{+|-}] digits [radix]

• Optional leading + or – sign

• binary, decimal, hexadecimal, or octal digits

• Common radix characters:

– h–hexadecimal – d–decimal (default) – b–binary

– r–encoded real – o–octal

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

Hexadecimal beginning with letter: 0A5h

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Integer expressions

• Operators and precedence levels:

• Examples:

Real number constants (encoded reals)

• Fixed point v.s. floating point

• Example 3F800000r=+1.0,37.75=42170000r

• double

S E M

1 8 23

S E M

1 11 52

±1.bbbb×2

^(E-127)

Real number constants (decimal reals)

• [sign]integer.[integer][exponent]

sign → {+|-}

exponent → E[{+|-}]integer

• Examples:

2.

+3.0

-44.2E+05 26.E5

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’

– "This isn't a test"

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Reserved words and identifiers

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

– Instruction mnemonics, directives, type attributes, operators, predefined symbols

• Identifiers

– 1-247 characters, including digits – case insensitive (by default)

– first character must be a letter, _, @, or $ – examples:

var1 Count $first _main MAX open_file

@@myfile xVal _12345

Directives

• Commands that are recognized and acted upon by the assembler

– Part of assembler’s syntax but not part of the Intel instruction set

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

– case insensitive

• Different assemblers have different directives

– NASM != MASM, for example

• Examples: .data .code PROC

Instructions

• Assembled into machine code by assembler

• Executed at runtime by the CPU

• Member of the Intel IA-32 instruction set

• Four parts

– Label (optional) – Mnemonic (required) – Operand (usually required) – Comment (optional)

Label: Mnemonic Operand(s) ;Comment

Labels

• Act as place markers

– marks the address (offset) of code and data

• Easier to memorize and more flexible mov ax, [0020] → mov ax, val

• Follow identifier rules

• Data label

– must be unique

– example: myArray BYTE 10

• Code label

– target of jump and loop instructions – example: L1: mov ax, bx

...

jmp L1

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Mnemonics and operands

• Instruction mnemonics

– "reminder"

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

• Operands

– constant (immediate value), 96 – constant expression, 2+4

– Register, eax

– memory (data label), count

• Number of operands: 0 to 3

– stc ; set Carry flag

– inc ax ; add 1 to ax

– mov count, bx ; move BX to count

Comments

• Comments are good!

– explain the program's purpose – tricky coding techniques

– application-specific explanations

• Single-line comments

– begin with semicolon (;)

• block comments

– begin with COMMENT directive and a programmer- chosen character and end with the same

programmer-chosen character COMMENT !

This is a comment

and this line is also a comment

!

Example: adding/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

main ENDP END main

directive marks comment comment

copy definitions from Irvine32.inc code segment. 3 segments: code, data, stack

beginning of a procedure source

destination

mark the last line and startup procedure

defined in Irvine32.inc to end a program

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

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Suggested coding standards

^{(1 of 2)}

• Some approaches to capitalization

– capitalize nothing – capitalize everything

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

– capitalize only directives and operators (used by the book)

• Other suggestions

– descriptive identifier names

– spaces surrounding arithmetic operators – blank lines between procedures

Suggested coding standards

^{(2 of 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 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

END main

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

main ENDP

; (insert additional procedures here) END main

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Assemble-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

Listing file

• Use it to see how your program is compiled

• Contains

– source code – addresses

– object code (machine language) – segment names

– symbols (variables, procedures, and constants)

• Example: addSub.lst

Defining data

• Intrinsic data types

• Data Definition Statement

• Defining BYTE and SBYTE Data

• Defining WORD and SWORD Data

• Defining DWORD and SDWORD Data

• Defining QWORD Data

• Defining TBYTE Data

• Defining Real Number Data

• Little Endian Order

• Adding Variables to the AddSub Program

• Declaring Uninitialized Data

Intrinsic data types

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• 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

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Intrinsic data types

^{(2 of 2)}

• REAL4

– 4-byte IEEE short real

• REAL8

– 8-byte IEEE long real

• REAL10

– 10-byte IEEE extended real

Data definition statement

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

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

• Only size matters, other attributes such as signed are just reminders for programmers.

• Syntax:

[name] directive initializer [,initializer] . . . At least one initializer is required, can be ?

• All initializers become binary data in memory

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 multiple 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:

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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 \

BYTE "Welcome to the Encryption Demo program "

BYTE "created by Kip Irvine.",0

Defining strings

^{(1 of 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:

Defining strings

^{(2 of 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

Idea: Define all strings used by your program in the same area of the data segment.

Using the DUP operator

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

• Counter and argument must be constants or constant expressions

var1 BYTE 20 DUP(0) ; 20 bytes, all 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

• Define storage for 16-bit integers – or double characters

– single value or multiple values

word1 WORD 65535 ; largest unsigned word2 SWORD –32768 ; smallest signed word3 WORD ? ; uninitialized,

; unsigned

word4 WORD "AB" ; double characters myList WORD 1,2,3,4,5 ; array of words array WORD 5 DUP(?) ; uninitialized array

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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

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 Endian 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 AddSub

TITLE Add and Subtract, (AddSub2.asm) 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

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Declaring unitialized data

• Use the .data? directive to declare an unintialized data segment:

.data?

• Within the segment, declare variables with "?"

initializers: (will not be assembled into .exe)

.data

smallArray DWORD 10 DUP(0) .data?

bigArray DWORD 5000 DUP(?)

Advantage: the program's EXE file size is reduced.

Mixing code and data

.code

mov eax, ebx .data

temp DWORD ? .code

mov temp, eax

Symbolic constants

• Equal-Sign Directive

• Calculating the Sizes of Arrays and Strings

• EQU Directive

• TEXTEQU Directive

Equal-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

– Easier to modify

– Easier to understand, ESC_key Array DWORD COUNT DUP(0) COUNT=5

mov al, COUNT COUNT=10

mov al, COUNT

COUNT = 500 .

mov al,COUNT

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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) list BYTE 10,20,30,40

ListSize = 4

list BYTE 10,20,30,40 Var2 BYTE 20 DUP(?) ListSize = ($ - list)

myString BYTE “This is a long string.”

myString_len = ($ - myString)

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

list DWORD 1,2,3,4

ListSize = ($ - list) / 4

EQU directive

• name EQU expression name EQU symbol name EQU <text>

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

• Can be useful for non-integer constant

• Cannot be redefined

EQU directive

PI EQU <3.1416>

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

.data

prompt BYTE pressKey

Matrix1 EQU 10*10 matrix1 EQU <10*10>

.data

M1 WORD matrix1 ; M1 WORD 100 M2 WORD matrix2 ; M2 WORD 10*10

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TEXTEQU directive

• name TEXTEQU <text>

name TEXTEQU textmacro name TEXTEQU %constExpr

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

• Called a text macro; can build on each other

• 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>

.code

setupAL ; generates: "mov al,10"

Chapter recap

• Basic Elements of Assembly Language

• Example: Adding and Subtracting Integers

• Assembling, Linking, and Running Programs

• Defining Data

• Symbolic Constants

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

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Real-Address Mode Programming

^{(1 of 2)}

• Generate 16-bit MS-DOS Programs

• Advantages

– enables calling of MS-DOS and BIOS functions – 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

Real-Address Mode Programming

^{(2 of 2)}

• Requirements

– INCLUDE Irvine16.inc

– Initialize DS to the data segment:

mov ax,@data mov ds,ax

Add and Subtract, 16-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