Computer Organization &
Computer Organization &
Assembly Languages Assembly Languages
Pu-Jen Cheng
Assembly Language Fundamentals
Adapted from the slides prepared by Kip Irvine for the book, Assembly Language for Intel-Based Computers, 5th Ed.
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
Integer constants
Integer expressions
Character and string constants
Reserved words and identifiers Di ti d i t ti
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:
¾ h – hexadecimal
¾ h hexadecimal
¾ d – decimal
¾ b – binary
¾ r – encoded real
Examples: 30d, 6Ah, 42, 1101b
Hexadecimal beginning with letter: 0A5h
Integer Expressions
Operators and precedence levels:
Examples:
Real Number Constants
[{+|-}] integer .[ integer ] [ exponent ]
Exponent: E[{+|-}] integer
Examples: 2., +3.0, -44.2E+05 E d d R l
Encoded Reals
¾ IEEE floating-point format (e.g. 3F800000r)
Character and String Constants
Enclose character in single or double quotes
¾ 'A', "x"
¾ ASCII character = 1 byte
Enclose strings in single or double quotes
"ABC"
¾ "ABC"
¾ 'xyz'
¾ Each character occupies a single byte
Embedded quotes:
¾ “This isn’t a test"
¾ 'Say "Goodnight," Gracie'
Reserved Words and Identifiers
Reserved words cannot be used as identifiers
¾ Instruction mnemonics (MOV), directives (.code), type attributes (BYTE, WORD), operators (=), predefined symbols (@data)
¾ See MASM reference in Appendix A
Id tifi
Identifiers
¾ 1-247 characters, including digits
¾ not case sensitive
¾ first character must be a letter, _, @, ?, or $
Examples: var1, Count, $first, _main, @@myfile
Directives
Commands that are recognized and acted upon by the assembler
¾ Not part of the Intel instruction set
¾ Used to declare code, data areas, select memory model declare procedures etc memory model, declare procedures, etc.
¾ not case sensitive
Different assemblers have different directives
¾ NASM not the same as MASM, for example
Examples: .data, .code
Instructions
Assembled into machine code by assembler
Executed at runtime by the CPU
We use the Intel IA-32 instruction set
An instruction contains:
L b l ( i l)
¾ Label (optional)
¾ Mnemonic (required)
¾ Operand (depends on the instruction)
¾ Comment (optional)
Label: Mnemonic Operand(s) ;Comment
Labels
Act as place markers
¾ marks the address (offset) of code and data
Follow identifer rules
Data label
¾ must be unique
¾ must be unique
¾ example: count DWORD 100 (not followed by colon)
Code label
¾ target of jump and loop instructions
¾ example: target: (followed by colon)
….
jmp target
Mnemonics and Operands
Instruction Mnemonics
¾ memory aid
¾ examples: MOV, ADD, SUB, MUL, INC, DEC
Operands
¾ constant (immediate value) 96
¾ constant (immediate value), 96
¾ constant expression, 2+4
¾ Register, eax
¾ memory (data label), count
Constants and constant expressions are often called immediate values
Comments
Comments are good!
¾ explain the program's purpose
¾ when it was written, and by whom
¾ revision information
¾ tricky coding techniquest c y cod g tec ques
¾ application-specific explanations
Single-line comments
¾ begin with semicolon (;)
Multi-line comments
¾ begin with COMMENT directive and a programmer-chosen character
¾ end with the same programmer-chosen character
COMMENT !
This is a comment
and this line is also a comment
!
Instruction Format Examples
No operands
¾ stc ; set Carry flag
One operand
¾ inc eax ; register
i B t
¾ inc myByte ; memory
Two operands
¾ add ebx, ecx ; register, register
¾ sub myByte, 25 ; memory, constant
¾ add eax, 36 * 25 ; register, constant-expression
NOP Instruction
¾ Used by compilers and assemblers to align codes
What's Next
Basic Elements of Assembly Language
Example: Adding and Subtracting Integers
Assembling, Linking, and Running Programs
Defining Data
Symbolic Constants
Symbolic Constants
Real-Address Mode Programming
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
main ENDP END main
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
Some approaches to capitalization
¾ capitalize nothing
¾ capitalize everything
¾ capitalize all reserved words, including
instruction mnemonics and register names
¾ capitalize only directives and operators
Other suggestions
¾ descriptive identifier names
¾ spaces surrounding arithmetic operators
¾ blank lines between procedures
Suggested Coding Standards (cont.)
Indentation and spacing
¾ code and data labels – no indentation
¾ executable instructions – indent 4-5 spaces
¾ comments: begin at column 40-45, aligned vertically
¾ 1-3 spaces between instruction and its operands
ex: mov ax,bx
¾ 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
E itP PROTO d E itC d DWORD 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
What's Next
Basic Elements of Assembly Language
Example: Adding and Subtracting Integers
Assembling, Linking, and Running Programs
Defining Data
Symbolic Constants
Real-Address Mode Programming
Assembling, Linking, and Running Programs
Assemble-Link-Execute Cycle
make32.bat
Listing File
Map File
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.
Link
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
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
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 File
Use it to see how your program is compiled
Contains
¾ source code
¾ addresses
object code (machine language)
¾ object code (machine language)
¾ segment names
¾ symbols (variables, procedures, and constants)
Example: addSub.lst
Map File
Information about each program segment:
¾ starting address
¾ ending address
¾ size
¾ segment typeseg e t type
Example: addSub.map (16-bit version)
What's Next
Basic Elements of Assembly Language
Example: Adding and Subtracting Integers
Assembling, Linking, and Running Programs
Defining Data
Symbolic Constants
Real-Address Mode Programming
Defining Data
Intrinsic Data Types
Data Definition Statement
Defining BYTE and SBYTE Data
Defining WORD and SWORD Data
Defining DWORD and SDWORD 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 (1 of 2)
BYTE, SBYTE
¾ 8-bit unsigned integer; 8-bit signed integer
WORD, SWORD
¾ 16-bit unsigned & signed integer
DWORD SDWORD
DWORD, SDWORD
¾ 32-bit unsigned & signed integer
QWORD
¾ 64-bit integer
TBYTE
¾ 80-bit integer
Intrinsic Data Types (2 of 2)
REAL4
¾ 4-byte IEEE short real
REAL8
¾ 8-byte IEEE long real
REAL10
REAL10
¾ 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
Syntax:
[name] directive initializer [,initializer] . . .
value1 BYTE 10
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
Each of the following defines a single byte of storage:
value5 SBYTE +127 ; largest signed byte value6 BYTE ? ; uninitialized byte
• A variable name is a data label that implies an offset (an address).
• If you declare a SBYTE variable, the Microsoft debugger will automatically display its value in decimal with a leading sign.
Defining Byte Arrays
list1 BYTE 10,20,30,40 list2 BYTE 10,20,30,40 BYTE 50,60,70,80
Examples that use multiple initializers:
, , , BYTE 81,82,83,84
list3 BYTE ?,32,41h,00100010b list4 BYTE 0Ah,20h,‘A’,22h
Defining Strings (1 of 3)
A string is implemented as an array of characters
¾ For convenience, it is usually enclosed in quotation marks
¾ It often will be null-terminated
Examples:
str1 BYTE "Enter your name",0
str2 BYTE 'Error: halting program',0 str3 BYTE 'A','E','I','O','U'
greeting 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 (cont.)
To continue a single string across multiple lines, end each line with a comma:
menu BYTE "Checking Account",0dh,0ah,0dh,0ah,
"1. Create a new account",0dh,0ah,
"2 Open an existing account" 0dh 0ah
"2. Open an existing account",0dh,0ah,
"3. Credit the account",0dh,0ah,
"4. Debit the account",0dh,0ah,
"5. Exit",0ah,0ah,
"Choice> ",0
Defining Strings (cont.)
End-of-line character sequence:
¾ 0Dh = carriage return
¾ 0Ah = line feed
str1 BYTE "Enter your name: ",0Dh,0Ah BYTE "E t dd " 0
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.
Syntax: counter DUP ( argument )
Counter and argument must be constants or constant expressions
var1 BYTE 20 DUP(0) ; 20 bytes, all equal to zero 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 ; 5 bytes
Defining WORD and SWORD Data
Define storage for 16-bit integers
¾ or double characters
¾ single value or multiple values
word1 WORD 65535 ; largest unsigned value word2 SWORD 32768 ; smallest signed 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
array WORD 5 DUP(?) ; uninitialized array
Defining DWORD and SDWORD Data
val1 DWORD 12345678h ; unsigned val2 SDWORD –2147483648 ; signed
Storage definitions for signed and unsigned 32-bit integers:
val3 DWORD 20 DUP(?) ; unsigned array val4 SDWORD –3,–2,–1,0,1 ; signed array
Defining QWORD, TBYTE, Real Data
quad1 QWORD 1234567812345678h
val1 TBYTE 1000000000123456789Ah rVal1 REAL4 -2 1
Storage definitions for quadwords, tenbyte values, and real numbers:
rVal1 REAL4 2.1
rVal2 REAL8 3.2E-260 rVal3 REAL10 4.6E+4096
ShortArray REAL4 20 DUP(0.0)
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, 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
Declaring Uninitialized Data
Use the .data? directive to declare an unintialized data segment:
.data?
Within the segment, declare variables with "?"
initializers:
llA DWORD 10 DUP(?) smallArray DWORD 10 DUP(?)
.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
What's Next
Basic Elements of Assembly Language
Example: Adding and Subtracting Integers
Assembling, Linking, and Running Programs
Defining Data
Symbolic Constants
Real-Address Mode Programming
Symbolic Constants
Equal-Sign Directive
Calculating the Sizes of Arrays and Strings
EQU Directive
TEXTEQU Directive
Equal-Sign Directive
name = expression
¾ expression is a 32-bit integer (expression or constant)
¾ may be redefined
¾ name is called a symbolic constant
good programming style to use symbols
good programming style to use symbols
¾ Easier to modify
¾ Easier to understand, ESC_key
¾ Array DWORD COUNT DUP(0)
¾ COUNT=5
Mov al, COUNT COUNT=10
Mov al, COUNT
COUNT = 500 .
.
mov al,COUNT
Calculating the Size of a Byte Array
Current location counter: $
¾ subtract address of list
¾ difference is the number of bytes
list BYTE 10,20,30,40 BYTE 100 DUP(0) ListSize = ($ - list)
Calculating the Size of a Word Array
Divide total number of bytes by 2 (the size of a word)
list WORD 1000h,2000h,3000h,4000h ListSize = ($ - list) / 2
Calculating the Size of a Doubleword Array
Divide total number of bytes by 4 (the size of a doubleword)
list DWORD 1,2,3,4
Li tSi ($ li t) / 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 matrix2 EQU <10*10>
.data
M1 WORD matrix1 ; M1 WORD 100 M2 WORD matrix2 ; M2 WORD 10*10
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 setupAL TEXTEQU <mov al,count>
.code
setupAL ; generates: "mov al,10"
What's Next
Basic Elements of Assembly Language
Example: Adding and Subtracting Integers
Assembling, Linking, and Running Programs
Defining Data
Symbolic Constants
Real-Address Mode Programming
Real-Address Mode Programming
Generate 16-bit MS-DOS Programs
Advantages
¾ enables calling of MS-DOS and BIOS functions
¾ no memory access restrictions
Disadvantages
Disadvantages
¾ must be aware of both segments and offsets
¾ cannot call Win32 functions (Windows 95 onward)
¾ limited to 640K program memory
Real-Address Mode Programming
(cont.)
Requirements
¾ INCLUDE Irvine16.inc
¾ Initialize DS to the data segment:
mov ax,@data mov ds,ax
Add and Subtract, 16-Bit Version
TITLE Add and Subtract, Version 2 (AddSub2r.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
Summary
Integer expression, character constant
directive – interpreted by the assembler
instruction – executes at runtime
code, data, and stack segments
li ti bj t t bl fil
source, listing, object, map, executable files
Data definition directives:
¾ BYTE, SBYTE, WORD, SWORD, DWORD, SDWORD, QWORD, TBYTE, REAL4, REAL8, and REAL10
¾ DUP operator, location counter ($)
Symbolic constant
¾ EQU and TEXTEQU
