Transcript Introduction to assembly programming

Introduction to Assembly language
Data Definition
Reserve words
Labels
Instruction Mnemonic
Hello World example
Sahar Mosleh
California State University San Marcos
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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.
Sahar Mosleh
California State University San Marcos
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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
hexadecimal
d
decimal
q/o
octal
b
binary
• Example:
26
42o
26d
0A3h
11010011b
42q
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decimal
octal
decimal
hexadecimal
binary
Octal
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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).
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Operator
Name
Precedence level
()
Parentheses
1
+, -
Unary plus, minus 2
*, /
Multiply, divide
3
Mod
Modulus
3
+, -
Add, subtract
4
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• Example:
4+5*2
12 – 1 MOD 5
-5 + 2
( 4+2) * 6
multiply, add
modulus, subtract
unary minus, add
add, multiply
• Example:
Expression
----------------16 / 5
-(3 +4) * (6-1)
-3 + 4 * 6 – 1
25 mod 3
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Value
--------?
?
?
?
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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’
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California State University San Marcos
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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.
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California State University San Marcos
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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
Max
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count
open
$first
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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.
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California State University San Marcos
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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:
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Mnemonic
Operand(s)
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;comment
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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
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California State University San Marcos
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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
• Sub
• Mul
• Jmp
• Call
• Mov
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add two values
subtract two values
multiply two values
jump to new location( label)
call a procedure
move (assign) one value to another
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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
----------96
2+4
Eax
Count
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Operand Type
-----------------constant (immediate value)
constant expression
register
memory
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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 comment
this line is also comment
&
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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
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California State University San Marcos
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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.
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California State University San Marcos
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Procedure
Clrscr
Crlf
Delay
DumpMem
DumpRegs
ReadChar
ReadHex
ReadInt
ReadStrng
WaitMsg
WritChar
WriteDec
WritInt
WriteString
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Description
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.
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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.
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California State University San Marcos
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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
mov
call
clrscr
edx, offset prompt1
writestring
call
crlf
exit
main ENDP
; (insert additional procedure here)
END main
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; clear the screen
; move the address of prompt to edx
; display the string that is stored at
;the address pointed by edx
; move the courser to next line
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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
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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
mov
call
call
clrscr
eax, 2156543
writeInt
crlf
; clear the screen
; move the integer to eax
; display the INT that is stored at eax
; move the courser to next line
exit
main ENDP
; (insert additional procedure here)
END main
Sahar Mosleh
California State University San Marcos
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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
Usage
BYTE
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
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California State University San Marcos
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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:
• Value1
• Value2
• Value3
• Value4
byte
byte
byte
sbyte
• Value5
sbyte +127 ; Largest signed byte
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‘A’
0
255
-128
; character constant
; Smallest unsigned byte
; Largest unsigned byte
; Smallest signed byte
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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:
• Value6
byte
?
• Variable name is a label that marks the offset of variable from
the beginning of it’s enclosing segment.
• .data
• Value1
• Value2
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Byte
Byte
10h
20h
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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
• word2
• word3
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sword -32768
word 65535
word ?
; Smallest signed value
; Largest unsigned value
; un initialized, unsigned
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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
• value3
dword
dword
12345678h
?
; unsigned value
; un initialized, unsigned
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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.
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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 10000
var2 dword 20000
Result byte
“ 10000 + 20000 = “,0
.code
main PROC
call
clrscr
mov
edx, offset Result
call
writestring
mov
eax, var1
add
eax, var2
call
writeint
call
crlf
exit
main ENDP
; (insert additional procedure here)
END main
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; clear the screen
; move the address of prompt to edx
; eax=10000
; eax=10000+20000
; display the content of eax (30000)
; move the courser to next line
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Example:
Subtraction
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 10000
var2 dword 20000
Result byte
“ 10000 - 20000 = “,0
.code
main PROC
call
clrscr
mov
edx, offset Result
call
writestring
mov
eax, var1
sub
eax, var2
call
writeint
call
crlf
exit
main ENDP
; (insert additional procedure here)
END main
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; clear the screen
; move the address of prompt to edx
; eax=10000
; eax=10000-20000
; display the content of eax (-10000)
; move the courser to next line
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Negate
Example:
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 27
Result byte
“ The negate of 27 is “,0
.code
main PROC
call
clrscr
mov
edx, offset Result
call
writestring
mov
eax, var1
neg
eax
call
writeint
call
crlf
exit
main ENDP
; (insert additional procedure here)
END main
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; clear the screen
; move the address of prompt to edx
; eax=27
; eax=-27
; display the content of eax (-27)
; move the courser to next line
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INC and DEC
Example:
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
10000
INC1
byte
“ increment of 10000 is“,0
DEC1
byte
“ Decrement of 10000 is“,0
.code
main PROC
call
clrscr
mov
edx, offset INC1
call
writestring
mov
eax, var1
inc
eax
call
writeint
call
crlf
mov
edx, offset DEC1
call
writestring
mov
eax, var1
dec
eax
call
writeint
exit
main ENDP
; (insert additional procedure here)
END main
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; clear the screen
; move the address of INC1 to edx
; eax=10000
; eax=10001
; display the content of eax (10001)
; move the courser to next line
; move the address of DEC1 to edx
; eax=10000
; eax=9999
; display the content of eax (9999)
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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
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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 edx
call
writestring
; display the string that is stored at
;the address pointed by edx
call
crlf
; move the courser to next line
call
Readint
;read the int from user and put it in eax
call
writeint
;display the content of eax
exit
main ENDP
; (insert additional procedure here)
END main
Sahar Mosleh
California State University San Marcos
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