Computer Organization and Assembly Languages p g z y g g Yung-Yu Chuang

Intel x86 Assembly Fundamentals

Computer Organization and Assembly Languages p g z y g g Yung-Yu Chuang

with slides by Kip Irvine

x86 Assembly Language x86 Assembly Language

Fundamentals

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)Label (optional)

– Mnemonic (required)

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

Label: Mnemonic Operand(s) ;Comment

3

Labels

• Act as place markers

marks the address (offset) of code and data – marks the address (offset) of code and data

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

• Follow identifier rules D l b l

• Data label

– must be unique l

– example: myArray BYTE 10

• Code label (ends with a colon)

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

...

jmp L1

Reserved words and identifiers

• Reserved words cannot be used as identifiers

Instruction mnemonics directives type attributes – Instruction mnemonics, directives, type attributes,

operators, predefined symbols

• Identifiers Identifiers

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

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

var1 Count $first

_main MAX open_file

@@myfile xVal _12345

5

Mnemonics and operands

• Instruction mnemonics

"reminder"

– reminder

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

• Operands

• Operands

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

– Register, eax

– memory (data label), county ( ), cou t

• Number of operands: 0 to 3

– stcstc ; set Carry flag; set Carry flag

– inc ax ; add 1 to ax

– mov count, bx, ; move BX to count

Directives

• Commands that are recognized and acted upon by the assembler

by the assembler

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

instruction set

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

model, declare procedures, etc.

– case insensitive

• Different assemblers have different directives

• Different assemblers have different directives

– NASM != MASM, for example

E l d d OC

• Examples: .data .code PROC

7

Comments

• Comments are good!

– explain the program's purposeexplain the program s purpose – tricky coding techniques

– application-specific explanationspp p p

• Single-line comments

– begin with semicolon (;)g (;)

• block comments

– begin with COMMENT directive and a programmer-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 and this line is also a comment

Example: adding/subtracting integers

directive marking a comment

TITLE Add and Subtract (AddSub.asm)

; This program adds and subtracts 32-bit integers.

comment

INCLUDE Irvine32.inc .code

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

main PROC

mov eax,10000h ; EAX = 10000h add eax,40000h ; EAX = 50000h

code seg e t. 3 seg e ts: code, data, stac beginning of a procedure

source d ti ti

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

call DumpRegs ; display registers exit

destination

defined in Irvine32 inc to end a program

exit main ENDP

END main marks the last line and

defined in Irvine32.inc to end a program

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define the startup procedure

Example output

Program output showing registers and flags:

Program output, showing registers and flags:

EAX 00030000 EBX 7FFDF000 ECX 00000101 EDX FFFFFFFF 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

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 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

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END main

Program template

TITLE Program Template (Template.asm)

; Program Description:

; Author:

; Creation Date:

;

; Revisions:

; Date: Modified by:

.data

; (insert variables here) code

.code

main PROC

; (insert executable instructions here) i

exit main ENDP

; (insert additional procedures here) END main

Assemble-link execute cycle

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

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

Link Library

St 2 Step 3: Step 4:

Source File

Object File

Executable

File Output

Step 2:

assembler

Step 3:

linker

Step 4:

OS loader

Listing File

Map

Step 1: text editor File

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Defining data

Intrinsic data types

• BYTE , SBYTE

8 bit i d i t 8 bit i d i t

– 8-bit unsigned integer; 8-bit signed integer

• WORD , SWORD

– 16-bit unsigned & signed integer

• DWORD , SDWORD

– 32-bit unsigned & signed integer

• QWORD Q

– 64-bit integer

• TBYTE

• TBYTE

– 80-bit integer

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Intrinsic data types

• REAL4

4 b t IEEE h t l – 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

memory for a variable.

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

• Only size matters other attributes such as signed are

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

• Syntax:Syntax:

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

• All initializers become binary data in memory

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Integer constants

• [{+|-}] digits [radix]

Optional leading + or sign

• Optional leading + or – sign

• binary, decimal, hexadecimal, or octal digits

C di h t

• Common radix characters:

– h – hexadecimal

d d i l (d f lt) – d – decimal (default) – b – binary

r encoded real – r – encoded real – o – octal

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

Hexadecimal beginning with letter: 0A5h

Hexadecimal beginning with letter: 0A5h

Integer expressions

• Operators and precedence levels:

• Examples:

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Real number constants (encoded reals)

• Fixed point v.s. floating point

1 8 23

S E M

1 8 23

±1.bbbb×2

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

• double

1 11 52

S E M

Real number constants (decimal reals)

• [sign]integer.[integer][exponent]

i { | } sign → {+|-}

exponent → E[{+|-}]integer

• Examples:

2 2.

+3.0

-44 2E+0544.2E+05 26.E5

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Character and string constants

• Enclose character in single or double quotes

'A' " "

– 'A', "x"

– ASCII character = 1 byte

l l d bl

• Enclose strings in single or double quotes

– "ABC"

– 'xyz'

– Each character occupies a single byte

• Embedded quotes:

– ‘Say "Goodnight," Gracie’y g – "This isn't a test"

Defining BYTE and SBYTE Data

Each of the following defines a single byte of storage:

value1 BYTE 'A‘ ; character constant

l 2 BYTE 0 ll t i d b t

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

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

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Defining multiple bytes

Examples that use multiple initializers:

list1 BYTE 10 20 30 40

Examples that use multiple initializers:

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

Defining strings

• A string is implemented as an array of characters

characters

– For convenience, it is usually enclosed in quotation marks

q

– It usually has a null byte at the end

• Examples:

str1 BYTE "Enter your name",0

str2 BYTE 'Error: halting program',0

Examples:

str3 BYTE 'A','E','I','O','U'

greeting1 BYTE "Welcome to the Encryption Demo program "

BYTE "created by Kip Irvine " 0 BYTE created by Kip Irvine. ,0 greeting2 \

BYTE "Welcome to the Encryption Demo program "

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BYTE "created by Kip Irvine.",0

Defining strings

• End-of-line character sequence:

0Dh = carriage return – 0Dh = carriage return – 0Ah = line feed

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

BYTE Enter your address: ,0 newLine BYTE 0Dh 0Ah 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

string.

• Counter and argument must be constants or constant expressionsp

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

i iti li d

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

;"STACKSTACKSTACKSTACK"

var4 BYTE 10 3 DUP(0) 20

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var4 BYTE 10,3 DUP(0),20

Defining WORD and SWORD data

• Define storage for 16-bit integers or double characters

– or double characters

– single value or multiple values

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

; unsignedg

word4 WORD "AB" ; double characters myList WORD 1,2,3,4,5y , , , , ; array of words; y

array WORD 5 DUP(?) ; uninitialized array

Defining DWORD and SDWORD data

Storage definitions for signed and unsigned 32-bit Storage definitions for signed and unsigned 32-bit integers:

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

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

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Defining QWORD, TBYTE, Real Data

Storage definitions for quadwords, tenbyte values, and real numbers:

quad1 QWORD 1234567812345678h and real numbers:

q Q

val1 TBYTE 1000000000123456789Ah rVal1 REAL4 -2.1a .

rVal2 REAL8 3.2E-260 rVal3 REAL10 4.6E+4096 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 individual bytes in reverse order. The least significant byte occurs at the first (lowest) memory address

memory address.

• Example:

val1 DWORD 12345678h

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Adding variables to AddSub

TITLE Add and Subtract, (AddSub2.asm) INCLUDE Irvine32 inc

INCLUDE Irvine32.inc .data

val1 DWORD 10000h val2 DWORD 40000h 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 main ENDP END main

Declaring unitialized data

• Use the .data? directive to declare an i ti li d d t t

unintialized data segment:

.data?

• Within the segment, declare variables with "?"

initializers: (will not be assembled into .exe)

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

.data

smallArray DWORD 10 DUP(0) .data?

bigArray DWORD 5000 DUP(?)

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Mixing code and data

.code

mov eax ebx mov eax, ebx .data

temp DWORD ? temp DWORD ? .code

mov temp eax

mov temp, eax

Symbolic constants

Equal-sign directive

• name = expression

i i 32 bit i t ( i t t)

– expression is a 32-bit integer (expression or constant) – may be redefined

i ll d b li t t – name is called a symbolic constant

• good programming style to use symbols

– Easier to modify

– Easier to understand, ESC_key COUNT = 500 Array DWORD COUNT DUP(0)

COUNT=5

l COUNT

.

mov al,COUNT mov al, COUNT

COUNT=10

mov al COUNT mov al, COUNT

Calculating the size of a byte array

• current location counter: $

bt t dd f li t – 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 = 4stS e stS e ($ st) list BYTE 10,20,30,40

var2 BYTE 20 DUP(?) ListSize = ($ - list)

myString BYTE “This is a long string.”

St i l ($ St i )

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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)

li t WORD 1000h 2000h 3000h 4000h 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> Q

• Define a symbol as either an integer or text expression

expression.

• Can be useful for non-integer constants C t b d fi d

• Cannot be redefined

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EQU directive

PI EQU <3.1416>Q

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

.data

prompt BYTE pressKey

matrix1 EQU 10*10 matrix2 EQU <10*10>

matrix2 EQU <10 10>

.data

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

Addressing

Addressing Modes

Addressing Modes

32-Bit Addressing Modes

• These addressing modes use 32-bit registers

Segment + Base + (Index * Scale) + displacement

Operand types

• Three basic types of operands:

I di t t t i t (8 16 32 bit ) – Immediate – a constant integer (8, 16, or 32 bits)

• value is encoded within the instruction R i t th f i t

– Register – the name of a register

• register name is converted to a number and encoded within the instruction

encoded within the instruction

– Memory – reference to a location in memory dd i d d ithi th

• memory address is encoded within the

instruction, or a register holds the address of a memory location

memory location

45

Instruction operand notation

Direct memory operands

• A direct memory operand is a named reference to storage in memory

reference to storage in memory

• The named reference (label) is automatically dereferenced by the assembler

dereferenced by the assembler

.data

1 BYTE 10h var1 BYTE 10h, .code

l 1 AL 10h

mov al,var1 ; AL = 10h mov al,[var1] ; AL = 10h alternate format; I prefer this one.

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Direct-offset operands

A constant offset is added to a data label to produce an effective address (EA) The address is dereferenced to get effective address (EA). The address is dereferenced to get the value inside its memory location. (no range checking)

.data

arrayB BYTE 10h,20h,30h,40h d

.code

mov al,arrayB+1 ; AL = 20h

mov al,[arrayB+1] ; alternative notation mov al,[arrayB+1] ; alternative notation mov al,arrayB+3 ; AL = 40h

Direct-offset operands

A constant offset is added to a data label to produce an effective address (EA) The address is dereferenced to

data

effective address (EA). The address is dereferenced to get the value inside its memory location.

.data

arrayW WORD 1000h,2000h,3000h arrayD DWORD 1,2,3,4

.code

mov ax,[arrayW+2] ; AX = 2000h

[ W 4] AX 3000h

mov ax,[arrayW+4] ; AX = 3000h

mov eax,[arrayD+4] ; EAX = 00000002h

; will the following assemble and run?

mov ax,[arrayW-2] ; ??

[ 16]

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mov eax,[arrayD+16] ; ??

Data-Related Operators and Directives

• OFFSET Operator

• PTR Operator

• TYPE Operator p

• LENGTHOF Operator

• SIZEOF Operator

• SIZEOF Operator

• LABEL Directive

OFFSET Operator

• OFFSET returns the distance in bytes, of a label from the beginning of its enclosing segment

from the beginning of its enclosing segment – Protected mode: 32 bits

– Real mode: 16 bits

offset offset

data segment:

myByte

The Protected-mode programs we write only have a single segment (we use the flat memory model).

51

g g ( y )

OFFSET Examples

Let's assume that bVal is located at 00404000h:

.data

bVal BYTE ? wVal WORD ? dVal DWORD ? dV l2 DWORD ? dVal2 DWORD ? .code

.code

mov esi,OFFSET bVal ; ESI = 00404000 mov esi,OFFSET wVal ; ESI = 00404001 mov esi,OFFSET dVal ; ESI = 00404003 mov esi,OFFSET dVal2; ESI = 00404007

Relating to C/C++

The value returned by OFFSET is a pointer. Compare the following code written for both C++ and assembly the following code written for both C++ and assembly language:

; C++ version:

char array[1000];

char * p = &array;

char * p = &array;

.data

array BYTE 1000 DUP(?) .code

mov esi OFFSET array ; ESI is p mov esi,OFFSET array ; ESI is p

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TYPE Operator

The TYPE operator returns the size, in bytes, of a single element of a data declaration

element of a data declaration.

.data

var1 BYTE ? var2 WORD ? var3 DWORD ? var3 DWORD ? var4 QWORD ? .code

mov eax,TYPE var1 ; 1

2 2

mov eax,TYPE var2 ; 2 mov eax,TYPE var3 ; 4 mov eax TYPE var4 ; 8 mov eax,TYPE var4 ; 8

LENGTHOF Operator

The LENGTHOF operator counts the number of elements in a single data declaration

.data LENGTHOF

in a single data declaration.

byte1 BYTE 10,20,30 ; 3 array1 WORD 30 DUP(?),0,0 ; 32

2 WORD 5 DUP(3 DUP(?)) 15 array2 WORD 5 DUP(3 DUP(?)) ; 15 array3 DWORD 1,2,3,4 ; 4 digitStr BYTE "12345678",0 ; 9 digitStr BYTE 12345678 ,0 ; 9 .code

mov ecx,LENGTHOF array1 ; 32

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SIZEOF Operator

The SIZEOF operator returns a value that is equivalent to multiplying LENGTHOF by TYPE

.data SIZEOF

multiplying LENGTHOF by TYPE.

byte1 BYTE 10,20,30 ; 3 array1 WORD 30 DUP(?),0,0 ; 64 array2 WORD 5 DUP(3 DUP(?)) ; 30 array3 DWORD 1,2,3,4 ; 16 digitStr BYTE "12345678" 0 ; 9 digitStr BYTE 12345678 ,0 ; 9 .code

mov ecx,SIZEOF array1 ; 64

ALIGN Directive

• ALIGN bound aligns a variable on a byte, word, doubleword or paragraph boundary for

doubleword, or paragraph boundary for efficiency. (bound can be 1, 2, 4, or 16.)

bVal BYTE ? ; 00404000 ALIGN 2

ALIGN 2

wVal WORD ? ; 00404002 bV l2 BYTE ? 00404004 bVal2 BYTE ? ; 00404004 ALIGN 4

dVal DWORD ? ; 00404008 dVal2 DWORD ? ; 0040400C

57

PTR Operator

Overrides the default type of a label (variable).

Provides the flexibility to access part of a variable .data

myDouble DWORD 12345678h

Provides the flexibility to access part of a variable.

myDouble DWORD 12345678h .code

mov ax,myDouble o a , y oub e ; error – why?; e o y?

mov ax,WORD PTR myDouble ; loads 5678h mov WORD PTR myDouble,4321h ; saves 4321h

To understand how this works, we need to know about little endian ordering of data in memory about little endian ordering of data in memory.

Little Endian Order

• Little endian order refers to the way Intel stores integers in memory

stores integers in memory.

• Multi-byte integers are stored in reverse order, with the least significant byte stored at the

with the least significant byte stored at the lowest address

• For example the doubleword 12345678h would

• For example, the doubleword 12345678h would be stored as:

offset byte

78 0000

56 0001

offset byte

When integers are loaded from memory into registers the bytes 56

34 12

0001 0002 0003

memory into registers, the bytes are automatically re-reversed into their correct positions.

59

12 0003

PTR Operator Examples

.data

myDouble DWORD 12345678h myDouble DWORD 12345678h

12345678 5678 78 0000

offset doubleword word byte

myDouble

12345678 5678 0000

1234

78 56 34

0001

myDouble myDouble + 1

1234 34 12

0002 0003

myDouble + 2 myDouble + 3

mov al,BYTE PTR myDouble ; AL = 78h mov al BYTE PTR [myDouble+1] ; AL = 56h mov al,BYTE PTR [myDouble+1] ; AL = 56h mov al,BYTE PTR [myDouble+2] ; AL = 34h mov ax,WORD PTR [myDouble]y ; AX = 5678h mov ax,WORD PTR [myDouble+2] ; AX = 1234h

PTR Operator

PTR can also be used to combine elements of a smaller

d d h i l d Th CPU

data type and move them into a larger operand. The CPU will automatically reverse the bytes.

.data

myBytes BYTE 12h,34h,56h,78h .code

mov ax WORD PTR [myBytes] ; AX = 3412h mov ax,WORD PTR [myBytes] ; AX = 3412h mov ax,WORD PTR [myBytes+1] ; AX = 5634h mov eax,DWORD PTR myBytes, y y ; EAX ;

; =78563412h

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Your turn . . .

Write down the value of each destination operand:

.data

varB BYTE 65h,31h,02h,05h varW WORD 6543h 1202h

varW WORD 6543h,1202h varD DWORD 12345678h .code

mov ax,WORD PTR [varB+2] ; a. 0502h mov bl,BYTE PTR varD ; b. 78h

mov bl,BYTE PTR [varW+2] ; c.

mov ax WORD PTR [varD+2] ; d

78h 02h 1234h mov ax,WORD PTR [varD+2] ; d.

mov eax,DWORD PTR varW ; e.

1234h

12026543h

Spanning Multiple Lines

A data declaration spans multiple lines if each line (except the last) ends with a comma The LENGTHOF (except the last) ends with a comma. The LENGTHOF and SIZEOF operators include all lines belonging to the declaration:

.data

array WORD 10,20, array WORD 10,20, 30,40, 50,60 .code

mov eax LENGTHOF array ; 6 mov eax,LENGTHOF array ; 6 mov ebx,SIZEOF array ; 12

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Spanning Multiple Lines

In the following example, array identifies only the first WORD declaration Compare the values returned by

WORD declaration. Compare the values returned by LENGTHOF and SIZEOF here to those in the previous slide:

.data

array WORD 10,20 WORD 30,40 WORD 50,60 .code

mov eax LENGTHOF array ; 2 mov eax,LENGTHOF array ; 2 mov ebx,SIZEOF array ; 4

LABEL Directive

• Assigns an alternate label name and type to an existing storage location

storage location

• LABEL does not allocate any storage of its own; it is just an alias.

j

• Removes the need for the PTR operator data

.data

dwList LABEL DWORD wordList LABEL WORDo d st O

intList BYTE 00h,10h,00h,20h .code

mov eax,dwList ; 20001000h mov cx,wordList ; 1000h

mov dl intList ; 00h

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mov dl,intList ; 00h

Indirect operands

An indirect operand holds the address of a variable, usually an array or string. It can be dereferenced (just

d

y y g (j

like a pointer). [reg] uses reg as pointer to access memory

.data

val1 BYTE 10h,20h,30h code

.code

mov esi,OFFSET val1

mov al,[esi] ; dereference ESI (AL = 10h) inc esi

l [ i] AL 20h mov al,[esi] ; AL = 20h inc esi

inc esi

mov al,[esi] ; AL = 30h

Indirect operands

Use PTR when the size of a memory operand is ambiguous.

.data

myCount WORD 0

bl t d t i th .code

mov esi OFFSET myCount

unable to determine the size from the context

mov esi,OFFSET myCount

inc [esi] ; error: ambiguous inc WORD PTR [esi] ; ok

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Array sum example

Indirect operands are ideal for traversing an array. Note that the register in brackets must be incremented by a g y value that matches the array type.

.data

arrayW WORD 1000h,2000h,3000h code

.code

mov esi,OFFSET arrayW mov ax,[esi]

add esi,2 ; or: add esi,TYPE arrayW add ax,[esi]

dd i 2 i t S b 2

add esi,2 ; increment ESI by 2

add ax,[esi] ; AX = sum of the array

Indexed operands

An indexed operand adds a constant to a register to

generate an effective address. There are two notational

d

generate an effective address. There are two notational

forms: [label + reg] label[reg]

.data

arrayW WORD 1000h,2000h,3000h code

.code

mov esi,0

mov ax,[arrayW + esi] ; AX = 1000hy

mov ax,arrayW[esi] ; alternate format add esi,2

dd [ W i]

add ax,[arrayW + esi]

etc.

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Index scaling

You can scale an indirect or indexed operand to the offset of an array element. This is done by multiplying .data

o set o a a ay ele e t. s s do e by ult ply g the index by the array's TYPE:

.data

arrayB BYTE 0,1,2,3,4,5 arrayW WORD 0 1 2 3 4 5 arrayW WORD 0,1,2,3,4,5 arrayD DWORD 0,1,2,3,4,5

code .code

mov esi,4

mov al arrayB[esi*TYPE arrayB] ; 04 mov al,arrayB[esi*TYPE arrayB] ; 04 mov bx,arrayW[esi*TYPE arrayW] ; 0004

mov edx arrayD[esi*TYPE arrayD] ; 00000004 mov edx,arrayD[esi*TYPE arrayD] ; 00000004

Pointers

You can declare a pointer variable that contains the offset of another variable

.data

1000 2000 3000 offset of another variable.

arrayW WORD 1000h,2000h,3000h ptrW DWORD arrayW

code .code

mov esi,ptrW

mov ax,[esi] ; AX = 1000h

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