Intel x86 Assembly Fundamentals

Intel x86 Assembly Fundamentals

Computer Organization and Assembly Languages Yung-Yu Chuang

2007/12/10

with slides by Kip Irvine

Intel microprocessor history

Early Intel microprocessors

• Intel 8080 (1972) – 64K addressable RAM – 8-bit registers

– CP/M operating system – 5,6,8,10 MHz

– 29K transistros

• Intel 8086/8088 (1978) – IBM-PC used 8088 – 1 MB addressable RAM – 16-bit registers

– 16-bit data bus (8-bit for 8088) – separate floating-point unit (8087) – used in low-cost microcontrollers now

my first computer

The IBM-AT

• Intel 80286 (1982) – 16 MB addressable RAM – Protected memory

– several times faster than 8086 – introduced IDE bus architecture – 80287 floating point unit

– Up to 20MHz – 134K transistors

Intel IA-32 Family

• Intel386 (1985)

– 4 GB addressable RAM – 32-bit registers

– paging (virtual memory) – Up to 33MHz

• Intel486 (1989)

– instruction pipelining – Integrated FPU – 8K cache

• Pentium (1993)

– Superscalar (two parallel pipelines)

Intel P6 Family

• Pentium Pro (1995)

– advanced optimization techniques in microcode – More pipeline stages

– On-board L2 cache

• Pentium II (1997)

– MMX (multimedia) instruction set – Up to 450MHz

• Pentium III (1999)

– SIMD (streaming extensions) instructions (SSE) – Up to 1+GHz

• Pentium 4 (2000)

– NetBurst micro-architecture, tuned for multimedia – 3.8+GHz

• Pentium D (2005, Dual core)

IA-32 Architecture

IA-32 architecture

• Lots of architecture improvements, pipelining, superscalar, branch prediction, hyperthreading and multi-core.

• From programmer’s point of view, IA-32 has not

changed substantially except the introduction

of a set of high-performance instructions

Modes of operation

• Protected mode

– native mode (Windows, Linux), full features, separate memory

• Real-address mode

– native MS-DOS

• System management mode

– power management, system security, diagnostics

• Virtual-8086 mode

• hybrid of Protected

• each program has its own 8086 computer

Addressable memory

• Protected mode

– 4 GB

– 32-bit address

• Real-address and Virtual-8086 modes

– 1 MB space – 20-bit address

General-purpose registers

CS SS DS

ES

EIP EFLAGS

16-bit Segment Registers EAX

EBX ECX EDX

32-bit General-Purpose Registers

FS GS EBP

ESP ESI EDI

Accessing parts of registers

• Use 8-bit name, 16-bit name, or 32-bit name

• Applies to EAX, EBX, ECX, and EDX

AH AL

16 bits 8

AX

EAX 8

32 bits 8 bits + 8 bits

Index and base registers

• Some registers have only a 16-bit name for their lower half (no 8-bit aliases). The 16-bit registers are usually used only in real-address mode.

Some specialized register uses

• General-Purpose

– EAX – accumulator (automatically used by division and multiplication)

– ECX – loop counter

– ESP – stack pointer (should never be used for arithmetic or data transfer)

– ESI, EDI – index registers (used for high-speed memory transfer instructions)

– EBP – extended frame pointer (stack)

Some specialized register uses

• Segment

– CS – code segment – DS – data segment – SS – stack segment

– ES, FS, GS - additional segments

• EIP – instruction pointer

• EFLAGS

– status and control flags

– each flag is a single binary bit (set or clear)

• Some other system registers such as IDTR, GDTR, LDTR etc.

Status flags

• Carry

– unsigned arithmetic out of range

• Overflow

– signed arithmetic out of range

• Sign

– result is negative

• Zero

– result is zero

• Auxiliary Carry

– carry from bit 3 to bit 4

• Parity

– sum of 1 bits is an even number

Floating-point, MMX, XMM registers

• Eight 80-bit floating-point data registers

– ST(0), ST(1), . . . , ST(7) – arranged in a stack

– used for all floating-point arithmetic

• Eight 64-bit MMX registers

• Eight 128-bit XMM registers for single-instruction multiple-data (SIMD) operations

ST(0) ST(1) ST(2) ST(3) ST(4) ST(5) ST(6) ST(7)

IA-32 Memory Management

Real-address mode

• 1 MB RAM maximum addressable (20-bit address)

• Application programs can access any area of memory

• Single tasking

• Supported by MS-DOS operating system

Segmented memory

Segmented memory addressing: absolute (linear) address is a combination of a 16-bit segment value added to a 16- bit offset

00000 10000 20000 30000 40000 50000 60000 70000 80000 90000 A0000 B0000 C0000 D0000 E0000 F0000

8000:0000 8000:FFFF

seg ofs

8000:0250 0250

linear addresses

one segment (64K)

Calculating linear addresses

• Given a segment address, multiply it by 16 (add a hexadecimal zero), and add it to the offset

• Example: convert 08F1:0100 to a linear address

Adjusted Segment value: 0 8 F 1 0 Add the offset: 0 1 0 0 Linear address: 0 9 0 1 0

• A typical program has three segments: code, data and stack. Segment registers CS, DS and SS are used to store them separately.

Example

What linear address corresponds to the segment/offset address 028F:0030?

028F0 + 0030 = 02920

Always use hexadecimal notation for addresses.

Protected mode

• 4 GB addressable RAM (32-bit address)

– (00000000 to FFFFFFFFh)

• Each program assigned a memory partition which is protected from other programs

• Designed for multitasking

• Supported by Linux & MS-Windows

Protected mode

• Segment descriptor tables

• Program structure

– code, data, and stack areas – CS, DS, SS segment descriptors – global descriptor table (GDT)

• MASM Programs use the Microsoft flat memory

model

Flat segmentation model

• All segments are mapped to the entire 32-bit physical address space, at least two, one for data and one for code

• global descriptor table (GDT)

Multi-segment model

• Each program has a local descriptor table (LDT) – holds descriptor for each segment used by the program

3000 RAM

00003000

Local Descriptor Table

0002 00008000 000A 00026000 0010

base limit access

8000 26000

multiplied by 1000h

Paging

• Virtual memory uses disk as part of the memory, thus allowing sum of all programs can be larger than physical memory

• Divides each segment into 4096-byte blocks called pages

• Page fault (supported directly by the CPU) – issued by CPU when a page must be loaded from disk

• Virtual memory manager (VMM) – OS utility that manages the loading and unloading of pages

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) – 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 (ends with a colon)

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

...

jmp L1

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

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

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

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 marking a comment comment

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

beginning of a procedure source

destination

marks the last line and define the 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

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

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

Defining data

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

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

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"

Intrinsic data types

• 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

Intrinsic data types

• 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:

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

• 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

• 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

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

main ENDP END main

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

• 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

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

Addressing

Operand types

• Three basic types of operands:

– Immediate – a constant integer (8, 16, or 32 bits)

• value is encoded within the instruction – Register – the name of a register

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

– Memory – reference to a location in memory

• memory address is encoded within the

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

Instruction operand notation Direct memory operands

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

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

.data

var1 BYTE 10h, .code

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

Direct-offset operands

.data

arrayB BYTE 10h,20h,30h,40h .code

mov al,arrayB+1 ; AL = 20h

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

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

Direct-offset operands

.data

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

.code

mov ax,[arrayW+2] ; AX = 2000h mov ax,[arrayW+4] ; AX = 3000h

mov eax,[arrayD+4] ; EAX = 00000002h A constant offset is added to a data label to produce an effective address (EA). The address is dereferenced to get the value inside its memory location.

; will the following assemble and run?

mov ax,[arrayW-2] ; ??

mov eax,[arrayD+16] ; ??

Your turn. . .

Write a program that rearranges the values of three doubleword values in the following array as: 3, 1, 2.

.data

arrayD DWORD 1,2,3

• Step 2: Exchange EAX with the third array value and copy the value in EAX to the first array position.

•Step1: copy the first value into EAX and exchange it with the value in the second position.

mov eax,arrayD

xchg eax,[arrayD+4]

xchg eax,[arrayD+8]

mov arrayD,eax

Evaluate this . . .

• We want to write a program that adds the following three bytes:

.data

myBytes BYTE 80h,66h,0A5h

• What is your evaluation of the following code?

mov ax,myBytes add ax,[myBytes+1]

add ax,[myBytes+2]

• What is your evaluation of the following code?

mov al,myBytes add al,[myBytes+1]

add al,[myBytes+2]

Evaluate this . . .

myBytes BYTE 80h,66h,0A5h

Yes: Move zero to BX before the MOVZX instruction.

• How about the following code. Is anything missing?

movzx ax,myBytes mov bl,[myBytes+1]

add ax,bx

mov bl,[myBytes+2]

add ax,bx ; AX = sum

Data-Related Operators and Directives

• OFFSET Operator

• PTR Operator

• TYPE Operator

• LENGTHOF Operator

• SIZEOF Operator

• LABEL Directive

OFFSET Operator

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

– Protected mode: 32 bits – Real mode: 16 bits

offset

myByte data segment:

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

OFFSET Examples

.data

bVal BYTE ? wVal WORD ? dVal DWORD ? dVal2 DWORD ? .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 Let's assume that bVal is located at 00404000h:

Relating to C/C++

; C++ version:

char array[1000];

char * p = &array;

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

.data

array BYTE 1000 DUP(?) .code

mov esi,OFFSET array ; ESI is p

ALIGN Directive

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

efficiency. (bound can be 1, 2, 4, or 16.) bVal BYTE ? ; 00404000

ALIGN 2

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

dVal DWORD ? ; 00404008

dVal2 DWORD ? ; 0040400C

PTR Operator

.data

myDouble DWORD 12345678h .code

mov ax,myDouble ; error – why?

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

Overrides the default type of a label (variable).

Provides the flexibility to access part of a variable.

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

Little Endian Order

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

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

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

78 0000 56 34 12

0001 0002 0003 offset byte

When integers are loaded from memory into registers, the bytes are automatically re-reversed into their correct positions.

PTR Operator Examples

.data

myDouble DWORD 12345678h

12345678 5678 0000

1234 78 56 34 12

0001 0002 0003 offset doubleword word byte

myDouble myDouble + 1

myDouble + 2 myDouble + 3

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

PTR Operator

.data

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

mov ax,WORD PTR [myBytes] ; AX = 3412h mov ax,WORD PTR [myBytes+2] ; AX = 5634h mov eax,DWORD PTR myBytes ; EAX

; =78563412h PTR can also be used to combine elements of a smaller data type and move them into a larger operand. The CPU will automatically reverse the bytes.

Your turn . . .

.data

varB BYTE 65h,31h,02h,05h varW WORD 6543h,1202h varD DWORD 12345678h .code

mov ax,WORD PTR [varB+2] ; a.

mov bl,BYTE PTR varD ; b.

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

mov ax,WORD PTR [varD+2] ; d.

mov eax,DWORD PTR varW ; e.

Write down the value of each destination operand:

0502h 78h 02h 1234h 12026543h

TYPE Operator

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

.data

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

mov eax,TYPE var1 ; 1 mov eax,TYPE var2 ; 2 mov eax,TYPE var3 ; 4 mov eax,TYPE var4 ; 8

LENGTHOF Operator

.data LENGTHOF

byte1 BYTE 10,20,30 ; 3 array1 WORD 30 DUP(?),0,0 ; 32 array2 WORD 5 DUP(3 DUP(?)) ; 15 array3 DWORD 1,2,3,4 ; 4 digitStr BYTE "12345678",0 ; 9 .code

mov ecx,LENGTHOF array1 ; 32

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

SIZEOF Operator

.data SIZEOF

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 .code

mov ecx,SIZEOF array1 ; 64

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

Spanning Multiple Lines

.data

array WORD 10,20, 30,40,

50,60 .code

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

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

Spanning Multiple Lines

.data

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

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

In the following example, array identifies only the first WORD declaration. Compare the values returned by LENGTHOF and SIZEOF here to those in the previous slide:

LABEL Directive

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

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

• Removes the need for the PTR operator .data

dwList LABEL DWORD wordList LABEL WORD

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

mov eax,dwList ; 20001000h mov cx,wordList ; 1000h mov dl,intList ; 00h

Indirect Operands

.data

val1 BYTE 10h,20h,30h .code

mov esi,OFFSET val1

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

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

mov al,[esi] ; AL = 30h

An indirect operand holds the address of a variable, usually an array or string. It can be dereferenced (just like a pointer). [reg] uses reg as pointer to access memory

Indirect Operands

.data

myCount WORD 0 .code

mov esi,OFFSET myCount

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

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

unable to determine the size from the context

Array Sum Example

.data

arrayW WORD 1000h,2000h,3000h .code

mov esi,OFFSET arrayW mov ax,[esi]

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

add esi,2 ; increment ESI by 2 add ax,[esi] ; AX = sum of the array Indirect operands are ideal for traversing an array. Note that the register in brackets must be incremented by a value that matches the array type.

Indexed Operands

.data

arrayW WORD 1000h,2000h,3000h .code

mov esi,0

mov ax,[arrayW + esi] ; AX = 1000h mov ax,arrayW[esi] ; alternate format add esi,2

add ax,[arrayW + esi]

etc.

An indexed operand adds a constant to a register to generate an effective address. There are two notational

forms: [label + reg] label[reg]

Index Scaling

.data

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

mov esi,4

mov al,arrayB[esi*TYPE arrayB] ; 04 mov bx,arrayW[esi*TYPE arrayW] ; 0004 mov edx,arrayD[esi*TYPE arrayD] ; 00000004

You can scale an indirect or indexed operand to the offset of an array element. This is done by multiplying the index by the array's TYPE:

Pointers

.data

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

.code

mov esi,ptrW

mov ax,[esi] ; AX = 1000h You can declare a pointer variable that contains the offset of another variable.

Data Transfers Instructions

MOV instruction

• Move from source to destination. Syntax:

MOVdestination, source

• Source and destination have the same size

• No more than one memory operand permitted

• CS, EIP, and IP cannot be the destination

• No immediate to segment moves

MOV instruction

.data

count BYTE 100 wVal WORD 2 .code

mov bl,count mov ax,wVal mov count,al

mov al,wVal ; error mov ax,count ; error mov eax,count ; error

Your turn . . .

Explain why each of the following MOV statements are invalid:

.data

bVal BYTE 100 bVal2 BYTE ? wVal WORD 2 dVal DWORD 5 .code

mov ds,45 ; a.

mov esi,wVal ; b.

mov eip,dVal ; c.

mov 25,bVal ; d.

mov bVal2,bVal ; e.

Memory to memory

.data

var1 WORD ? var2 WORD ? .code

mov ax, var1 mov var2, ax

Copy smaller to larger

.data

count WORD 1 .code

mov ecx, 0 mov cx, count .data

signedVal SWORD -16 ; FFF0h .code

mov ecx, 0 ; mov ecx, 0FFFFFFFFh mov cx, signedVal

MOVZX and MOVSX instructions take care of extension for both sign and unsigned integers.

Zero extension

mov bl,10001111b

movzx ax,bl ; zero-extension When you copy a smaller value into a larger destination, the MOVZX instruction fills (extends) the upper half of the destination with zeros.

1 0 0 0 1 1 1 1

1 0 0 0 1 1 1 1

Source

Destination 0 0 0 0 0 0 0 0

0

The destination must be a register.

movzx r32,r/m8 movzx r32,r/m16 movzx r16,r/m8

Sign extension

mov bl,10001111b

movsx ax,bl ; sign extension

The MOVSX instruction fills the upper half of the destination with a copy of the source operand's sign bit.

1 0 0 0 1 1 1 1

1 0 0 0 1 1 1 1

Source

Destination 1 1 1 1 1 1 1 1

The destination must be a register.

MOVZX MOVSX

From a smaller location to a larger one

mov bx, 0A69Bh

movzx eax, bx ; EAX=0000A69Bh movzx edx, bl ; EDX=0000009Bh movzx cx, bl ; EAX=009Bh mov bx, 0A69Bh

movsx eax, bx ; EAX=FFFFA69Bh movsx edx, bl ; EDX=FFFFFF9Bh movsx cx, bl ; EAX=FF9Bh

LAHF/SAHF (load/store status flag from/to AH) .data

saveflags BYTE ? .code

lahf

mov saveflags, ah ...

mov ah, saveflags sahf

S,Z,A,P,C flags are copied.

XCHG Instruction

XCHGexchanges the values of two operands. At least one operand must be a register. No immediate operands are permitted.

.data

var1 WORD 1000h var2 WORD 2000h .code

xchg ax,bx ; exchange 16-bit regs xchg ah,al ; exchange 8-bit regs xchg var1,bx ; exchange mem, reg xchg eax,ebx ; exchange 32-bit regs xchg var1,var2 ; error 2 memory operands

Exchange two memory locations

.data

var1 WORD 1000h var2 WORD 2000h .code

mov ax, val1 xchg ax, val2 mov val1, ax

Arithmetic Instructions

Addition and Subtraction

• INC and DEC Instructions

• ADD and SUB Instructions

• NEG Instruction

• Implementing Arithmetic Expressions

• Flags Affected by Arithmetic

– Zero – Sign – Carry – Overflow

INC and DEC Instructions

• Add 1, subtract 1 from destination operand

– operand may be register or memory

• INC destination

• Logic: destination ← destination + 1

• DEC destination

• Logic: destination ← destination – 1

INC and DEC Examples

.data

myWord WORD 1000h

myDword DWORD 10000000h .code

inc myWord ; 1001h dec myWord ; 1000h inc myDword ; 10000001h mov ax,00FFh

inc ax ; AX = 0100h

mov ax,00FFh

inc al ; AX = 0000h

Your turn...

Show the value of the destination operand after each of the following instructions executes:

.data

myByte BYTE 0FFh, 0 .code

mov al,myByte ; AL = mov ah,[myByte+1] ; AH =

dec ah ; AH =

inc al ; AL =

dec ax ; AX =

FFh 00h FFh 00h FEFF

ADD and SUB Instructions

•ADD destination, source

• Logic: destination ← destination + source

•SUB destination, source

• Logic: destination ← destination – source

• Same operand rules as for the MOV instruction

ADD and SUB Examples

.data

var1 DWORD 10000h var2 DWORD 20000h

.code ; ---EAX---

mov eax,var1 ; 00010000h add eax,var2 ; 00030000h add ax,0FFFFh ; 0003FFFFh add eax,1 ; 00040000h sub ax,1 ; 0004FFFFh

NEG (negate) Instruction

.data

valB BYTE -1 valW WORD +32767 .code

mov al,valB ; AL = -1

neg al ; AL = +1

neg valW ; valW = -32767 Reverses the sign of an operand. Operand can be a register or memory operand.

Suppose AX contains –32,768 and we apply NEG to it.

Will the result be valid?

Implementing Arithmetic Expressions

Rval DWORD ? Xval DWORD 26 Yval DWORD 30 Zval DWORD 40 .code

mov eax,Xval

neg eax ; EAX = -26 mov ebx,Yval

sub ebx,Zval ; EBX = -10 add eax,ebx

mov Rval,eax ; -36

HLL compilers translate mathematical expressions into assembly language. You can do it also. For example:

Rval = -Xval + (Yval – Zval)

Your turn...

mov ebx,Yval neg ebx

add ebx,Zval mov eax,Xval sub ebx

mov Rval,eax

Translate the following expression into assembly language.

Do not permit Xval, Yval, or Zval to be modified:

Rval = Xval - (-Yval + Zval) Assume that all values are signed doublewords.

Flags Affected by Arithmetic

• The ALU has a number of status flags that

reflect the outcome of arithmetic (and bitwise) operations

– based on the contents of the destination operand

• Essential flags:

– Zero flag – destination equals zero – Sign flag – destination is negative – Carry flag – unsigned value out of range – Overflow flag – signed value out of range

• The MOV instruction never affects the flags.

Concept Map

status flags

ALU conditional

jumps

branching logic arithmetic &

bitwise operations

part of

used by provide attached to

affect

CPU

executes

executes

Zero Flag (ZF)

mov cx,1

sub cx,1 ; CX = 0, ZF = 1 mov ax,0FFFFh

inc ax ; AX = 0, ZF = 1 inc ax ; AX = 1, ZF = 0

Whenever the destination operand equals Zero, the Zero flag is set.

A flag is set when it equals 1.

A flag is clear when it equals 0.

Sign Flag (SF)

mov cx,0

sub cx,1 ; CX = -1, SF = 1 add cx,2 ; CX = 1, SF = 0 The Sign flag is set when the destination operand is negative. The flag is clear when the destination is positive.

The sign flag is a copy of the destination's highest bit:

mov al,0

sub al,1 ; AL=11111111b, SF=1 add al,2 ; AL=00000001b, SF=0

Carry Flag (CF)

The Carry flag is set when the result of an operation generates an unsigned value that is out of range (too big or too small for the destination operand).

mov al,0FFh

add al,1 ; CF = 1, AL = 00

; Try to go below zero:

mov al,0

sub al,1 ; CF = 1, AL = FF

In the second example, we tried to generate a negative value. Unsigned values cannot be negative, so the Carry flag signaled an error condition.

Carry Flag (CF)

• Addition and CF: copy carry out of MSB to CF

• Subtraction and CF: copy inverted carry out of MSB to CF

• INC/DEC do not affect CF

• Applying NEG to a nonzero operand sets CF

Your turn . . .

mov ax,00FFh

add ax,1 ; AX= SF= ZF= CF=

sub ax,1 ; AX= SF= ZF= CF=

add al,1 ; AL= SF= ZF= CF=

mov bh,6Ch

add bh,95h ; BH= SF= ZF= CF=

mov al,2

sub al,3 ; AL= SF= ZF= CF=

For each of the following marked entries, show the values of the destination operand and the Sign, Zero, and Carry flags:

0100h 0 0 0 00FFh 0 0 0 00h 0 1 1 01h 0 0 1

FFh 1 0 1

Overflow Flag (OF)

The Overflow flag is set when the signed result of an operation is invalid or out of range.

; Example 1 mov al,+127

add al,1 ; OF = 1, AL = ??

; Example 2

mov al,7Fh ; OF = 1, AL = 80h add al,1

The two examples are identical at the binary level because 7Fh equals +127. To determine the value of the destination operand, it is often easier to calculate in hexadecimal.

A Rule of Thumb

• When adding two integers, remember that the Overflow flag is only set when . . .

– Two positive operands are added and their sum is negative

– Two negative operands are added and their sum is positive

What will be the values of OF flag?

mov al,80h

add al,92h ; OF = mov al,-2

add al,+127 ; OF =

Your turn . . .

mov al,-128

neg al ; CF = OF =

mov ax,8000h

add ax,2 ; CF = OF = mov ax,0

sub ax,2 ; CF = OF = mov al,-5

sub al,+125 ; CF = OF =

What will be the values of the Carry and Overflow flags after each operation?

0 1

0 0

1 0

0 1

Signed/Unsigned Integers: Hardware Viewpoint

• All CPU instructions operate exactly the same on signed and unsigned integers

• The CPU cannot distinguish between signed and unsigned integers

• YOU, the programmer, are solely responsible for using the correct data type with each instruction

Overflow/Carry Flags: Hardware Viewpoint

• How the ADD instruction modifies OF and CF:

– CF = (carry out of the MSB)

– OF = (carry out of the MSB) XOR (carry into the MSB)

• How the SUB instruction modifies OF and CF:

– NEG the source and ADD it to the destination – CF = INVERT (carry out of the MSB)

– OF = (carry out of the MSB) XOR (carry into the MSB)

Auxiliary Carry (AC) flag

• AC indicates a carry or borrow of bit 3 in the destination operand.

• It is primarily used in binary coded decimal (BCD) arithmetic.

mov al, oFh

add al, 1 ; AC = 1

Parity (PF) flag

• PF is set when LSB of the destination has an even number of 1 bits.

mov al, 10001100b

add al, 00000010b ; AL=10001110, PF=1 sub al, 10000000b ; AL=00001110, PF=0

Jump and Loop

JMP and LOOP Instructions

• Transfer of control or branch instructions

– unconditional – conditional

• JMP Instruction

• LOOP Instruction

• LOOP Example

• Summing an Integer Array

• Copying a String

JMP Instruction

top:

. .

jmp top

• JMPis an unconditional jump to a label that is usually within the same procedure.

• Syntax: JMP target

• Logic: EIP ← target

• Example:

LOOP Instruction

• The LOOP instruction creates a counting loop

• Syntax: LOOP target

• Logic:

• ECX ← ECX – 1

• if ECX != 0, jump to target

• Implementation:

• The assembler calculates the distance, in bytes, between the current location and the offset of the target label. It is called the relative offset.

• The relative offset is added to EIP.

LOOP Example

The following loop calculates the sum of the integers 5 + 4 + 3 +2 + 1:

When LOOP is assembled, the current location = 0000000E.

Looking at the LOOP machine code, we see that –5 (FBh) is added to the current location, causing a jump to location 00000009:

00000009 ← 0000000E + FB

00000000 66 B8 0000 mov ax,0 00000004 B9 00000005 mov ecx,5 00000009 66 03 C1 L1:add ax,cx 0000000C E2 FB loop L1 0000000E

offset machine code source code

Your turn . . .

If the relative offset is encoded in a single byte, (a) what is the largest possible backward jump?

(b) what is the largest possible forward jump?

(a) −128 (b) +127

Average sizes of machine instructions are about 3 bytes, so a loop might contain, on average, a maximum of 42 instructions!

Your turn . . .

What will be the final value of AX?

mov ax,6 mov ecx,4 L1:

inc ax loop L1

How many times will the loop

execute? mov ecx,0

X2:

inc ax loop X2 10

4,294,967,296

Nested Loop

If you need to code a loop within a loop, you must save the outer loop counter's ECX value. In the following example, the outer loop executes 100 times, and the inner loop 20 times.

.data

count DWORD ? .code

mov ecx,100 ; set outer loop count L1:

mov count,ecx ; save outer loop count mov ecx,20 ; set inner loop count L2:...

loop L2 ; repeat the inner loop mov ecx,count ; restore outer loop count loop L1 ; repeat the outer loop

Summing an Integer Array

.data

intarray WORD 100h,200h,300h,400h .code

mov edi,OFFSET intarray ; address mov ecx,LENGTHOF intarray ; loop counter

mov ax,0 ; zero the sum

L1:

add ax,[edi] ; add an integer add edi,TYPE intarray ; point to next loop L1 ; repeat until ECX = 0 The following code calculates the sum of an array of 16-bit integers.

Copying a String

.data

source BYTE "This is the source string",0 target BYTE SIZEOF source DUP(0),0

.code

mov esi,0 ; index register mov ecx,SIZEOF source ; loop counter L1:

mov al,source[esi] ; get char from source mov target[esi],al ; store in the target inc esi ; move to next char loop L1 ; repeat for entire string

good use of SIZEOF

The following code copies a string from source to target.

Conditional Processing

Status flags - review

• The Zero flag is set when the result of an operation equals zero.

• The Carry flag is set when an instruction generates a result that is too large (or too small) for the

destination operand.

• The Sign flag is set if the destination operand is negative, and it is clear if the destination operand is positive.

• The Overflow flag is set when an instruction generates an invalid signed result.

• Less important:

– The Parity flag is set when an instruction generates an even number of 1 bits in the low byte of the destination operand.

– The Auxiliary Carry flag is set when an operation produces a carry out from bit 3 to bit 4

NOT instruction

• Performs a bitwise Boolean NOT operation on a single destination operand

• Syntax: (no flag affected)

NOT destination

• Example:

mov al, 11110000b not al

NOT

0 0 1 1 1 0 1 1 1 1 0 0 0 1 0 0

NOT

inverted

AND instruction

• Performs a bitwise Boolean AND operation between each pair of matching bits in two operands

• Syntax: (O=0,C=0,SZP)

AND destination, source

• Example:

mov al, 00111011b and al, 00001111b 0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 0 1 1

AND

unchanged cleared

AND

bit extraction

OR instruction

• Performs a bitwise Boolean OR operation between each pair of matching bits in two operands

• Syntax: (O=0,C=0,SZP)

OR destination, source

• Example:

mov dl, 00111011b or dl, 00001111b

OR

0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 0 0 1 1 1 1 1 1

OR

set unchanged

XOR instruction

• Performs a bitwise Boolean exclusive-OR operation between each pair of matching bits in two operands

• Syntax: (O=0,C=0,SZP)

XOR destination, source

• Example:

mov dl, 00111011b xor dl, 00001111b

XOR

0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 1 0 0

XOR

inverted unchanged

XOR is a useful way to invert the bits in an operand and data encryption

Applications

mov al,'a‘ ; AL = 01100001b and al,11011111b ; AL = 01000001b

• Task: Convert the character in AL to upper case.

• Solution: Use the AND instruction to clear bit 5.

Applications

mov al,6 ; AL = 00000110b or al,00110000b ; AL = 00110110b

• Task: Convert a binary decimal byte into its equivalent ASCII decimal digit.

• Solution: Use the OR instruction to set bits 4 and 5.

The ASCII digit '6' = 00110110b

Applications

mov ax,wordVal

and ax,1 ; low bit set?

jz EvenValue ; jump if Zero flag set

• Task: Jump to a label if an integer is even.

• Solution: AND the lowest bit with a 1. If the result is Zero, the number was even.

Applications

or al,al

jnz IsNotZero ; jump if not zero

• Task: Jump to a label if the value in AL is not zero.

• Solution: OR the byte with itself, then use the JNZ (jump if not zero) instruction.

ORing any number with itself does not change its value.

TEST instruction

• Performs a nondestructive AND operation between each pair of matching bits in two operands

• No operands are modified, but the flags are affected.

• Example: jump to a label if either bit 0 or bit 1 in AL is set. test al,00000011b

jnz ValueFound

• Example: jump to a label if neither bit 0 nor bit 1 in AL is set.

test al,00000011b jz ValueNotFound

CMP instruction

• Compares the destination operand to the source operand

– Nondestructive subtraction of source from destination (destination operand is not changed)

• Syntax: (OSZCAP)

CMP destination, source

• Example: destination == source mov al,5

cmp al,5 ; Zero flag set

• Example: destination < source mov al,4

cmp al,5 ; Carry flag set

CMP instruction

• Example: destination > source mov al,6

cmp al,5 ; ZF = 0, CF = 0 (both the Zero and Carry flags are clear)

The comparisons shown so far were unsigned.

CMP instruction

• Example: destination > source mov al,5

cmp al,-2 ; Sign flag == Overflow flag The comparisons shown here are performed with signed integers.

• Example: destination < source mov al,-1

cmp al,5 ; Sign flag != Overflow flag

Conditions

0 1

destination=source

0 0

destination>source

1 0

destination<source

CF ZF

unsigned

ZF=1 destination=source

SF == OF destination>source

SF != OF destination<source

flags signed

Setting and clearing individual flags

and al, 0 ; set Zero or al, 1 ; clear Zero or al, 80h ; set Sign and al, 7Fh ; clear Sign

stc ; set Carry

clc ; clear Carry

mov al, 7Fh

inc al ; set Overflow or eax, 0 ; clear Overflow

Conditional jumps

Conditional structures

• There are no high-level logic structures such as if-then-else, in the IA-32 instruction set. But, you can use combinations of comparisons and jumps to implement any logic structure.

• First, an operation such as CMP, AND or SUB is executed to modified the CPU flags. Second, a conditional jump instruction tests the flags and changes the execution flow accordingly.

CMP AL, 0 JZ L1 :

L1:

J cond instruction

• A conditional jump instruction branches to a label when specific register or flag conditions are met

Jcond destination

• Four groups: (some are the same) 1. based on specific flag values

2. based on equality between operands

3. based on comparisons of unsigned operands 4. based on comparisons of signed operands

Jumps based on specific flags Jumps based on equality

Jumps based on unsigned comparisons

＞≧＜≦

Jumps based on signed comparisons

Examples

mov Large,bx cmp ax,bx jna Next mov Large,ax Next:

• Compare unsigned AX to BX, and copy the larger of the two into a variable named Large

mov Small,ax cmp bx,ax jnl Next mov Small,bx Next:

• Compare signed AX to BX, and copy the smaller of the two into a variable named Small

Examples

.date

intArray DWORD 7,9,3,4,6,1 .code

...

mov ebx, OFFSET intArray mov ecx, LENGTHOF intArray L1: test DWORD PTR [ebx], 1

jz found add ebx, 4 loop L1 ...

• Find the first even number in an array of unsigned integers

BT (Bit Test) instruction

• Copies bit n from an operand into the Carry flag

• Syntax: BT bitBase, n

– bitBase may be r/m16 or r/m32 – n may be r16, r32, or imm8

• Example: jump to label L1 if bit 9 is set in the AX register:

bt AX,9 ; CF = bit 9

jc L1 ; jump if Carry

• BTC bitBase, n: bit test and complement

• BTR bitBase, n: bit test and reset (clear)

• BTS bitBase, n: bit test and set

Conditional loops

LOOPZ and LOOPE

• Syntax:

LOOPE destination LOOPZ destination

• Logic:

– ECX ← ECX – 1

– if ECX != 0 and ZF=1, jump to destination

• The destination label must be between -128 and +127 bytes from the location of the following instruction

• Useful when scanning an array for the first element that meets some condition.

LOOPNZ and LOOPNE

• Syntax:

LOOPNZ destination LOOPNE destination

• Logic:

– ECX ← ECX – 1;

– if ECX != 0 and ZF=0, jump to destination