Lesson 05 ASSEMBLY LANGUAGE FUNDAMENTALS 1 Assembly Language

Lesson 05 ASSEMBLY LANGUAGE FUNDAMENTALS 1

Assembly Language Fundamentals • • Contents Basic Elements of Assembly Language Assembling Linking Running Programs Defining Data Symbolic Constants Real-Address Mode Programming 2

BASIC ELEMENTS OF ASSEMBLY LANGUAGE ü ü ü ü ü Integer constants Integer expressions Character and string constants Reserved words and identifiers Directives and instructions Labels Mnemonics and Operands Comments Examples 3

BASICS OF ASSEMBLY LANGUAGE • Assembly Language code is NOT case sensitive but use of upper case is preferred. • Mnemonic • All instructions are represented in numbers by processor. Mnemonic is easy word (symbol) used instead of instruction, which will be converted into numbers. • Assembler • Convert mnemonic into numbers (instructions) 4

ASSEMBLY LANGUAGE SIMPLE PROGRAM Write a simple program in assembly language which show character ‘A’. (new. File. asm). model small. stack 256. code start: mov dl, ‘A’ mov ah, 2 h int 21 h mov ax, 4 c 00 h int 21 h end start 5

INTEGER CONSTANTS • Integer constants: (Also called Integer Literal) • Elements within […] are optional. • Elements within {…} require a choice of one of the enclosed elements (Separated by | sign) • Elements in italics have known description. • It is made up of three parts 1. Optional leading + or – sign 2. One or More digits 3. Optional Suffix indicating base (Called Radix) • Syntax: • [{+|-}] digits [radix] 6

INTEGER CONSTANTS • Radix: • It is used to denote base of number. • By default base is Decimal. • Radix may be binary(b), decimal(d), hexadecimal(h), or octal digits(o or q). • Examples: 30 d, 6 Ah, 42, 1101 b • A hexadecimal constant beginning with a letter must have leading zero to prevent the assembler from interpreting it as an identifier. • 0 A 5 h 7

INTEGER EXPRESSIONS • Integer Expressions: • Mathematical expressions involving integer values and arithmetic operators. • Can be stored in 32 bits (0 to FFFFh) • Arithmetic Operators: Operator Name Precedence Level () Parentheses 1 +, - Unary plus, Minus (+5 -6) 2 *, / Multiply, Divide 3 MOD Modulus 3 +, - Add, subtract 4 8

REAL NUMBER CONSTANTS • Real Number Constants: • Include decimal reals or (encoded) hexadecimal reals. • Syntax: • [sign]integer. [integer][exponent] • Example: • 2. • +3. 0 • -44. 2 E+05 • 26. E 5 • Note: At least one digit and a decimal point are required. 9

CHARACTER CONSTANTS • A character constant is a single character enclosed in single or double quotes. • MASM stores the value in memory as the character’s binary ASCII code. • Example • ‘A’ • ‘B’ • ‘C’ 10

STRING CONSTANTS • A string constant is a sequence of characters (including spaces) enclosed in single or double quotes. • Example • 'ABC' • 'X' • "Good night, Gracie" • '4096' • Embedded quotes are permitted when used in the manner shown by the following examples: • "This isn't a test" • 'Say "Good night, " Gracie' 11

RESERVED WORDS • Reserved words have special meaning in MASM and can only be used in their correct context. • There are different types of reserved words: • Instruction mnemonics, such as MOV, ADD, and MUL • Register names, such as AH, CR 0, EAX, EBX. • Attributes, which provide size and usage information for variables and operands such as, BYTE and WORD • Operators, used in constant expressions such as +, -. • Predefined symbols, such as @data, which return constant integer values at assembly time 12

IDENTIFIERS • 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 a letter (A. . Z, a. . z), underscore (_), @ , ? , or $. Subsequent characters may also be digits. • An identifier cannot be the same as an assembler reserved word. • Eg. var 1, count, $first, _main, MAX, _1234 13

DIRECTIVES • It is a command embedded in the source code, used by assembler. • Not case sensitive • Do not execute at run time. • Perform the work of assigning names to memory segments and other tasks for MASM. • Assembler for Intel processors share the same instruction set, but different directives. • Directives play a role to define program sections or segments. • . data (It identifies the area of a program containing variables. ) • . Code (Identifies the area of a program containing executable instructions. • . STACK 100 h (Identifies area of the program holding the runtime stack and its size. • The DWORD directive tells the assembler to reserve space in the program for a double word variable. • my. Var DWORD 26 ; DWORD directive 14

INSTRUCTIONS • Statement that becomes executable when a program is assembled. (Run time) • Assembler translate instructions into machine language, they are loaded and executed by CPU at runtime. • Parts of instruction 1. 2. 3. 4. Label (optional) Instruction Mnemonic (required) Operand(s) (usually required) Comments (optional) • Syntax • [label: ] mnemonic [operands] [; comment] • The MOV instruction, executes at runtime, copying the contents of my. Var to the EAX register: • mov eax, my. Var ; MOV instruction 15

INSTRUCTIONS (1. LABEL) • Label is an identifier that acts as a place marker for instructions and data. • Label placed before instructions denotes instruction’s address. • Label placed before variable denotes variable address. 16

INSTRUCTIONS (1. LABEL) • Data Labels: • Identifies the location of the variable (Unique) • Example: count DWORD 100 • Assembler assigns a numeric address to each label. • Multiple data items label • It is possible to define multiple data items following a label. • Array defines the location of the first number (1024) • Other numbers following in memory immediately afterward. • Array DWORD 1024, 2048 DWORD 4096, 8192 (Ends without colon) 17

INSTRUCTIONS (1. LABEL) • Code Labels: • A label in the code area of a program must end with colon (: ) character. • Code area: Where instructions are located. • Code Labels are used as target of jumping and looping instructions. • E. g. Target: mov ax, bx …. Jump target (Ends with colon) ; jump instruction transfer the control to label target § Note: Within any procedure, name of label must be unique. § Procedure is equivalent to function in modern languages. § Naming rules are applied on label. 18

INSTRUCTIONS (2. MNEMONIC) • Mnemonic: A device that assist memory. It is a word used instead of simple instructions. • There are four types of instructions: 1. Data Movement Instructions • Move data from one place to another • Eg: Data Movement – Add ax, 1234 ; AND 1234 with ax – Add bx, 0534 ; ADD 0534 to bx – Add bx, [1200] ; ADD data at address 1200 to bx – Add ax, [1234] ; ADD data from address 1234 to ax 19

INSTRUCTIONS (2. MNEMONIC) 2. Program control instruction • Controls pointer of instruction. As, sometime sequence of execution is executed and in between if new sequence of instruction needs to be executed than this is the job of program control instruction. • Eg: • cmp • jne • Jne ax, 0 ; 1234 ; [1234] ; compare ax with 0 jump if not equal to 1234 address 20

INSTRUCTIONS (2. MNEMONIC) 3. Arithmetic / Logic Instructions • Add, sub, mul, div, and, or, not, etc • Eg: Arithmatic • Mov ax, • Lda 0234 bx ; Move data from bx to ax ; Load 0234 into accumulator 4. Special instructions • These are not common, used to get direct access of microprocessor. • Eg: • Cli ; • Sti ; Clear the interrupt flag (means no interrupt is allowed) Set the interrupt flag (allow interrupts to processor) 21

INSTRUCTIONS (3. OPERANDS) • Assembly language instructions can have between zero and three operands. • Operands can be: • Register, memory operand, constant expression, input output port. • Memory Operand can be specified by: • Name of variable • Address of a variable(placed in one or more registers) Example Operand Type 96 Constant (Immediate Value) 2+4 Constant Expression Eax Register 22

INSTRUCTIONS (3. OPERANDS) • Examples of instructions having operands • Example: § STC statement without operand stc ; set Carry flag § INC statement has one operand: inc eax ; add to EAX § MOV instruction has two operands: mov count, ebx ; move EBX to count 23

INSTRUCTIONS (3. OPERANDS) • In two-operand instruction: • First operand is called the destination. (modified by instruction) • Second operand is called the source. • In three-operand instruction: • First is called as destination, remaining two are called as source. imul eax, ebx, 5 In above instruction EBX is multiplied by 5 and the product is stored in EAX register. 24

INSTRUCTIONS (4. COMMENTS) • Comments are description of the code written in assembly language, ignored by assembler. • Purpose of comments • • Description of the program’s purpose Name of programmer Program dates Technical details • Comments could be of following type: • Single-line comments: Beginning with a semicolon character (; ). • Block comments: Starts with COMMENT directive and a userspecified symbol (e. g. & or !). 25

INSTRUCTIONS (4. COMMENTS) • For Example: COMMENT ! THIS IS A FIRST LINE OF COMMENT. THIS IS SECOND LINE OF COMMENT. ! OR COMMENT & THIS IS A FIRST LINE OF COMMENT. THIS IS SECOND LINE OF COMMENT. & 26