Computer Organization and Assembly Language


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Computer Organization and Assembly Language
Lecture 3 – Assembly Language Fundamentals
Basic Elements of Assembly Language
An assembly language program is composed of : • Constants • Expressions • Literals • Reserved Words • Mnemonics • Identifiers • Directives • Instructions • Comments

Integer Constants

• Integer constants can be written in decimal, hexadecimal, octal or binary, by adding a radix (or number base) suffix to the end .

• Radix Suffices:

–d

decimal (the default)

–h

hexadecimal

– q or o

octal

–b

binary

Examples of Integer Constants

• 26 • 1Ah • 1101b • 36q • 2Bh • 42Q • 36D • 47d

Decimal Hexadecimal Binary Octal Hexadecimal Octal Decimal Decimal

Integer Expressions

• An integer expressions is a mathematical expressions involving integer values and integer operators.

• The expressions must be one that can be stored in 32 bits (or less).

• The precedence:

– ()

Expressions in Parentheses

– +, -

Unary Plus and minus

– *, /, Mod Multiply, Divide, Modulus

– +, -

Add, Subtract

Examples of Integer Expressions

Expression
16 / 5 - (3 + 4) * (6 – 1) -3 + 4 * 6 – 1 4 + 5 * 2 -5 + 2 12 – 1 MOD 5 (4 + 2) * 6

Value
3 -35 20
1

Real Number Constants
• There are two types of real number constants: – Decimal reals, which contain a sign followed by a number with decimal fraction and an exponent:
[sign] integer.[integer][exponent]
Examples:
2. +3.0 -44.2E+05 26.E5
– Encoded reals, which are represented exactly as they are stored:
3F80000r
Characters Constants
• A character constant is a single character enclosed in single or double quotation marks.
• The assembler converts it to the equivalent value in the binary code ASCII:
‘A’ “d”

String Constants
• A string constant is a string of characters enclosed in single or double quotation marks:
‘ABC’ “x” “Goodnight, Gracie” ‘4096’ “This isn’t a test” ‘Say “Goodnight, ” Gracie’
Reserved Words
• Reserved words have a special meaning to the assembler and cannot be used for anything other than their specified purpose.
• They include: – Instruction mnemonics – Directives – Operators in constant expressions – Predefined symbols such as @data which return constant values at assembly time.

Identifiers
• Identifiers are names that the programmer chooses to represent variables, constants, procedures or labels.
• Identifiers: – can have 1 to 247 characters – are not case-sensitive – begin with a letter , underscore, @ or $ and can also contain digits after the first character. – cannot be reserved words

Examples of Identifiers

var1 _main @@myfile Count xVal

open_file _12345 $first MAX

Directives
• Directives are commands for the assembler, telling it how to assemble the program.
• Directives have a syntax similar to assembly language but do not correspond to Intel processor instructions.
• Directives are also case-insensitive: • Examples
.data .code name PROC
Instructions
• An instruction in Assembly language consists of a name (or label), an instruction mnemonic, operands and a comment
• The general form is: [name] [mnemonic] [operands] [; comment]
• Statements are free-form; i.e, they can be written in any column with any number of spaces between in each operand as long as they are on one line and do not pass column 128.

Labels
• Labels are identifiers that serve as place markers within the program for either code or data.
• These are replaces in the machine-language version of the program with numeric addresses.
• We use them because they are more readable: mov ax, [9020] vs. mov ax, MyVariable

Code Labels

• Code labels mark a particular point within

the program’s code.

• Code labels appear at the beginning and are

immediately followed by a colon:

target:

mov

ax, bx

… …

jmp target

Data Labels
• Labels that appear in the operand field of an instruction:
mov first, ax
• Data labels must first be declared in the data section of the program:
first BYTE 10
Instruction Mnemonics
• Instruction mnemonics are abbreviations that identify the operation carried out by the instruction:
mov - move a value to another location add - add two values sub - subtract a value from another jmp - jump to a new location in the program mul - multiply two values call - call a procedure

Operands

• Operands in an assembly language

instruction can be:

– constants

96

– constant expressions 2 + 4

– registers

eax

– memory locations

count

Operands and Instructions

• All instructions have a predetermined number of operands.

• Some instructions use no operands:

stc

; set the Carry Flag

• Some instructions use one operand:

inc

ax ; add 1 to AX

• Some instructions use two operands:

mov

count, bx ; add BX to count

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Computer Organization and Assembly Language