4. BASIC ELEMENTS OF ASSEMBLY
• Integer Constants
• Integer Expression
• Real Number Constants
• Character Constants
• String Constants
• Reserved Words
• Identifiers
• Directives
• Instructions
5. INTEGER CONSTANTS
• one or more digits, and an optional suffix character (called a radix)
indicating the number‟s base
[{+| −}] digits [radix]
• If no radix is given, the integer constant is assumed to be decimal
6. RADIX
• Radix may be one of the following (uppercase or lowercase):
Radix
Type
Radix
Type
H
Hexadecimal
R
Real
q/o
Octal
T
Decimal
D
Decimal
Y
Binary
B
Binary
7. INTEGER EXPRESSION
• An integer expression is a mathematical expression involving integer
values and arithmetic operators.
8. REAL NUMBER CONSTANTS
• Real number constants are represented as decimal reals or encoded
(hexadecimal) reals.
• A decimal real contains an optional sign followed by an integer
• A decimal point, an optional integer that expresses a fraction, and an
optional exponent
9. CHARACTER AND STRING CONSTANTS
• A character constant is a single character enclosed in single or double
quotes.
• „A‟
• “A”
• A string constant is a sequence of characters (including spaces)
enclosed in single or double quotes
• „Hi All‟
• “Welcome”
10. RESERVED WORDS
• Reserved words have special meaning Assembly and can only be
used in their correct context.
• Instruction mnemonics (MOV, ADD, SUB)
• Register names
• Directives
• Attributes, which provide size and usage information for variables and operands
• Operators, used in constant expressions
• Predefined symbols (@)
11. IDENTIFIERS
• An identifier is a programmer-chosen name. It might identify a
variable, a constant, a procedure, or a code label.
12. DIRECTIVES
• A directive is a command embedded in the source code that is
recognized and acted upon by the assembler.
• Directives do not execute at runtime.
• The .DATA directive identifies the area of a program containing variables
• The .CODE directive identifies the area of a program containing executable
instructions
13. INSTRUCTIONS
• An instruction is a statement that becomes executable when a
program is assembled.
• Instructions are translated by the assembler into machine language
bytes.
• Then loaded and executed by the CPU at runtime.
15. INSTRUCTIONS - LABEL
• A label is an identifier that acts as a place marker for instructions and
data.
• Data Labels A data label identifies the location of a variable, providing
a convenient way to reference the variable in code.
• count DWORD 100
• The assembler assigns a numeric address to each label
16. INSTRUCTIONS - LABEL
• Code Labels A label in the code area of a program (where
instructions are located) must end with a colon (:) character.
• Code labels are used as targets of jumping and looping instructions.
• target:
• mov ax,bx
• ...
• jmp target
17. INSTRUCTION - INSTRUCTION MNEMONIC
• An instruction mnemonic is a short word that identifies an instruction.
Mnemonic
Description
MOV
Move (assign) one value to another
ADD
Add two values
SUB
Subtract one value from another
MUL
Multiply two values
JMP
Jump to a new location
CALL
Call a procedure
18. INSTRUCTION - OPERANDS
• Assembly language instructions can have between zero and three
operands.
• Each of which can be a register, memory operand, constant
expression, or input-output port.
19. INSTRUCTION - OPERANDS
• The INC instruction has one operand:
inc eax
; add 1 to EAX
• The MOV instruction has two operands:
mov count,ebx ; move EBX to count
20. INSTRUCTION - COMMENTS
• Comments are an important way for the writer of a program to
communicate information about the program‟s design to a person
reading the source code.
• The following information is typically included at the top of a program
listing:
• Description of the program‟s purpose
• Names of persons who created and/or revised the program
• Program creation and revision dates
• Technical notes about the program‟s implementation
21. INSTRUCTION - COMMENTS
• Comments can be specified in two ways:
• Single-line comments, beginning with a semicolon character (;).
• Block comments, beginning with the COMMENT directive and a user-specified
symbol
COMMENT !
This line is a comment.
This line is also a comment.
!
22. INSTRUCTION EXECUTION
• Fetch:
• The control unit fetches the next instruction from the instruction queue and
increments the instruction pointer (IP).
• Decode:
• The control unit decodes the instruction‟s function to determine what the
instruction will do.
• Fetch operands:
• If the instruction uses an input operand located in memory, the control unit uses
a read operation to retrieve the operand and copy it into internal registers
23. INSTRUCTION EXECUTION
• Execute:
• The ALU executes the instruction using the named registers and internal
registers as operands and sends the output to named registers and/or memory
• Store output operand:
• If the output operand is in memory, the control unit uses a write operation to
store the data.
24. INSTRUCTION EXECUTION
loop
fetch next instruction
advance the instruction pointer (IP)
decode the instruction
if memory operand needed, read value from memory
execute the instruction
if result is memory operand, write result to memory
continue loop
