Module 3.4: Assembly - Variables and Constant

Variables

NASM provides various define directives for reserving storage space for __________.

[variable-name]    define-directive    initial-value   [,initial-value]

Syntax for storage allocation statement for initialized data.

DB
DW
DD
DQ
DT

Five basic forms of the define directive

DB

Define Byte (1 byte)

DW

Define Word (2 bytes)

DD

Define Doubleword (4 bytes)

DQ

Define Quadword (8 bytes)

DT

Define Ten Bytes (10 bytes)

Little-endian byte ordering

Processor uses the _______.

RESB
RESW
RESD
RESQ
REST

Used for reserving space for uninitialized data. Take a single operand that specifies the number of units of space to be reserved.

RESB

Reserve a Byte

RESW

Reserve a Word

RESD

Reserve a Doubleword

RESQ

Reserve a Quadword

REST

Reserve a Ten Bytes

Multiple Initializations

The TIMES directive allows multiple initializations to the same value.

Constants

There are several directives provided by NASM that define constants.

EQU
%assign
%define

Three directives

The EQU Directive

Used for defining constants.

CONSTANT_NAME EQU expression

The syntax of the EQU directive

The %assign Directives

Can be used to define numeric constants like the EQU directive. This directive allows redefinition.

%assign TOTAL 10

The example of the %assign Directive

The %define Directive

Allows defining both numeric and string constants. This directive is similar to the #define in C.