NASM variables reserve storage space for storing any data with a specific name. Whenever there is an instructional need for that data or value, that data can be easily accessed by the name of the variable.
Variable in NASM Assembly Language
Allocating storage space for data
Variable Syntax - of the storage allocation statement for data
[variable-name] define-directive initial-value [,initial-value]...Where, variable-name is the identifier for each storage location. The assembler adds an offset value to each variable name defined in the data segment.
Allocating Storage Space for Initialized Data
There are five basic forms of the define directive −
| Directive | Purpose | Storage Space |
|---|---|---|
| DB | Define Byte | allocates 1 byte |
| DW | Define Word | allocates 2 bytes |
| DD | Define Doubleword | allocates 4 bytes |
| DQ | Define Quadword | allocates 8 bytes |
| DT | Define Ten Bytes | allocates 10 bytes |
Following are some examples of using define directives −
choice DB 'P'
number DW 12345
neg_number DW -12345
big_number DQ 123456789
real_number1 DD 1.234
real_number2 DQ 123.456- Each byte of character is stored as its ASCII value in hexadecimal.
- Each decimal value is automatically converted to its 16-bit binary equivalent and stored as a hexadecimal number.
- Processor uses the little-endian byte ordering.
- Negative numbers are converted to its 2's complement representation.
- Short and long floating-point numbers are represented using 32 or 64 bits, respectively.
The following program shows the use of define directive −
section .text
global _start ;must be declared for linker (gcc)
_start: ;tell linker entry point
mov edx,1 ;message length
mov ecx,choice ;message to write
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
section .data
choice DB 'P'
When the above code is compiled and executed, it produces the following result −
PAllocating Storage Space for Uninitialized Data
The reserve directives are used for reserving space for uninitialized data. The reserve directives take a single operand that specifies the number of units of space to be reserved. Each define directive has a related reserve directive.
There are five basic forms of the reserve directive −
| Directive | Purpose |
|---|---|
| RESB | Reserve a Byte |
| RESW | Reserve a Word |
| RESD | Reserve a Doubleword |
| RESQ | Reserve a Quadword |
| REST | Reserve a Ten Bytes |
Multiple Definitions
You can have multiple data definition statements in a program. For example −
choice DB 'a' ;ASCII of y = 79H
number1 DW 12345 ;12345D = 3039H
number2 DD 12345679 ;123456789D = 75BCD15HThe assembler allocates contiguous memory for multiple variable definitions.
Multiple Initializations
The TIMES directive allows multiple initializations to the same value. For example, an array named marks of size 9 can be defined and initialized to zero using the following statement −
Example
section .text
global _start ;must be declared for linker (ld)
_start: ;tell linker entry point
mov edx,9 ;message length
mov ecx, stars ;message to write
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
section .data
stars times 9 db 'a'When the above code is compiled and executed, it produces the following result −
aaaaaaaaa
