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NASM provides various define directives for reserving storage space for variables. The define assembler directive is used for allocation of storage space. It can be used to reserve as well as initialize one or more bytes.
Allocating Storage Space for Initialized Data
The syntax for storage allocation statement for initialized data is −
[variable-name] define-directive initial-value [,initial-value]...
Where, variable-name is the identifier for each storage space. The assembler associates an offset value for each variable name defined in the data segment.
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 ''y'' number DW 12345 neg_number DW -12345 big_number DQ 123456789 real_number1 DD 1.234 real_number2 DQ 123.456
Please note that −
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Each byte of character is stored as its ASCII value in hexadecimal.
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Each decimal value is automatically converted to its 16-bit binary equivalent and stored as a hexadecimal number.
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Processor uses the little-endian byte ordering.
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Negative numbers are converted to its 2”s complement representation.
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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 ''y''
When the above code is compiled and executed, it produces the following result −
y
Allocating 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 ''Y'' ;ASCII of y = 79H number1 DW 12345 ;12345D = 3039H number2 DD 12345679 ;123456789D = 75BCD15H
The 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 −
marks TIMES 9 DW 0
The TIMES directive is useful in defining arrays and tables. The following program displays 9 asterisks on the screen −
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 ''*''
When the above code is compiled and executed, it produces the following result −
*********
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