Assembly Variables
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:
Where, variab le-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:
Following are some examples of using define directives:
Please note that:
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 use of the define directive:
When the above code is compiled and executed, it produces fol owing result:
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:
Multiple Definitions
You can have multiple data definition statements in a program. For example:
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:
The TIMES directive is useful in defining arrays and tables. The following program displays 9 asterisks on the screen:
When the above code is compiled and executed, it produces fol owing result:
