Variables and Data Types

all initialized variables are defined in the .data section of code

Data Types

Numeric Values

Number values may be specified in decimal, hex, or octal.

When specifying hex or base-16 values, they are preceded with a 0x . For example, to specify 127 as hex, it would be 0x7f .

When specifying octal, or-base-8 values, they are followed by a q . For example,to specify 511 as octal, it would be 777q .

The default radix (base) is decimal, so no special notation is required for decimal (base-10) numbers.

Characters, Strings and Integers

in assembly there is no need to define the variable type like C, instead the only thing that matters here is the variable length:

section .data

string  db    "hello world!",0    --> 0 is the null terminator
number  dw    2535333
character    db    'c'
float    dd    23.2323r

Arrays

Initialized arrays are defined with comma separated values.

as you can see the 'arr' array has 3 elements in index 0 to 2. the 'arr' variable will point to the first element and for next elements we increment the the index. for example element number 1 will be arr+1.

Defining Constants

Constants are defined with equ . The general format is:

<name> equ <value>
SIZE equ 10000

The value of a constant cannot be changed during program execution.

The constants are substituted for their defined values during the assembly process. As such, a constant is not assigned a memory location. This makes the constant more flexible since it is not assigned a specific type/size (byte, word, double-word, etc.). The values are subject to the range limitations of the intended use.

example:

section .data
msg1	db	"this is a test", 0xa
len1	equ	$ -msg1
msg2	db	"and this is another test :)", 0xa
len2	equ	$ -msg2
STDIN	equ	2		; for ebx when taking input
STDOUT	equ	1		; for ebx when printing output
SYS_EXIT	equ	1	; system call for sys_exit (kernel opcode 0)
SYS_WRITE	equ	4	; system call for sys_write (kernel opcode 4)
SYS_READ	equ	3	; system call for sys_read (kernel opcode 3)

section .text
global _start


_start:
; print first message
mov	eax, SYS_WRITE
mov	ebx, STDOUT
mov	ecx, msg1
mov	edx, len1
int	80h

; print second message
mov     eax, SYS_WRITE
mov     ebx, STDOUT
mov     ecx, msg2
mov     edx, len2
int     80h

; exit
mov	eax, SYS_EXIT
mov	ebx, 0
int	80h

Length of Variables

when working with strings the length will be the length of the string minus one (the null byte):

msg  db  “Hello, World!”,10,0
msgLen  equ   $-msg-1         → minus the NULL byte (0)

when working with integers and floats the length is equal to the length of the variable:

num  db  100
numLen equ $-num

the length calculation and declaration in the .data section will be constant when using 'equ'.

Newline Character

we can use both hex and decimal representation of ASCII characters including the newline character:

10 
0xa
0ah

example:

msg    db    "hello world!",0ah    --> printing 'msg' will add a newline after

Uninitialized Variables

Uninitialized data is declared in the "section .bss" section.

The general format is:

Some simple examples include:

Multiple Initialization

The TIMES directive allows multiple initializations to the same value

marks TIMES 9 DW 0   --> marks = 9 * '0'