section .data newline db 10 num_format db "%d", 0xA, 0xD, 0 section .text global _start _start: mov rsi, 1 ; file descriptor: STDOUT mov rdx, 2 ; number of bytes to write (for each number) print_loop: ; Print the current number mov rax, 1 ; syscall: write mov rdi, rsi ; file descriptor lea rdx, [num_format] ; pointer to the format string mov rcx, 2 ; number of bytes to write syscall ; call kernel ; Print newline mov rax, 1 ; syscall: write mov rdi, rsi ; file descriptor lea rdx, [newline] ; pointer to the newline character mov rcx, 1 ; number of bytes to write syscall ; call kernel ; Increment counter inc rsi ; Check if we have printed all numbers cmp rsi, 10 jne print_loop ; Exit the program mov rax, 60 ; syscall: exit xor rdi, rdi ; status: 0 syscall ; call kernel
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Assembly language(asm) is a low-level programming language, where the language instructions will be more similar to machine code instructions.
Every assembler may have it's own assembly language designed for a specific computers or an operating system.
Assembly language requires less execution time and memory. It is more helful for direct hardware manipulation, real-time critical applications. It is used in device drivers, low-level embedded systems etc.
Assembly language usually consists of three sections,
Data section
To initialize variables and constants, buffer size these values doesn't change at runtime.
bss section
To declare variables
text section
_start
specifies the starting of this section where the actually code is written.
There are various define directives to allocate space for variables for both initialized and uninitialized data.
variable-name define-directive initial-value
Define Directive | Description | Allocated Space |
---|---|---|
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 |
Define Directive | Description |
---|---|
RESB | Reserve a Byte |
RESW | Reserve a Word |
RESD | Reserve a Doubleword |
RESQ | Reserve a Quadword |
REST | Reserve a Ten Bytes |
Constants can be defined using
CONSTANT_NAME EQU regular-exp or value
%assign constant_name value
%define constant_name value
Loops are used to iterate a set of statements for a specific number of times.
mov ECX,n
L1:
;<loop body>
loop L1
where n specifies the no of times loops should iterate.
Procedure is a sub-routine which contains set of statements. Usually procedures are written when multiple calls are required to same set of statements which increases re-usuability and modularity.
procedure_name:
;procedure body
ret