HelloWorld.asm

 .globl fact_do               # Make the function visible to the linker
fact_do:                     
    pushq %rbp               # Save the base pointer
    movq %rsp, %rbp          # Set the base pointer to the current stack pointer
    pushq %rbx               # Save RBX register which will be used to store 'result'

    movq %rdi, %rbx          # Move the input 'n' into RBX, RBX will store 'result'
    movq $1, %rax            # Initialize 'result' to 1 in RAX

    .Ldo:                    # Label for the beginning of the do-while loop
        imulq %rbx, %rax     # RAX *= RBX (result *= n)
        subq $1, %rbx        # RBX -= 1 (n = n - 1)
        cmpq $1, %rbx        # Compare n with 1
        jg .Ldo              # If n > 1, jump back to the start of the loop

    popq %rbx                # Restore the old RBX
    popq %rbp                # Restore the old base pointer
    ret                      # Return with the result in RAX

created 7 months ago

Assembly Online Compiler

Write, Run & Share Assembly code online using OneCompiler's Assembly online compiler for free. It's one of the robust, feature-rich online compilers for Assembly language. Getting started with the OneCompiler's Assembly compiler is simple and pretty fast. The editor shows sample boilerplate code when you choose language as Assembly and start coding.

About Assembly

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.

Syntax help

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.

Variables

There are various define directives to allocate space for variables for both initialized and uninitialized data.

1. To allocate storage space to Initialized data

Syntax

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

2. To allocate storage space to un-initialized data

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

Constants can be defined using

1. equ

To define numeric constants

CONSTANT_NAME EQU regular-exp or value

2. %assign

To define numeric constants.

%assign constant_name value

3. %define

To define numeric or string constants.

%define constant_name value

Loops

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.

Procedures

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