 section .data
  sys_write  equ 1
  std_out    equ 1
  sys_exit   equ 60
  exit_code  equ 0
  
  new_line   db 0xa
  input      db 'Hello World!'

section .bss
  output resb 12
  
section .text
	global _start

_start:
	mov rsi, input            
	xor rcx, rcx
	cid 
	mov rdi, $ + 15
	call calculateStrLength
	xor rax, rax     
	xor rdi, rdi                     
	jmp reverseStr

calculateStrLength:
  cmp byte[rsi], 0
  je  exitFromRoutine
  lodsb
  push rax
  inc rcx
  jmp calculateStrLength
  
exitFromRoutine:
  push rdi
  ret
  

reverseStr:
	;; check is it end of string
	cmp	rcx, 0
	;; if yes print result string
	je	printResult
	;; get symbol from stack
	pop	rax
	;; write it to output buffer
	mov	[output + rdi], rax
	;; decrease length counter
	dec	rcx
	;; increase additional length counter (for write syscall)
	inc	rdi
	;; loop again
	jmp	reverseStr

;;
;; Print result string
;;
printResult:
	mov	rdx, rdi
	mov	rax, 1
	mov	rdi, 1
	mov	rsi, output
	syscall
	jmp	printNewLine

;;
;; Print new line
;;
printNewLine:
	mov	rax, sys_write
	mov	rdi, std_out
	mov	rsi, new_line
	mov	rdx, 1
	syscall
	jmp	exit

;;
;; Exit from program
;;
exit:
	;; syscall number
	mov	rax, sys_exit
	;; exit code
	mov	rdi, exit_code
	;; call sys_exit
	syscall

created 3 years ago by destizakni

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