 .section .data
# permanent constants and variables for program
input_prompt	:	.asciz "Input a string\: \n"
input_spec		:	.asciz "%s"
word			:	.space 8
output			:	.asciz "Your string is\: %s\n"
length_spec		:	.asciz "String length\: \n"
#palindrome_spec:	.asciz "String is a palindrome (T/F)\: %c\n"

.section .text
.global main

main:
			# load x0 with input_prompt string
			ldr x0, =input_prompt
			# branch to printf
			bl printf
			
			# load x0 with format specifier for input and x1 with word variable
			ldr x0, =input_spec
			ldr x1, =word
			# branch to scanf
			bl scanf
			
			# load register with address of word
			# ldr x19, =word
			
			# load output and num to argument registers
			ldr x0, =output
			# load x1 to address x19 points to
			ldr x1, =word
			# branch to printf
			bl printf
	
			# calc length
			# set counter to zero
			add x19, xzr, xzr
			# load by byte string array into x9
	L1: 	add x10, x19, x1
			ldurb w11, [x10, #0]
			
			# branch exit if zero
			cbz w11, L2
			# increment counter
			add x19, x19, #1
			# branch to loop
			b L1
	L2:		ldr x0, =length_spec
			#ldr x1, [x19]
			bl printf
	
			# branch to exit	
			b exit

# standard ARM exit instruction
exit:
	mov x0, 0
	mov x8, 93
	svc 0
	ret

created 3 years ago

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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