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+title: ARM64 process command line arguments in assembly
+keywords:raspi5,asm,c,kernel,arm64
+
+# ARM64 process command line arguments in assembly
+
+## Intro
+
+Goal is to learn how to get command line arguments in assembly 
+and do basic processing for Linux kernel, all is tested with Raspberry Pi4 on 64bit OS.
+
+## Location of arguments
+
+Operating system provides number of arguments, command line arguments and
+environment variables. Pointers for those located in stack and location
+of actual values also reference in allocated stack from operating system.
+
+## Checking for argument number
+
+Number of arguments passed to command line utility is located
+at stack pointers first value. Argument number located at
+_sp_ address, after that there is arguments here we check number
+of arguments if there is less then 1, it always more then 1 as
+first argument is always program calling path
+
+```c
+	ldr x3, [sp]
+	cmp x3, #1
+	b.LE exit_program  
+```
+
+
+## Printing out all arguments
+
+Pointers to the argument values are after the stack pointers first value.
+As first value in stack pointer is number of arguments passed then need to 
+ensure to not go over it.
+
+### Write strlen function
+
+There is no default strlen instruction in assembly, so need to write
+one our-self. I used _x0_ as input argument, _x1_ as internal counter
+_w2_ to get and compare string char and _x1_ is where final amount
+of characters is saved. 
+
+```c
+//param 0: x0 pointer to string
+//using registers: x2
+//return result in x1:
+strlen:
+	mov x1, #0 //number of arguments
+loop_strlen_till_0:
+	//check if str[] element is 0
+	ldrb w2, [x0]	//ldrb w2, [x2]
+	//if element of string is zero then exit
+	cbz w2, string_element_eq_zero
+	//if element isnt 0 then add +1 to counted size
+	add x1, x1, 1
+	//increase str pointer +1
+	add x0, x0, #1
+	b loop_strlen_till_0
+string_element_eq_zero:
+	ret
+```
+
+
+### Printing out all arguments passed
+
+Next step is to utilize strlen function and printout all arguments
+passed from command line
+
+```c
+argument_print_loop:
+	//number of arguments now in x4
+	//get the value of the first argument
+	//add x0, sp, #8
+	mov x0, x4
+	ldr x0, [x0]
+	 //x0 points to firt argument of programm name
+	bl strlen
+
+	//write
+	mov x2, x1
+	mov x0, #1 //set stdout
+	ldr x1, [x4] //point to programm name
+	//ldr x5, =program_len //hopefully correct size of print_arg
+	//ldr x2, [x5]
+	mov w8, #64
+	svc #0
+
+	//print new line
+	mov x0, #1 //set stdout
+	ldr x1, =new_line //point to newline
+	mov x2, #1
+	mov w8, #64
+	svc #0
+
+	add x4, x4, #8
+	sub x3, x3, 1
+	cmp x3, 1
+	b.GE argument_print_loop
+```
+
+All programm seems to work ok
+
+## Source code
+
+Final source code of this example
+
+```c
+.data
+
+hello_world:
+	.ascii "print arg program\n"
+hello_world_len = . - hello_world
+
+new_line:
+	.ascii "\n"
+
+program_len: .quad 11
+
+.text
+.global _start
+_start:
+	//save to x0 number of arguments
+	ldr x3, [sp]
+	//save to x1 address of first argument 
+	add x4, sp, #8
+
+	//write hello world
+	mov x0, #1
+	ldr x1, =hello_world
+	ldr x2, =hello_world_len
+	mov w8, #64
+	svc #0
+
+argument_print_loop:
+	//number of arguments now in x4
+	
+	//get the value of the first argument
+	mov x0, x4
+	ldr x0, [x0] //x0 points to firt argument of programm name
+	bl strlen
+
+	//write
+	mov x2, x1
+	mov x0, #1 //set stdout
+	ldr x1, [x4] //point to programm name
+	mov w8, #64
+	svc #0
+
+	//print new line
+	mov x0, #1 //set stdout
+	ldr x1, =new_line //point to newline
+	mov x2, #1
+	mov w8, #64
+	svc #0
+
+
+	add x4, x4, #8
+	sub x3, x3, 1
+	cmp x3, 1
+	b.GE argument_print_loop
+
+
+exit_program:
+	//exit(0)
+	ldr x0, #0
+	mov w8, #93
+	svc #0
+
+//param 0: x0 pointer to string
+//using registers: x2
+//return result in x1:
+strlen:
+	mov x1, #0 //number of arguments
+loop_strlen_till_0:
+	//check if str[] element is 0
+	ldrb w2, [x0]	//ldrb w2, [x2]
+	//if element of string is zero then exit
+	cbz w2, string_element_eq_zero
+	//if element isnt 0 then add +1 to counted size
+	add x1, x1, 1
+	//increase str pointer +1
+	add x0, x0, #1
+	b loop_strlen_till_0
+string_element_eq_zero:
+	ret
+```
+
+## Comilation size
+
+|Program name |Uncompressed size|Compressed size|
+|---|---|---|
+|print_arg| 1552 bytes | 481 bytes |
+
+
+## Links
+
+[Make tiny binary](/writeup/arm64_make_tiny_binary.md)
+\ No newline at end of file