#Assembly Basics

Assembly program code is written in various segments

	1. .data   ---> All Initialized Data
	2. .bss	   ---> All uninitialized Data
	3. .text   ---> Program Instructions
		3.1  .globl _start  ---> Externally Callable Routines
		3.2  _start:        ---> Main() routine 

System calls are the library with the kernel exposes to the User space for getting various task done

List of system calls can be checked under /usr/include/asm_generic/unistd.h
System calls are invoked by processes using a software interrupt - INT 0x80

#Whenever system calls are invoked, we need to load some registers with the appropriate arguments which the system call may require

	1. EAX : System Call number
	2. EBX : First Argument
	3. ECX : Second Argument
	4. EDX : Third Argument
	5. ESI : Fourth Argument
	6. EDI : fifth argument

	1. movl %eax, %ebx (moves a 32 bit value)
		move eax value into ebx however eax itself doesn't change

	2. movw %ax, %bx (moves a 16 bit value)

	3. movb %ah, %bh (moves a 8 bit value)

Moving Data

Between Registers

	movl %eax, %ebx

Between Registers and Memory

	location:
			.int 10
	movl %eax, location
	movl location, %ebx

Moving Immediate value in register movl $10, %ebx

Immediate value into memory location

	location:
			.byte 0

	movb $10, location

Moving data into an indexed memory location

	IntegerArray:
		.int 10, 20, 30, 40, 50

	movl %eax, IntegerArray(0, 2, 4)

Above we are selecting the 3rd integer "30"
BaseAddress(Offset, Index, Size) = IntegerArray(0, 2, 4)

Calling exit(0) to exit program

	> Function definition
		void _exit(int status)

	1. Sys call number for exit() is 1, so load EAX with 1

		movl $1, %eax
				number is defined by $1			

	2. "status" is lets say 0, EBX must be loaded with 0
		movl $0, %ebx

	3. Calling the System call by Raising the Software interrupt

		int $0x80 

#gdb commands

	1. list <line Number> --> Lists the code from the given line number
		list 1

	2. disassamble <function Name> ---> Shows the assembly code of the function

	3. break <line Number>  ---> Setting the break point 

	4. info registers ---> For looking at the registers

	5. Stack can be viewed in gdb using examine command
		x/10xb <memory address>   ---> Starting with the memory address, 10 bytes are showed in hex format.
		x/10xb 0xbffff5f0

		
		10 - units
		x - hex form
		b - bytes

		x/10xw 0xbffff5f0 ---> 10 words in hex format
				
	6. info variables

	7. 

Hello world in assembly

My First Assembly Program

.data

HelloWorldString: .ascii "Hello World\n"

.text

.globl _start

_start: # Load all the arguments for write() function call

movl $4, %eax
movl $1, %ebx
movl $HelloWorldString, %ecx
movl $12, %edx
int $0x80

# Need to exit the program

movl $1, %eax
movl $0, %ebx
int $0x80