💻 About the project

AsmHelp a simple basic help for those who are starting in assembly I am a simple student of assembly and I want to share my knowledge for those who have doubts or want to start. We're covering here in this project just the Linux(ELF) Assembly Intel x86. Feel free to have a look at Assembly x86_64

Note: To this guide, we gonna use `nasm` and `ld`. Search how to install them in you linux distribution. You can use `nasm` and `ld` manually, but I made a script to automate the building process of our executable. You dont need to use, but is there anyway. Just do the following `./_nasm file.asm out.out`

Translations

Leia em PT-BR!

Index

Basic Syntax
Sections
Registers
Syscalls
Types-Variables-Allocation
Stack
Heap

Basic Syntax

Basically a assembly instruction is made by a mnemonic register1, register2, and we just keep putting and taking out values form these registers. That mnemonic is a assembly instruction that will make some operation between (usually) two registers.

mov eax, ebx ; Will move the value from ebx to eax

There you know two basics registers, eax and ebx also that the ; can be used to create comments

Basic Sections

"Roughly, a section is a range of addresses, with no gaps; all data "in" those addresses is treated the same for some particular purpose.". Keeping it simple, basically are memory spaces that you can separate your instructions. Below we have some basics sections that must have in you code. We define them using the keyword section <name>.

Sections	Description
.data	.data - section is used for declaring initialized data or constants. This data does change at runtime you can declare various constant values, file names, or buffer size, etc., in this section.
.bss	.bss - section is used for declaring uninitialized variables
.text	.text - section is used for keeping the actual code This section must begin with the declaration `global _start`, which tells to the linker the entry point of the program and then it'll starts the execution from that point.

Registers

First of all, what is a register? "A processor register is a quickly accessible location available to a computer's processor. Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-only.". But following the logic of being simple, its basically a fast access "variable" that you can put or remove values. In the x86 architecture we have 8 general purpose registers. Strong recommend this and this for a better understanding, not only for registers, but assembly in general. The registers are the following:

Register	Description
eax	Accumulator register - Used in arithmetic operations
ebx	Base register - Used as a pointer to data
ecx	Counter register - Used in shift/rotate instructions and loops
edx	Data register - Used in arithmetic operations and I/O operations
esi	Source Index register - Used as a pointer to a source in stream operations
edi	Destination Index register - Used as a pointer to a destination in stream operations
esp	Stack Pointer register - Pointer to the top of the stack
ebp	Stack Base Pointer register - Used to point to the base of the stack

But most of the registers have lost their special purposes in the modern instruction set, except for esp and ebp.

SysCalls

what is a syscall? well a syscall or system call are APIs for the interface between user space and kernel space. We already use system calls.we've already seen registers and the mov instruction respectively.

You can make use of Linux system calls in your assembly programs. You need to take the following steps for using Linux system calls in your program -

* Put the system call number in the EAX register.
* Store the arguments to the system call in the registers EBX, ECX, etc.
* Call the relevant interrupt (80h).
* The result is usually returned in the EAX register.

The following table shows some of the system calls used in this tutorial -

%eax	Name	%ebx	%ecx	%edx	%esx	%edi
1	sys_exit	int	-	-	-	-
2	sys_fork	struct pt_regs	NULL	-	-	-
3	sys_read	unsigned int	char *	size_t	-	-
4	sys_write	unsigned int	const char *	size_t	-	-
5	sys_open	const char *	int	int	-	-
6	sys_close	unsigned int	-	-	-	-

let's go to our first assembly code, let's make a folder with all the codes used here, it will be in the codes folder lets go !!

section .data
   msg db "Hello, Wolrd!!", 0xa ;Declaring_variable 
   len equ $-msg ;Size of bytes 

section .text
   global _start

_start:
  
   mov edx, len ; Moving len to edx
   mov ecx, msg ; Moving msg to ecx
   mov ebx, 1 ; Stdout to ebx
   mov eax, 4 ; Moving sys_write to eax 
   int 0x80; Call Kernel

   mov eax, 1; moving sys_exit to eax
   int 0x80; Call Kernel

congratulations on your ability to create your first assembly code !!

Types-Variables-Allocation

Storage space for variables. Assembly policy is used for storage space allocation. It can be used to reserve and initialize one or more bytes such as db dw dd dq dt.

the allocation for boot data is as follows.

syntax	-	-
variable-name	define-type	initial-value

as well as other languages, make the correct use of the allocation, to avoid errors when it is compiled. next we will see the instructions and their functions in the allocation.

type	Meaning	bytes allocated
DB	Define Byte	allocates 1 byte
DW	Define Word	allocates 2 bytes
DD	Define Doubleword	allocates 4 bytes
DQ	Define Quadword	allocates 8 bytes
DT	Define Ten Bytes	allocates 10 bytes

what does each allocate? we will see in the following example.

choice		DB	'y'

it allocates characters in, it can be a string like in the example of "hello, wolrd" remember?

number		DW	12345

allocates positive numbers

number_negative	DW	-12345

allocates numbers negatives

big_number	DQ	123456789

allocates numbers bigs

real_number1	DD	1.234

allocates numbers floats

real_number2	DQ	123.456

allocates numbers floats big

VitorMob/AsmHelp