ASM

1. Registers in x86 Assembly

Register	Common Use	Size	Description
eax	General accumulator	32-bit	Holds operation results, return values
ebx	Base register	32-bit	Often used as an index or scratch register
ecx	Counter	32-bit	Used in loops and iteration
edx	Data	32-bit	Used in division and system calls
esi	Source Index	32-bit	Used for traversing source buffers
edi	Destination Index	32-bit	Used for writing into destination buffers

2. Register Subsets (Bytes)

Each 32-bit register can be split:

32-bit	16-bit	High Byte	Low Byte
eax	ax	ah	al
ebx	bx	bh	bl

• al, bl: used to hold 1 byte (great for XOR operations)

• Used when processing byte-by-byte payloads

3. Core Assembly Instructions

Instruction	Meaning	Example
mov	Move value/data	mov eax, 5
add	Add	add eax, ebx
sub	Subtract	sub eax, 2
xor	Exclusive OR (XOR)	xor al, bl
cmp	Compare values	cmp eax, ebx
je	Jump if equal	je label
jne	Jump if not equal	jne label
jl	Jump if less	jl label
jg	Jump if greater	jg label
jmp	Unconditional jump	jmp loop_start
dec	Decrease by 1	dec ecx
loop	Loop with ECX counter	loop decode_loop
push	Push onto stack	push eax
pop	Pop from stack	pop ebx

4. XOR Concept (Cyber Gold)

• XOR is a bitwise operation:

  • 1 ^ 1 = 0

  • 1 ^ 0 = 1

  • 0 ^ 1 = 1

  • 0 ^ 0 = 0

• Perfect for encryption/decryption

• Reversible: value ^ key ^ key = value

• Often used to hide payloads, strings, shellcode

5. Special Characters.

Carácter	ASCII Decimal	En C
\n	10	salto de línea
\r	13	retorno de carro
\t	9	tabulación
\0	0	null terminator