Assembly x86 NASM and Registers

The processor has many register that are used to perform different operations. Bellow is a list with the processor register (prefixed with %) and the operation corresponding to the data inside of it.

NR	syscall name	references	%eax	arg0 (%ebx)	arg1 (%ecx)	arg2 (%edx)	arg3 (%esi)	arg4 (%edi)	arg5 (%ebp)
0	restart_syscall	man/ cs/	0x00	-	-	-	-	-	-
1	exit	man/ cs/	0x01	int error_code	-	-	-	-	-
2	fork	man/ cs/	0x02	-	-	-	-	-	-
3	read	man/ cs/	0x03	unsigned int fd	char *buf	size_t count	-	-	-

Here is a simple Assembly

section .data
    hello db 'Hello, World!', 0

section .text
    global _start

_start:
    ; write system call
    mov eax, 4           ; system call number for write
    mov ebx, 1           ; file descriptor (stdout)
    mov ecx, hello       ; message to write
    mov edx, 13          ; message length
    int 0x80             ; invoke the kernel

    ; exit system call
    mov eax, 1           ; system call number for exit
    xor ebx, ebx         ; exit status (0)
    int 0x80             ; invoke the kernel

Data Section The data section, also known as the “.data” section, is used to declare and allocate memory for initialized data values. This section is typically used for storing static variables, strings, arrays, and other data that have predefined values.

The db directive stands for “define byte” and is used to allocate memory for individual bytes. It allows you to declare and initialize byte-sized data values.