[SOLVED] Creating a boot disk

[gerard4143]

Hi I'm trying to create a boot disk and I have a few questions

In the code below I basically copy some assembler code from the boot file into the array ch and then I set ch[510] = 0x55, ch[511] = 0xaa. Why do I set the the byte ch[510] = 0x55 and ch[511] = 0xaa?

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

int main(int argc, char**argv)
{
	char ch[512];

	int fd = open("boot", O_RDONLY);
	read(fd, ch, 512);
	close(fd);

	fd = open("/dev/fd0", O_RDWR);
	lseek(fd, 0, SEEK_CUR);

	ch[510] = 0x55;
	ch[511] = 0xaa;

	write(fd, ch, 512);

	close(fd);

	exit(EXIT_SUCCESS);
}

and question two - how could I get the assembler code below to display some characters. The examples I found on the Internet said to:

movw $0xb800, %ax
movw %ax, %es
to here I have no problems
but the next two lines???
I have to move $0x41 into the address 0000 with offset %es or $0xb800
and move $0x1f into address 0001 with offset %es or $0xb800

movb $0x41, into address 0000 with offset 0xb800
movb $0x1f, into address 0000 with offset 0xb8000

this I don't know how to do??

.section .data

.section .bss

.section .text
	.global _start
_start:

loop1:
		jmp	loop1

[gerard4143]

Found the answer...

.code16

.section .data

.section .bss

.section .text
	.global _start
_start:
		movw	$0xb800, %ax
		movw	%ax, %es

		movb	$0x41, %es:0
		movb	$0x1f, %es:1
loop1: 		
		jmp	loop1