Writing bootsector - The GNU "as" (aka gas) way

My friend Haynes pointed me to this quick hands on tutorial..

http://susam.in/articles/boot-sector-code.php

Simple, brief and neatly explained tutorial on how to write bootsectors with GNU tools for the x86 architecture.


Also, objcopy can be used to convert between binary formats.

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Sunday 2008-03-30 02:19 UTC+5:30

I used the snippet of code given at the above webpage

# Author: Susam Pal 
#
# To assemble and link this code, execute the following commands:-
# as -o char.o char.s
# ld --oformat binary -o char char.o
#
# To write this code into the boot sector of a device, say /dev/sdb:-
# dd if=char of=/dev/sdb
# echo -ne "\x55\xaa" | dd seek=510 bs=1 of=/dev/sdb

.code16
.section .text
.globl _start
_start:
 mov $0xb800, %ax
 mov %ax, %ds
 movb $'A', 0
 movb $0x1e, 1
idle:
 jmp idle

Assembled and linked the same using the commands given in the comment part of the code.
Then, had the ".com" file pushed into a floppy image file. And it worked !

When I pushed the binary into a floppy image formatted as "mkfs -t vfat", the filesystem went corrupt
even though the booting was successful. When I tried formatting the floppy as ext3, everything worked
and the filesystem was also preserved. Wondering why writing the bootsector corrupted the vfat filesystem.

Used qemu to test the stuff !!

.