Bootsector authoring by Gareth Owen
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
-=- gaz@athene.co.uk -=-
Requirements
============
During this article I assume that you have good knowledge of the
assembly language and intel architecture.
If not, read an assembly tutorial, they aren't hard to find...
Start
=====
Creating your own bootsector is simpler than you may think,
the only requirement is that the bootsector is 512 bytes long, and at
offset 0x1FE (decimal=510), the word 0xAA55 is placed. This is the first
thing the BIOS does when the PC boots up, it first looks on the first
floppy drive at the first sector for 0xAA55 at the end, and if it finds it
then it loads it into memory, and starts executing it, otherwise it trys the
primary harddisk, and if that isn't found it just bombs out with an error.
You should place your boot sector at:
Sector 1
Cylinder 0
Head 0
I recommend you start playing about with floppys first instead of your hard disk
because the hard disk bootsector stores information about the file system if you
are running DOS/Windows, if you overrite that, then you have just lost your
hard disk contents :-)
The BIOS loads the bootsector at linear offset 0x7C00, the state of
the registers are:
DL = Boot drive, 1h = floppy1, 80h = primary harddisk, etc
CS = 0
IP = 0x7c00
So instead of adding [ORG 7C00h] to the top of your file, you can add:
mov ax, 0x7C0
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
And that will set-up the segment registers so they point to the start of
your bootsector..
Most boot sectors usually just store the boot drive, load the kernel
from disk, and jump to it.. Some will also load protected mode.
Since most people find it easier looking at source code and figuring
it out than reading documentation i have included sources for a boot
sector and a boot sector writter.
Here is the bootsector...
;******************* START ************************
; Boot sector authoring example by Gareth Owen (gaz@athene.co.uk)
; This should be accompanied with an article explaining bootsectors
[BITS 16] ; the bios starts out in 16-bit real mode
[ORG 0]; Data offset = 0
jmp start ; skip over our data and functions, we cannot execute data :-),
; well, you can, but i am not held responsible for the results :)
; Boot sector authoring example by Gareth Owen (gaz@athene.co.uk)
; This should be accompanied with an article explaining bootsectors
[BITS 16] ; the bios starts out in 16-bit real mode
[ORG 0]; Data offset = 0
jmp start ; skip over our data and functions, we cannot execute data :-),
; well, you can, but i am not held responsible for the results :)
; -------------------------------------
; Data used in the boot-loading process
; ------------------------------------------------------------------------
bootdrv db 0
bootmsg db 'Gareth Owen',39,'s Boot Sector Example',13,10,0
rebootmsg db 'Press any key to reboot',13,10,0
; these are used in the processor identification
processormsg db 'Checking for 386+ processor: ',0
need386 db 'Sorry... 386+ required!',13,10,0
found386 db 'Found!',13,10,0
whateverdb 'Insert your code to do something here',13,10,0
;*******************************************
; Functions we are going to use ...
;*******************************************
detect_cpu:
mov si, processormsg ; tell the user what we're doing
call message
; test if 8088/8086 is present (flag bits 12-15 will be set)
pushf ; save the flags original value
xor ah,ah ; ah = 0
push ax ; copy ax into the flags
popf ; with bits 12-15 clear
pushf ; Read flags back into ax
pop ax
and ah,0f0h ; check if bits 12-15 are set
cmp ah,0f0h
je no386 ; no 386 detected (8088/8086 present)
; check for a 286 (bits 12-15 are clear)
mov ah,0f0h ; set bits 12-15
push ax ; copy ax onto the flags
popf
pushf ; copy the flags into ax
pop ax
and ah,0f0h ; check if bits 12-15 are clear
jz no386 ; no 386 detected (80286 present)
popf ; pop the original flags back
mov si, found386
call message
ret ; no 8088/8086 or 286, so ateast 386
no386:
mov si,need386 ; tell the user the problem
call message
jmp reboot ; and reboot when key pressed
; ********************************************************************
message: ; Dump ds:si to screen.
lodsb ; load byte at ds:si into al
or al,al ; test if character is 0 (end)
jz done
mov ah,0eh ; put character
mov bx,0007 ; attribute
int 0x10 ; call BIOS
jmp message
done:
ret
; ********************************************************************
getkey:
mov ah, 0 ; wait for key
int 016h
ret
; ********************************************************************
reboot:
mov si, rebootmsg ; be polite, and say we're rebooting
call message
call getkey ; and even wait for a key :)
db 0EAh ; machine language to jump to FFFF:0000 (reboot)
dw 0000h
dw 0FFFFh
; no ret required; we're rebooting! (Hey, I just saved a byte :)
; *******************************************
; The actual code of our boot loading process
; *******************************************
start:
mov ax,0x7c0 ; BIOS puts us at 0:07C00h, so set DS accordinly
mov ds,ax ; Therefore, we don't have to add 07C00h to all our
data
mov [bootdrv], dl ; quickly save what drive we booted from
cli ; clear interrupts while we setup a stack
mov ax,0x9000 ; this seems to be the typical place for a stack
mov ss,ax
mov sp,0xffff ; let's use the whole segment. Why not? We can :)
sti ; put our interrupts back on
; Interestingly enough, apparently the processor will disable
; interupts itself when you directly access the stack segment!
; Atleast it does in protected mode, I'm not sure about real mode.
mov si,bootmsg ; display our startup message
call message
call detect_cpu ; check if we've got a 386
.386 ; use 386 instructions from now on (I don't want to manually include
; operand-size(66h) or address-size(67h) prefixes... it's annoying :)
mov si,whatever ; tell the user we're not doing anything interesting here
call message
call getkey
call reboot
times 510-($-$$) db 0
dw 0xAA55
;******************** GBOOTSECT END *************************
Here is the code for writting the bootsector to a floppy disk.
It has been compiled with DJGPP for DOS.
It writes the file 'bootsect', onto Sector 1, Cylinder 0, Head 0 of
the floppy drive.
//***************START****************
#include <bios.h>
#include <stdio.h>
void main()
{
FILE *in;
unsigned char buffer[520];
if((in = fopen("bootsect", "rb"))==NULL)
{
printf("Error loading file\n");
exit(0);
}
fread(&buffer, 512, 1, in);
while(biosdisk(3, 0, 0, 0, 1, 1, buffer));
fclose(in);
}
//*************END****************************
Well, if you still don't understand something, then mail me
at gaz@athene.co.uk and i'll help you out
- Gareth Owen
