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