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|
;;; Hello emacs, this looks like -*- asm -*- code, doesn't it?
;;;
;;; This forms a simple dummy boot program for use with a tool to
;;; format DOS floppies. All it does is displaying a message, and
;;; recover gracefully by re-initializing the CPU.
;;;
;;; Written by Joerg Wunsch, Dresden. Placed in the public domain.
;;; This software is provided as is, neither kind of warranty applies.
;;; Use at your own risk.
;;;
;;; (This is written in as86 syntax. as86 is part of Bruce Evans'
;;; bcc package.)
;;;
;;; $FreeBSD$
;;;
;;; This code must be linked to address 0x7c00 in order to function
;;; correctly (the BIOS boot address).
;;;
;;; It's 16-bit code, and we don't care for a data segment.
use16
.text
entry _begin
_begin: jmp init ; jump to boot prog
nop ; historical baggage ;-)
;;;
;;; Reserve space for the "BIOS parameter block".
;;; This will be overwritten by the actual formatting routine.
;;;
bpb: .ascii "BSD 4.4" ; "OEM" name
.word 512 ; sector size
.byte 2 ; cluster size
.word 1 ; reserved sectors (just the boot sector)
.byte 2 ; FAT count
.word 112 ; # of entries in root dir
.word 1440 ; total number of sectors, MSDOS 3.3 or below
.byte 0xf9 ; "media descriptor"
.word 3 ; FAT size (sectors)
.word 9 ; sectors per track
.word 2 ; heads per cylinder
.word 0 ; hidden sectors
;; MSDOS 4.0++ -- only valid iff total number of sectors == 0
.word 0 ; unused
.long 0 ; total number of sectors
.short 0 ; physical drive (0, 1, ..., 0x80) %-)
.byte 0 ; "extented boot signature"
.long 0 ; volume serial number (i.e., garbage :)
.ascii " " ; label -- same as vol label in root dir
.ascii "FAT12 " ; file system ID
;;;
;;; Executable code starts here.
;;;
init:
;; First, display our message.
mov si, *message
lp1: seg cs
movb al, [si]
inc si
testb al, al
jz lp2 ; null-terminated string
movb bl, *7 ; display with regular attribute
movb ah, *0x0e ; int 0x10, fnc 0x0e -- emulate tty
int 0x10
jmp lp1
lp2: xorb ah, ah ; int 0x16, fnc 0x00 -- wait for keystroke
int 0x16
mov ax, *0x40 ; write 0x1234 to address 0x472 --
push ax ; tell the BIOS that this is a warm boot
pop dx
mov 0x72, *0x1234
jmpf 0xfff0,0xf000 ; jump to CPU initialization code
message:
.byte 7
.byte 0xc9
.byte 0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd
.byte 0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd
.byte 0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd
.byte 0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd
.byte 0xcd,0xcd,0xcd,0xcd,0xcd,0xcd
.byte 0xbb, 13, 10, 0xba
.ascii " Sorry, this disc does actually not contain "
.byte 0xba, 13, 10, 0xba
.ascii " a bootable system. "
.byte 0xba, 13, 10, 0xba
.ascii " Press any key to reboot. "
.byte 0xba, 13, 10, 0xc8
.byte 0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd
.byte 0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd
.byte 0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd
.byte 0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd,0xcd
.byte 0xcd,0xcd,0xcd,0xcd,0xcd,0xcd
.byte 0xbc, 13,10
.byte 0
;; Adjust the value below after changing the length of
;; the code above!
.space 0x1fe-0x161 ; pad to 512 bytes
.byte 0x55, 0xaa ; yes, we are bootable (cheating :)
end
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