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