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261 lines
4.9 KiB
261 lines
4.9 KiB
!
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! SYS_SIZE is the number of clicks (16 bytes) to be loaded.
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! 0x3000 is 0x30000 bytes = 196kB, more than enough for current
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! versions of linux
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!
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SYSSIZE = 0x3000
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!
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! bootsect.s (C) 1991 Linus Torvalds
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!
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! bootsect.s is loaded at 0x7c00 by the bios-startup routines, and moves
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! iself out of the way to address 0x90000, and jumps there.
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!
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! It then loads 'setup' directly after itself (0x90200), and the system
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! at 0x10000, using BIOS interrupts.
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!
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! NOTE! currently system is at most 8*65536 bytes long. This should be no
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! problem, even in the future. I want to keep it simple. This 512 kB
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! kernel size should be enough, especially as this doesn't contain the
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! buffer cache as in minix
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!
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! The loader has been made as simple as possible, and continuos
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! read errors will result in a unbreakable loop. Reboot by hand. It
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! loads pretty fast by getting whole sectors at a time whenever possible.
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.globl begtext, begdata, begbss, endtext, enddata, endbss
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.text
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begtext:
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.data
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begdata:
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.bss
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begbss:
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.text
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SETUPLEN = 4 ! nr of setup-sectors
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BOOTSEG = 0x07c0 ! original address of boot-sector
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INITSEG = 0x9000 ! we move boot here - out of the way
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SETUPSEG = 0x9020 ! setup starts here
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SYSSEG = 0x1000 ! system loaded at 0x10000 (65536).
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ENDSEG = SYSSEG + SYSSIZE ! where to stop loading
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! ROOT_DEV: 0x000 - same type of floppy as boot.
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! 0x301 - first partition on first drive etc
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ROOT_DEV = 0x306
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entry start
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start:
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mov ax,#BOOTSEG
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mov ds,ax
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mov ax,#INITSEG
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mov es,ax
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mov cx,#256
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sub si,si
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sub di,di
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rep
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movw
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jmpi go,INITSEG
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go: mov ax,cs
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mov ds,ax
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mov es,ax
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! put stack at 0x9ff00.
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mov ss,ax
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mov sp,#0xFF00 ! arbitrary value >>512
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! load the setup-sectors directly after the bootblock.
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! Note that 'es' is already set up.
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load_setup:
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mov dx,#0x0000 ! drive 0, head 0
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mov cx,#0x0002 ! sector 2, track 0
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mov bx,#0x0200 ! address = 512, in INITSEG
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mov ax,#0x0200+SETUPLEN ! service 2, nr of sectors
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int 0x13 ! read it
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jnc ok_load_setup ! ok - continue
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mov dx,#0x0000
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mov ax,#0x0000 ! reset the diskette
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int 0x13
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j load_setup
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ok_load_setup:
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! Get disk drive parameters, specifically nr of sectors/track
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mov dl,#0x00
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mov ax,#0x0800 ! AH=8 is get drive parameters
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int 0x13
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mov ch,#0x00
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seg cs
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mov sectors,cx
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mov ax,#INITSEG
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mov es,ax
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! Print some inane message
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mov ah,#0x03 ! read cursor pos
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xor bh,bh
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int 0x10
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mov cx,#24
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mov bx,#0x0007 ! page 0, attribute 7 (normal)
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mov bp,#msg1
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mov ax,#0x1301 ! write string, move cursor
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int 0x10
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! ok, we've written the message, now
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! we want to load the system (at 0x10000)
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mov ax,#SYSSEG
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mov es,ax ! segment of 0x010000
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call read_it
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call kill_motor
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! After that we check which root-device to use. If the device is
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! defined (!= 0), nothing is done and the given device is used.
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! Otherwise, either /dev/PS0 (2,28) or /dev/at0 (2,8), depending
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! on the number of sectors that the BIOS reports currently.
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seg cs
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mov ax,root_dev
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cmp ax,#0
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jne root_defined
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seg cs
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mov bx,sectors
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mov ax,#0x0208 ! /dev/ps0 - 1.2Mb
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cmp bx,#15
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je root_defined
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mov ax,#0x021c ! /dev/PS0 - 1.44Mb
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cmp bx,#18
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je root_defined
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undef_root:
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jmp undef_root
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root_defined:
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seg cs
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mov root_dev,ax
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! after that (everyting loaded), we jump to
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! the setup-routine loaded directly after
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! the bootblock:
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jmpi 0,SETUPSEG
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! This routine loads the system at address 0x10000, making sure
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! no 64kB boundaries are crossed. We try to load it as fast as
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! possible, loading whole tracks whenever we can.
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!
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! in: es - starting address segment (normally 0x1000)
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!
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sread: .word 1+SETUPLEN ! sectors read of current track
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head: .word 0 ! current head
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track: .word 0 ! current track
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read_it:
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mov ax,es
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test ax,#0x0fff
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die: jne die ! es must be at 64kB boundary
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xor bx,bx ! bx is starting address within segment
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rp_read:
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mov ax,es
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cmp ax,#ENDSEG ! have we loaded all yet?
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jb ok1_read
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ret
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ok1_read:
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seg cs
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mov ax,sectors
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sub ax,sread
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mov cx,ax
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shl cx,#9
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add cx,bx
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jnc ok2_read
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je ok2_read
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xor ax,ax
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sub ax,bx
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shr ax,#9
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ok2_read:
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call read_track
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mov cx,ax
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add ax,sread
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seg cs
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cmp ax,sectors
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jne ok3_read
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mov ax,#1
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sub ax,head
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jne ok4_read
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inc track
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ok4_read:
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mov head,ax
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xor ax,ax
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ok3_read:
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mov sread,ax
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shl cx,#9
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add bx,cx
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jnc rp_read
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mov ax,es
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add ax,#0x1000
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mov es,ax
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xor bx,bx
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jmp rp_read
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read_track:
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push ax
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push bx
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push cx
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push dx
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mov dx,track
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mov cx,sread
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inc cx
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mov ch,dl
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mov dx,head
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mov dh,dl
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mov dl,#0
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and dx,#0x0100
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mov ah,#2
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int 0x13
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jc bad_rt
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pop dx
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pop cx
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pop bx
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pop ax
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ret
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bad_rt: mov ax,#0
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mov dx,#0
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int 0x13
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pop dx
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pop cx
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pop bx
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pop ax
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jmp read_track
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/*
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* This procedure turns off the floppy drive motor, so
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* that we enter the kernel in a known state, and
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* don't have to worry about it later.
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*/
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kill_motor:
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push dx
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mov dx,#0x3f2
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mov al,#0
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outb
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pop dx
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ret
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sectors:
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.word 0
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msg1:
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.byte 13,10
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.ascii "Loading system ..."
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.byte 13,10,13,10
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.org 508
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root_dev:
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.word ROOT_DEV
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boot_flag:
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.word 0xAA55
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.text
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endtext:
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.data
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enddata:
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.bss
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endbss:
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