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https://github.com/reactos/reactos.git
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256 lines
7.8 KiB
NASM
256 lines
7.8 KiB
NASM
; BOOTSECT.ASM
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; FAT12/16 Boot Sector
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; Copyright (c) 1998 Brian Palmer
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org 7c00h
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segment .text
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bits 16
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start:
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jmp short main
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nop
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OEMName db 'FreeLDR!'
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BytesPerSector dw 512
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SectsPerCluster db 1
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ReservedSectors dw 1
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NumberOfFats db 2
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MaxRootEntries dw 512
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TotalSectors dw 2880
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MediaDescriptor db 0f0h
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SectorsPerFat dw 9
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SectorsPerTrack dw 18
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NumberOfHeads dw 2
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HiddenSectors dd 0
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TotalSectorsBig dd 0
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BootDrive db 0
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Reserved db 0
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ExtendSig db 29h
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SerialNumber dd 00000000h
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VolumeLabel db 'FreeLoader!'
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FileSystem db 'FAT12 '
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main:
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cli
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cld
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xor ax,ax
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mov ss,ax
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mov sp,7c00h ; Setup a stack
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mov ax,cs ; Setup segment registers
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mov ds,ax ; Make DS correct
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mov es,ax ; Make ES correct
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sti ; Enable ints now
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mov [BootDrive],dl ; Save the boot drive
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xor ax,ax ; Zero out AX
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; Reset disk controller
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int 13h
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jnc Continue1
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jmp BadBoot ; Reset failed...
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Continue1:
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; Now we must find our way to the first sector of the root directory
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xor ax,ax
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xor cx,cx
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mov al,[NumberOfFats] ; Number of fats
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mul WORD [SectorsPerFat] ; Times sectors per fat
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add ax,WORD [HiddenSectors]
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adc dx,WORD [HiddenSectors+2] ; Add the number of hidden sectors
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add ax,[ReservedSectors] ; Add the number of reserved sectors
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adc dx,cx ; Add carry bit
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push ax ; Store it on the stack
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push dx ; Save 32-bit logical start sector
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push ax
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push dx ; Save it for later use also
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; DX:AX now has the number of the starting sector of the root directory
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; Now calculate the size of the root directory
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mov ax,0020h ; Size of dir entry
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mul WORD [MaxRootEntries] ; Times the number of entries
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mov bx,[BytesPerSector]
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add ax,bx
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dec ax
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div bx ; Divided by the size of a sector
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; AX now has the number of root directory sectors
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xchg ax,cx ; Now CX has number of sectors
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pop dx
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pop ax ; Restore logical sector start
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push cx ; Save for later use
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mov bx,7c0h ; We will load the root directory
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add bx,20h ; Right after the boot sector in memory
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mov es,bx
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xor bx,bx ; We will load it to [0000:7e00h]
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call ReadSectors ; Read the sectors
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jnc Continue2 ; BadBoot on error
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jmp BadBoot
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Continue2:
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; Now we have to find our way through the root directory to
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; The OSLOADER.SYS file
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mov bx,[MaxRootEntries]; Search entire root directory
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mov ax,7e0h ; We loaded at 07e0:0000
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mov es,ax
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xor di,di
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mov si,filename
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mov cx,11
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rep cmpsb ; Compare filenames
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jz FoundFile ; If same we found it
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dec bx
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jnz FindFile
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jmp ErrBoot
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FindFile:
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mov ax,es ; We didn't find it in the previous dir entry
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add ax,2 ; So lets move to the next one
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mov es,ax ; And search again
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xor di,di
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mov si,filename
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mov cx,11
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rep cmpsb ; Compare filenames
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jz FoundFile ; If same we found it
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dec bx ; Keep searching till we run out of dir entries
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jnz FindFile ; Last entry?
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jmp ErrBoot
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FoundFile:
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xor di,di ; ES:DI has dir entry
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xor dx,dx
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mov ax,WORD [es:di+1ah] ; Get start cluster
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dec ax
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dec ax
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xor ch,ch
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mov cl,BYTE [SectsPerCluster] ; Times sectors per cluster
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mul cx
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pop cx ; Get number of sectors for root dir
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add ax,cx
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adc dx,0
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pop cx ; Get logical start sector of
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pop bx ; Root directory
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add ax,bx ; Now we have DX:AX with the logical start
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adc dx,cx ; Sector of OSLOADER.SYS
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push ax
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push dx
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mov ax,WORD [es:di+1ch]
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mov dx,WORD [es:di+1eh]
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mov bx,[BytesPerSector]
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dec bx
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add ax,bx
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adc dx,0
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div WORD [BytesPerSector]
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xchg ax,cx ; Now CX has number of sectors of OSLOADER.SYS
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pop dx
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pop ax
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mov bx,800h
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mov es,bx
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xor bx,bx ; Load ROSLDR at 0000:8000h
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call ReadSectors ; Load it
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jc BadBoot
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mov dl,[BootDrive]
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xor ax,ax
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push ax
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mov ax,8000h
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push ax ; We will do a far return to 0000:8000h
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retf ; Transfer control to ROSLDR
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; Reads logical sectors into [ES:BX]
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; DX:AX has logical sector number to read
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; CX has number of sectors to read
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; CarryFlag set on error
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ReadSectors:
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push ax
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push dx
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push cx
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xchg ax,cx
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xchg ax,dx
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xor dx,dx
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div WORD [SectorsPerTrack]
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xchg ax,cx
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div WORD [SectorsPerTrack] ; Divide logical by SectorsPerTrack
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inc dx ; Sectors numbering starts at 1 not 0
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xchg cx,dx
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div WORD [NumberOfHeads] ; Number of heads
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mov dh,dl ; Head to DH, drive to DL
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mov dl,[BootDrive] ; Drive number
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mov ch,al ; Cylinder in CX
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ror ah,1 ; Low 8 bits of cylinder in CH, high 2 bits
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ror ah,1 ; in CL shifted to bits 6 & 7
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or cl,ah ; Or with sector number
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mov ax,513
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int 13h ; DISK - READ SECTORS INTO MEMORY
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; AL = number of sectors to read, CH = track, CL = sector
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; DH = head, DL = drive, ES:BX -> buffer to fill
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; Return: CF set on error, AH = status (see AH=01h), AL = number of sectors read
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pop cx
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pop dx
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pop ax
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jc ReadFail
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inc ax ;Increment Sector to Read
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jnz NoCarry
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inc dx
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NoCarry:
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push bx
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mov bx,es
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add bx,20h
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mov es,bx
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pop bx
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; Increment read buffer for next sector
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loop ReadSectors ; Read next sector
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ReadFail:
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ret
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; Displays a bad boot message
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; And reboots
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BadBoot:
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mov si,msgDiskError ; Bad boot disk message
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call PutChars ; Display it
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mov si,msgAnyKey ; Press any key message
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call PutChars ; Display it
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jmp Reboot
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; Displays an error message
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; And reboots
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ErrBoot:
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mov si,msgFreeLdr ; FreeLdr not found message
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call PutChars ; Display it
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mov si,msgAnyKey ; Press any key message
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call PutChars ; Display it
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Reboot:
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xor ax,ax
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int 16h ; Wait for a keypress
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int 19h ; Reboot
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PutChars:
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lodsb
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or al,al
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jz short Done
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mov ah,0eh
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mov bx,07h
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int 10h
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jmp short PutChars
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Done:
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retn
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msgDiskError db 'Disk error',0dh,0ah,0
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msgFreeLdr db 'FREELDR.SYS not found',0dh,0ah,0
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msgAnyKey db 'Press any key to continue.',0dh,0ah,0
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filename db 'FREELDR SYS'
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times 510-($-$$) db 0 ; Pad to 510 bytes
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dw 0aa55h ; BootSector signature
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