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324
boot/bootsect.asm
Executable file
324
boot/bootsect.asm
Executable file
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; $Id: bootsect.asm 5001 2012-10-12 11:34:05Z os $
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;******************************************************************************
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;* Betriebssysteme *
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;*----------------------------------------------------------------------------*
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;* *
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;* B O O T S E C T *
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;* *
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;*----------------------------------------------------------------------------*
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;* Code fuer den Disketten-Bootblock des System-Images. Das BIOS laedt den *
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;* ersten Block einer Diskette (den Bootblock) beim Starten des Rechner in *
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;* den Hauptspeicher und fuehrt ihn aus. Der Programmcode des Bootblocks *
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;* laedt nun das restliche System und fuehrt es aus. *
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;******************************************************************************
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;
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; Konstanten
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;
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BIOSSEG equ 0x07c0 ; Hierher wird der Bootsector
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; vom BIOS geladen
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BOOTSEG equ 0x0060 ; Hierher verschiebt sich der
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; Boot-Code
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SETUPSEG equ 0x9000 ; Hierher laedt der Boot-Code den
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; Setup-Code (max. 64K inkl. Stack)
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SYSTEMSEG equ 0x1000 ; System-Code (max. 512K)
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SECTORSZ equ 512 ; Groesse eines Sektors in Bytes
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[SECTION .text]
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;
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; Boot-Code
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;
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bootsector:
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jmp skip_data
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;------------------------------------------------------------------------------
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;
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; Datenbereich, der von 'build' beim Erzeugen der Boot-Diskette
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; gefuellt wird.
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;
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pad:
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times 4+bootsector-$ db 0 ; Bytes zum Auffuellen, damit 'total_tracks' an einer
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; geraden und tools/build.c bekannten Adresse liegt
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total_tracks:
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dw 0 ; Anzahl der Tracks der Diskette
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total_heads:
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dw 0 ; Anzahl der Seiten der Diskette
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total_sectors:
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dw 0 ; Anzahl der Sektoren pro Track
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setup_sectors:
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dw 0 ; Anzahl der Sektoren, die der Setup-Code einnimmt
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system_sectors:
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dw 0 ; Anzahl der Sektoren, die das System einnimmt
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bootdevice:
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db 0 ; BIOS Geraetecode: 00: Disk A, 01: Disk B, ..., 0x80 HD0, ...
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curr_track:
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db 0 ; Track, bei dem die Diskette/Partition beginnt
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curr_head:
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db 0 ; Head, bei dem die Diskette/Partition beginnt
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curr_sector:
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db 0 ; Sector, bei dem die Diskette/Partition beginnt
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;-----------------------------------------------------------------------
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;
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; Kopieren des Bootsectors
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;
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skip_data:
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mov bl,dl ; vom BIOS uebergebenes Boot-Device sichern
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mov ax,BIOSSEG
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mov ds,ax
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xor si,si
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mov ax,BOOTSEG
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mov es,ax
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xor di,di
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mov cx,SECTORSZ/2
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rep movsw
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;
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; Ausfuehrung durch die Kopie fortsetzen
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;
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jmp BOOTSEG:start
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;
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; Segmentregister initialisieren und Platz fuer den Stack schaffen
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;
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start:
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mov ax,cs ; Daten-, Stack- und Codesegment sollen
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mov ds,ax ; hierher zeigen.
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mov ss,ax
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mov sp,4*SECTORSZ ; Drei Sektoren als Stack freilassen
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mov [bootdevice],bl ; zuvor gesichertes Boot-Device permanent ablegen
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;
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; Ausgabe einer Meldung mit Hilfe eines BIOS-Aufrufs
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;
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mov ah,0x03 ; Feststellen der Cursor-Position
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xor bh,bh
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int 0x10
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mov cx,13
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mov bx,0x0007 ; page 0, attribute 7 (normal)
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mov ax,ds
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mov es,ax
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mov bp,bootmessage
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mov ax,0x1301 ; Ausgabe des Textes, Cursor bewegen
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int 0x10
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;
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; Nachladen des Setup-Codes und des Systems selbst.
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;
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xor ah,ah ; Reset des Disketten-/Plattencontrollers
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mov dl,[bootdevice]
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int 0x13
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;
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; Informationen ueber die Laufwerksgeometrie holen
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;
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hdd_probe:
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mov dl,[bootdevice]
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test dl,0x80
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jz load_setup ; Floppy mit den Standardparametern laden
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mov ah,0x8
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int 0x13
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jc load_setup ; CF bei Fehler gesetzt
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mov [total_heads],dh
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mov ax,cx ; CX sichern
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and ax,0x3f
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mov [total_sectors],ax
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mov ax,cx
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shr ax,6
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mov [total_tracks],ax
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;
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; Weiterstellen der Disketten-/Plattenposition um 1 (Bootblock)
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;
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load_setup:
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call step_disk
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;
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; Laden des Setup-Codes
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;
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mov word [curr_segment],SETUPSEG
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mov word [curr_offset],0
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mov ax,[setup_sectors]
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call load
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;
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; Laden des Kernels
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;
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mov word [curr_segment],SYSTEMSEG
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mov word [curr_offset],0
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mov ax,[system_sectors]
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call load
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;
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; Floppy wieder abschalten
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;
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call stop_floppy_motor
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;
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; Start des Setup-Codes
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;
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mov ax, [system_sectors] ; Speichere Anzahl an System-Sektoren in AX.
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jmp SETUPSEG:0
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;
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; load
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;
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; Die 'ax' Sektoren von der Diskette in den Hauptspeicher. Die Position auf
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; der Diskette muss vorher in curr_head/curr_track/curr_sector und die
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; Position im Hauptspeicher in curr_segment/curr_offset stehen. Die Positionen
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; werden entsprechend der geladenen Sektoren weitergestellt.
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;
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load:
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mov [to_load],ax
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l_next_part:
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mov al,[curr_head]
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mov [last_head],al
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mov al,[curr_track]
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mov [last_track],al
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mov al,[curr_sector]
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mov [last_sector],al
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mov ax,[curr_segment]
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mov [last_segment],ax
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mov ax,[curr_offset]
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mov [last_offset],ax
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mov al,0
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l_loop: call step
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cmp byte [curr_sector],0x01
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je l_now
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cmp word [curr_offset],0x0000
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je l_now
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cmp al,[to_load]
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jne l_loop
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l_now:
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push ax
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mov dl,[bootdevice]
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mov dh,[last_head]
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mov ch,[last_track]
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mov cl,[last_sector]
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mov bx,[last_segment]
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mov es,bx
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mov bx,[last_offset]
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mov ah,0x02 ; Funktionscode fuer 'Lesen'
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int 0x13
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pop ax
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push ax
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call print_dot
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pop ax
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mov ah,0
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sub [to_load],ax
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jne l_next_part
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ret
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;
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; step
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;
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; Stellt die aktuelle Position im Hauptspeicher und auf der Diskette
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; um einen Sektor (512 Byte) weiter.
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;
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step: add al,1
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call step_disk
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call step_memory
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ret
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step_disk:
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mov bl,[curr_sector]
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add bl,1
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mov [curr_sector],bl
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cmp bl,[total_sectors]
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jle l_1
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mov byte [curr_sector],1
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mov bl,[curr_head]
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add bl,1
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mov [curr_head],bl
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cmp bl,[total_heads]
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jne l_1
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mov byte [curr_head],0
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mov bl,[curr_track]
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add bl,1
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mov [curr_track],bl
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l_1: ret
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step_memory:
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mov bx,[curr_offset]
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add bx,SECTORSZ
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mov [curr_offset],bx
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test bx,0xffff
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jne l_2
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mov bx,[curr_segment]
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add bx,0x1000 ; 64 KByte weiterstellen
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mov [curr_segment],bx
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l_2 ret
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;
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; Ausgabe eines Stern ('*') mit Hilfe eines BIOS-Aufrufs
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;
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print_dot:
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mov ah,0x03 ; Feststellen der Cursor-Position
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xor bh,bh
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int 0x10
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mov cx,1
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mov bx,0x0007 ; page 0, attribute 7 (normal)
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mov ax,ds
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mov es,ax
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mov bp,dot
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mov ax,0x1301 ; Ausgabe des Textes, Cursor bewegen
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int 0x10
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ret
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;
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; stop_floppy_motor
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;
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; Stopt den Motor der Floppy, da das BIOS dazu in Kuerze nicht mehr in
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; der Lage sein wird. Egal, ob von Floppy oder Platte gebootet wurde.
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;
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stop_floppy_motor:
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mov dx,0x3f2
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xor al,al
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out dx,al
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ret
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;
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; Datenbereich
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;
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bootmessage:
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db 13,10
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db 'booting ... '
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dot:
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db '*'
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to_load:
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dw 0
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curr_segment:
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dw 0
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curr_offset:
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dw 0
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last_head:
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db 0
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last_track:
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db 0
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last_sector:
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db 0
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last_segment:
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dw 0
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last_offset:
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dw 0
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unused:
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times bootsector+510-$ db 0
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mark:
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dw 0xaa55
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