;************************************************************************************************************;;	FritzOS Prekernel 0.7.1, Copyright (C) 2002 Tom Fritz;;	Made to be compiled on a Linux system with GCC, LD & nasm. Also can be compiled on Win32 with;		DGJPP and NASM. For more information on compiling and installing, look at the INSTALLING;		file. It should be in the "info" directory.;	;	boot.asm: FritzOS Prekernel 0.7 bootloader: Start FritzOS->Load GDTR->Set Protected Mode->Load Second;	Stage Loader->Load The FritzOS C++ Kernel.;;	Not much kernels are commented very well, so I'll comment this one well.;;************************************************************************************************************;; * This program is a part of the FritzOS kernel, and may be freely; * copied under the terms of the GNU General Public License (GPL),; * version 2, or at your option any later version.; Start Of Assembly Code:; The BIOS loads this at 7C00h, so tell the assembler ( NASM ) that that's where it's loaded.	ORG 7c00h; Now that NASM knows where this is loaded, we jump to the main bootsector code:	jmp Start      ; Goto Start; StartStart:	mov [ drive ],		dl        ; Get the floppy FritzOS booted from	; Update the segment registers	xor ax,		ax		; XOR ax	mov ds,		ax		; Mov AX into DS	jmp Running      ; Go to the running part of the bootsector; This is where the boot program runs.Running:; Clear text mode screen	mov ax,		3		; Set text-mode 3.	int 10h				; Text-mode 3 set. The screen is cleared.; Load the FritzOS C++ Kernel.; Reset the floppy drive. Need to do this to read the C++ kernel.ResetFloppy:	mov ax,		0x00		; Select Floppy Reset BIOS Function        mov dl,		[ drive ]	; Select the floppy FritzOS booted from        int 13h				; Reset the floppy drive        jc ResetFloppy		; If there was a error, try again.; Read the floppy drive for loading the FritzOS C+ KernelReadFloppy:         mov bx,	9000h		; Load FritzOS at 9000h.         mov ah,	0x02		; Load disk data to ES:BX         mov al,	17		; Load two floppy head full's worth of data.         mov ch,	0		; First Cylinder         mov cl,	2		; Start at the 2nd Sector, so you don't load the bootsector, you load					;  the C++ Kernel/Second Stage Loader ( they are linked together ).         mov dh,	0		; Use first floppy head         mov dl,	[ drive ]	; Load from the drive FritzOS booted from.         int 13h			; Read the floppy disk.	 jc ReadFloppy			; Error, try again.; Now, since FritzOS is over 17 sectors ( one floppy head has 17 sectors ( 18, sector 0 is included in the;  number ), we load another floppy head.ReadFloppy2:	mov al,		17	; The Second Head Full	inc dh			; Set it to the second head	int 13h			; Read the floppy disk.	jc ReadFloppy2		; If there was a error, try again.; Get ready to set PMode ( Protected Mode)SetPMode:        cli			; Stop BIOS interrups - Protected Mode can't handle interrupts without a				;  PMode IDT. The BIOS loads a Real-Mode IDT, and Protected Mode has different				;   exceptions and other things. So FritzOS can't handle interrupts and IRQ's				;    currently.        ; Load a temporary GDTR. You need a GDT to set up the PMode selectors, and to allow the C++ kernel's	;  stack to function properly.	lgdt [ GDTR ]		; Load the GDTR.        ; Set Protected Mode ( PMode )        mov eax,	cr0        or eax,		1        mov cr0,	eax		; PMode bit set.	; Here we are now in Protected Mode	; BUT, we need to do this to make the CPU set the registers correctly:        jmp dword CodeSel:PMode; We are in protected mode here:[bits 32]PMode:        ; Put the Data Selector into eax for setting up other registers.        mov eax,	DataSel		; Data Selector.	; Make SS, DS, ES, FS and GS = the Data Selector ( In the GDT )        mov ss, eax        mov ds, eax        mov es, eax        mov fs, eax        mov gs, eax	; Set up the PMode stack:        mov ax, 0x10        mov ss, ax        mov esp, 0xFFFF ; To Fix...FritzOS will work correctly, all that I need to fix is make the stack			; larger.	; Stop the floppy motor from spinning ( since we are in PMode, the BIOS can't stop it so we stop it )         mov dl,		[ drive ]	; Select which motor to stop ( the floppy drive FritzOS booted from )	; Select Stop Floppy Motor function:	mov edx, 0x3f2	mov al, 0x0c	; Stop floppy motor:	out dx, al      ; Floppy Motor stopped!	        jmp dword CodeSel:9000h	; Jump to the C++ Kernel. The C++ Kernel is loaded at 9000h. So go there.; The global descriptor table this tells the computer where all the segments ( now selectors ) are.; This is just a temporary GDT. We will load a better one in the 2nd stage kernel.GDTR    dw GDTEnd-1      dd GDTGDTnullsel equ $-GDTGDT0          dd 0          dd 0CodeSel equ $-GDT          dw 0ffffh          dw 0          db 0          db 09ah          db 0cfh          db 0hDataSel equ $-GDT        dw 0ffffh        dw 0h        db 0h        db 092h        db 0cfh        db 0GDTEnd; This part makes sure the bootsector is 512 bytes.  times 509-($-$$) db 0	; 509 bytes because drive takes 1 byte and 0xAA55 takes 2, so correctly set the value			;  Otherwise, the bootsector will be too big or small, if that happens, FritzOS will			;   not work correctly.; This tells the BIOS to load the bootsector, ALWAYS need this. The BIOS sees this as a signal that this is;  a bootable disk.	dw 0xAA55; For storing which floppy drive FritzOS booted from	drive db 0; End of boot.asm;_____________________________________________________________________________________________________________