bootloader

; Edited version of the FritzOS boot code.
; Start Of Assembly Code:

; The BIOS loads the code into address 0x7C00, so tell NASM where our code is going to be running from.
    ORG 0x7c00

    jmp Start
    db "MSWIN4.1" ; OEM
    dw 512        ; bytes per sector
    db 1          ; sectors per cluster
    dw 1          ; number of reserved sectors ( including sector zero )
    db 2          ; number of file allocation tables
    dw 224        ; total number of directory entries
                  ; (must be set so that the root directory occupies entire sectors)
    dw 2880       ; total sectors in logical volume
    db 42         ; media descriptor type (?)
    dw 9          ; sectors per FAT (FAT12/16 only)
    dw 18         ; sectors per track
    dw 2          ; number of heads or sides on storage media
    dd 0          ; number of hidden sectors
    dd 1337       ; large amount of sector on disk

; total sectors 2880

    db $00        ; drive number
    db $00        ; flags
    db $28        ; $28 or $29
    db "YIFF"     ; volume serial number
    db "WahovipabOS" ; volume label
    db "FAT12   " ;system identifier

[BITS 16]
Start:  mov [drive], dl     ; Save the index of the floppy we booted from
        cli
        mov ax, 0x03        ; Start in text mode
        int 0x10

; ------------------------------------------------------
        mov ah, 02h ; set cursor pos
        mov bh,0
        mov dh, 5  ; row
        mov dl, 15 ; col
        int 10h

        mov ah, 0eh
        mov bh, 0
        mov bl, 15

        mov al, 'B'
        int 10h
        mov al, 'o'
        int 10h
        mov al, 'o'
        int 10h
        mov al, 't'
        int 10h
        mov al, ' '
        int 10h
        mov al, 'O'
        int 10h
        mov al, 'K'
        int 10h
        mov al, '!'
        int 10h
        mov al, ' '
        int 10h
        mov al, 3
        int 10h

        mov ah, 0
        int 16h
; ------------------------------------------------------


ResetFloppy:                ; Get ready to load the kernel from the floppy. First, we reset it.
        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.

; Now talk to the floppy drive
ReadFloppy:
        mov bx,    0x9000   ; Load FritzOS at 0x9000.
        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
        mov dh,    0        ; Use first floppy head
        mov dl,    [drive]  ; Load from the drive FritzOS booted from.
        int 0x13            ; Read the floppy disk.
        jc ReadFloppy       ; Error, try again.
ReadFloppy2:                ; Read another floppy head
        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.
SetPMode:                   ; Get ready to set PMode ( Protected Mode)

; ------------------------------------------------------
        mov ah, 02h ; set cursor pos
        mov bh,0
        mov dh, 6  ; row
        mov dl, 15 ; col
        int 10h

        mov ah, 0eh
        mov bh, 0
        mov bl, 15

        mov al, 'L'
        int 10h
        mov al, 'o'
        int 10h
        mov al, 'a'
        int 10h
        mov al, 'd'
        int 10h
        mov al, ' '
        int 10h
        mov al, 'O'
        int 10h
        mov al, 'K'
        int 10h
        mov al, '!'
        int 10h
        mov al, ' '
        int 10h
        mov al, 3
        int 10h

        mov ah, 0
        int 16h

        mov ax, 0x13        ; Change into mode 13h now
        int 0x10

;        call do_e820

; ------------------------------------------------------
; http://wiki.osdev.org/Detecting_Memory_(x86)

        ; Load a temporary GDTR.
        lgdt [GDTR]         ; Load the GDTR.

        ; Set Protected Mode ( PMode )
        mov eax,    cr0     ; Get the first control register.
        or eax,     1       ; Poke the 'Protected mode' bit into it.
        mov cr0,    eax     ; Store the modified control register.

        ; The CPU needs to jump at least once to get set up the rest of the way for protected mode
        jmp dword CodeSel:PMode

[bits 32] ; Tell NASM that we're in 32-bit protected mode now
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 ; Init stack

    ; 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 dx, 0x3f2
        mov al, 0x0c
        out dx, al      ; Floppy Motor stopped!

        jmp dword CodeSel:0x9000    ; Jump to the main kernel


; use the INT 0x15, eax= 0xE820 BIOS function to get a memory map
; inputs: es:di -> destination buffer for 24 byte entries
; outputs: bp = entry count, trashes all registers except esi
do_e820:
    xor ebx, ebx        ; ebx must be 0 to start
    xor bp, bp      ; keep an entry count in bp
    mov edx, 0x0534D4150    ; Place "SMAP" into edx
    mov eax, 0xe820
    mov [es:di + 20], dword 1   ; force a valid ACPI 3.X entry
    mov ecx, 24     ; ask for 24 bytes
    int 0x15
    jc short .failed    ; carry set on first call means "unsupported function"
    mov edx, 0x0534D4150    ; Some BIOSes apparently trash this register?
    cmp eax, edx        ; on success, eax must have been reset to "SMAP"
    jne short .failed
    test ebx, ebx       ; ebx = 0 implies list is only 1 entry long (worthless)
    je short .failed
    jmp short .jmpin
.e820lp:
    mov eax, 0xe820     ; eax, ecx get trashed on every int 0x15 call
    mov [es:di + 20], dword 1   ; force a valid ACPI 3.X entry
    mov ecx, 24     ; ask for 24 bytes again
    int 0x15
    jc short .e820f     ; carry set means "end of list already reached"
    mov edx, 0x0534D4150    ; repair potentially trashed register
.jmpin:
    jcxz .skipent       ; skip any 0 length entries
    cmp cl, 20      ; got a 24 byte ACPI 3.X response?
    jbe short .notext
    test byte [es:di + 20], 1   ; if so: is the "ignore this data" bit clear?
    je short .skipent
.notext:
    mov ecx, [es:di + 8]    ; get lower dword of memory region length
    or ecx, [es:di + 12]    ; "or" it with upper dword to test for zero
    jz .skipent     ; if length qword is 0, skip entry
    inc bp          ; got a good entry: ++count, move to next storage spot
    add di, 24
.skipent:
    test ebx, ebx       ; if ebx resets to 0, list is complete
    jne short .e820lp
.e820f:
    mov [mmap_ent], bp  ; store the entry count
    clc         ; there is "jc" on end of list to this point, so the carry must be cleared
    ret
.failed:
    stc         ; "function unsupported" error exit
    ret

; Protected mode needs a list of where the segments are in memory and what privelages they get.
GDTR    dw GDTEnd-1
        dd GDT
GDT
nullsel equ $-GDT
GDT0    dd 0
        dd 0
CodeSel equ $-GDT
        dw 0ffffh
        dw 0
        db 0
        db 09ah
        db 0cfh
        db 0h
DataSel equ $-GDT
        dw 0ffffh
        dw 0h
        db 0h
        db 092h
        db 0cfh
        db 0
GDTEnd

    mmap_ent db 0
; This part makes sure the bootsector is 512 bytes.
    drive db 0  ; For storing which drive we boot from
    ; The original version from FritzOS had AA55 in the wrong place!
    times 510-($-$$) db 0   ; moved over
    dw 0xAA55   ; BIOS checks this value to make sure it is AA55