os-k-team/os-k
os-k-team/os-k
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os-k/boot/mbr/mbr.asm

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;=----------------------------------------------------------------------------=;
; GNU GPL OS/K ;
; ;
; Authors: spectral` ;
; NeoX ;
; ;
; Desc: Bootsector for OS/K ;
; (x86_64 architecture only) ;
;=----------------------------------------------------------------------------=;
;; BOOT "SEGMENT"
%define BOOT_SEG 0x07c0 ; (BOOT_SEG << 4) + BOOT_OFF = 0x007c00
%define BOOT_OFF 0x0000
;; STACK "SEGMENT"
%define STACK_SEG 0x0600 ; (STACK_SEG << 4) + STACK_OFF = 0x007000
%define STACK_OFF 0x1000
;; DISK BUFFER "SEGMENT"
%define BUFFER_SEG 0x2000 ; (BUFFER_SEG << 4) + BUFFER_OFF = 0x020000
%define BUFFER_OFF 0x0000
;; SECOND STAGE LOADER "SEGMENT"
%define LOAD_SEG 0x0000 ; (LOAD_SEG << 4) + LOAD_OFF = 0x001000
%define LOAD_OFF 0x1000
[BITS 16] ; Ensure 16-bit code (because fuck UEFI)
Intro:
jmp short _start ; Jump over the BIOS PARAMETER BLOCK
nop ; Required by BIOS to recognize the Disk loul
BPB:
;---------------------------------------------------;
; Disk description table ;
;---------------------------------------------------;
%define OEMName bp+0x03 ; Disk label
%define bytesPerSector bp+0x0b ; Bytes per sector
%define sectorsPerCluster bp+0x0d ; Sectors per cluster
%define reservedSectors bp+0x0e ; Reserved sectors
%define fats bp+0x10 ; Number of fats
%define rootDirEntries bp+0x11 ; Number of entries in root dir
%define sectors bp+0x13 ; Logical sectors
%define mediaType bp+0x15 ; Media descriptor byte
%define fatSectors bp+0x16 ; Sectors per FAT
%define sectorsPerTrack bp+0x18 ; Sectors per track
%define heads bp+0x1a ; Number of sides/heads
%define hiddenSectors bp+0x1c ; Hidden sectors
%define hugeSectors bp+0x20 ; LBA sectors
%define biosBootdrvNum bp+0x24 ; Bootdrv number
%define reserved bp+0x25 ; This is not used
%define bootSignature bp+0x26 ; Bootdrv signature
%define volumeId bp+0x27 ; Volume ID
%define volumeLabel bp+0x2b ; Volume Label
%define fatTypeLabel bp+0x36 ; File system type
times 0x3b db 0x00 ; ALLOCATE THE SECTORS FOR THE BPB
;; ENTRY POINT
_start:
jmp BOOT_SEG:$+5 ; Fix the cs:ip registers with a vaudou magical trip
bootstrap:
jmp go
;; LOVELY DATA
FileNotFound db "First Stage ERROR : NO LOADER", 0
DiskError db "First Stage ERROR : DISK", 0
UserData dw 0
Bootdrv db 0
filename db "LOADER BIN"
;; GO !
go:
mov ax, BOOT_SEG ; Set segments to the location of the bootloader
mov ds, ax
mov es, ax
;; INIT STACK
cli
mov ax, STACK_SEG ; Init the stack
mov ss, ax ; Continue init the stack
mov sp, STACK_OFF ; Ok man, the stack is in 4K :O
sti
mov bp, (0x7c0-STACK_SEG) << 4 ; Correct bp (the disk description table)
;; INITIALIZE BOOT DISK
or dl, dl ; Verifying dl points actually to the boot drive
jz load_root
mov byte [Bootdrv], dl ; Another soul (the disk) saved!
mov ah, 0x08
int 0x13 ; int 0x13 : read drive parameters/geom
jc load_root
and cx, 0x003f ; Maximum sector number is the high bits 6-7 of cl
mov word [sectorsPerTrack], cx ; And whose low 8 bits are in ch
mov dl, dh ; Convert the maximum head number to a word with another vaudou magical trip
xor dh, dh
inc dx ; because head numbers start at zero
mov word [heads], dx ; Another soul (the heads number) saved!
;; LOAD THE ROOT DIRECTORY FROM DISK
load_root:
xor cx, cx
mov ax, 32 ; Size of root dir = (rootDirEntries * 32) / bytesPerSector
mul word [rootDirEntries] ; multiply by the total size of the root directory
div word [bytesPerSector] ; divide by the number of bytes used per sector
xchg cx, ax
mov al, byte [fats] ; Location of root dir = (fats * fatSectors) + reservedSectors
mul word [fatSectors] ; multiply by the sectors used
add ax, word [reservedSectors] ; increase ax by the reserved sectors
mov word [UserData], ax ; Start of user data = startOfRoot + numberOfRoot
add word [UserData], cx ; Add the size and location of the root directory
mov di, BUFFER_SEG ; Set the extra segment to the disk buffer
mov es, di
mov di, BUFFER_OFF ; Set es:di and load the root directory into the disk buffer
call read_sectors ; Read the sectors
;; FIND THE SECOND STAGE LOADER
mov di, BUFFER_OFF ; Set es:di to the disk buffer
mov cx, word [rootDirEntries] ; Search through all of the root dir entries
xor ax, ax ; Clear ax for the file entry offset
search_root:
xchg cx, dx ; Save cx because it's a loop counter
mov si, filename ; Load the filename
mov cx, 11 ; Compare first 11 bytes
rep cmpsb ; Compare si and di cx times
je load_fat ; We found the loader
add ax, 32 ; File entry offset
mov di, BUFFER_OFF ; Point back to the start of the entry
add di, ax ; Add the offset to point to the next entry
xchg dx, cx
loop search_root ; Continue to search for the file
;; ERROR...
mov si, FileNotFound ; Could not find the file
call print
call reboot
;; LOAD THE FAT FROM THE FILE
load_fat:
mov ax, word [es:di + 15] ; Get the file cluster at offset 26
push ax ; Store the FAT cluster
xor ax, ax ; Size of fat = (fats * fatSectors)
mov al, byte [fats] ; Move number of fats into al
mul word [fatSectors] ; Move fat sectors into bx
mov cx, ax ; Store in cx
mov ax, word [reservedSectors] ; Convert the first fat on the disk
mov di, BUFFER_OFF ; Set es:di and load the fat sectors into the disk buffer
call read_sectors ; Read the sectors
;; LOAD THE CLUSTERS OF THE LOADER AND JUMP
mov di, LOAD_SEG
mov es, di ; Set es:bx to where the file will load
mov di, LOAD_OFF
pop ax ; File cluster restored
call read_clusters ; Read clusters from the file
mov dl, byte [Bootdrv] ; Pass the boot Bootdrv into dl
jmp LOAD_SEG:LOAD_OFF ; Jump to the file loaded!
hlt ; This should never be hit...
; ...
; ...
; I hope....
; ...
call reboot
%include "boot/mbr/mbr.inc"
;; END
times 510 - ($ - $$) db 0 ; Pad remainder of boot sector with zeros
dw 0xaa55 ; Boot signature
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