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|
use16
ORG $7C00
LOAD_SEGMENT = 0x1000
FAT_SEGMENT = 0x0ee0
;===========================================
start:
jmp main ;jump to beginning of code
nop
;----------------------------------------
OEM: db "DevOS " ;OEM String
SectSize: dw 0x200 ;Bytes per sector
ClustSize: db 1 ;Sectors per cluster
ResSect: dw 1 ;#of reserved sectors
FatCnt: db 2 ;#of fat copies
RootSize: dw 224 ;size of root directory
TotalSect: dw 2880 ;total ;of sectors if below 32 MB
Media: db 0xF0 ;Media Descriptor
FatSize: dw 9 ;Size of each FAT
TrackSect: dw 9 ;Sectors per track
HeadCnt: dw 2 ;number of read-write heads
HiddenSect: db 0 ;number of hidden sectors
Sect32: db 0 ;;sectors if over 32 MB
BootDrive: db 0 ;holds drive that the boot sector came from
Reserved: db 0 ;reserved, empty
BootSign: db 0x29 ;extended boot sector signature
VolID: db "seri" ;disk serial
acVolLabel: db "MYVOLUME " ;just placeholder. We don't yet use volume labels.
acFSType: db "FAT16 " ;file system type
;===============================
main:
cli
;mov dl, BootDrive ;save what drive we booted from (should be 0x0)
mov ax, cs ;CS is set to 0x0, because that is where boot sector is loaded (0:07c00)
mov ds, ax ;DS = CS = 0x0
mov es, ax ;ES = CS = 0x0
mov ss, ax ;SS = CS = 0x0
mov sp, 0x7C00 ;Stack grows down from offset 0x7C00 toward 0x0000.
;sti ; we don't need hardware interrupts
;-----------------------
mov dl, [BootDrive] ;drive to reset
xor ax, ax ;subfunction 0
int 0x13 ;call interrupt 13h
jc bootFailure ;display error message if carry set (error)
;-------------
; display loading message
mov si, loadmsg
call WriteString
;---------------------
mov ax, LOAD_SEGMENT
mov es, ax
mov ax, 32
xor dx, dx
mul word [RootSize]
div word [SectSize] ;Divide (dx:ax,sectsize) -> (ax,dx)
mov cx, ax
mov [root_scts], cx
xor ax, ax ;find the root directory
mov al, [FatCnt] ;ax = number of FAT tables
mov bx, word [FatSize] ;bx = sectors per FAT
mul bx ;ax = #FATS * sectors per FAT
add ax, word [HiddenSect] ;Add hidden sectors to ax
adc ax, word [HiddenSect+2]
add ax, word [ResSect] ;Add reserved sectors to ax
mov [root_strt], ax
read_next_sector:
push cx
push ax
xor bx, bx
call ReadSector
check_entry:
mov cx, 11 ;Directory entries filenames are 11 bytes.
mov di, bx ;es:di = Directory entry address
mov si, kernelname ;ds:si = Address of kernelname we are looking for.
repz cmpsb ;Compare kernelname to memory.
je found_file ;If found, jump away.
add bx, word 32 ;Move to next entry. Complete entries are 32 bytes.
cmp bx, word [SectSize] ;Have we moved out of the sector yet?
jne check_entry ;If not, try next directory entry.
pop ax
inc ax ;check next sector when we loop again
pop cx
loopnz read_next_sector ;loop until either found or not
jmp bootFailure ;could not find file: abort
found_file:
mov ax, [es:bx+0x1a]
mov [file_strt], ax
;------------------------------
mov ax, FAT_SEGMENT
mov es, ax
;Calculate offset of FAT:
mov ax, word [ResSect] ;Add reserved sectors to ax
add ax, word [HiddenSect] ;Add hidden sectors to ax
adc ax, word [HiddenSect+2]
;Read all FAT sectors into memory:
mov cx, word [FatSize] ;Number of sectors in FAT
xor bx, bx ;Memory offset to read into (es:bx)
read_next_fat_sector:
push cx
push ax
call ReadSector
pop ax
pop cx
inc ax
add bx, word [SectSize]
loopnz read_next_fat_sector ;continue with next sector
;----------------------------
;Set memory segment that will receive the file:
mov ax, LOAD_SEGMENT
mov es, ax
;Set memory offset for loading to 0.
xor bx, bx
;Set memory segment for FAT:
mov cx, [file_strt] ;CX now points to file's first FAT entry
read_file_next_sector:
;Locate sector:
mov ax, cx ;Sector to read is equal to current FAT entry
add ax, [root_strt] ;Plus the start of the root directory
add ax, [root_scts] ;Plus the size of the root directory
sub ax, 2 ;... but minus 2
;Read sector:
push cx ;Read a sector from disk, but save CX
call ReadSector ;as it contains our FAT entry
pop cx
add bx, [SectSize] ;Move memory pointer to next section
;Get next sector from FAT:
push ds ;Make DS:SI point to FAT table
mov dx, FAT_SEGMENT ;in memory.
mov ds, dx
mov si, cx ;Make SI point to the current FAT entry
mov dx, cx ;(offset is entry value * 1.5 bytes)
;shr dx
add si, dx
mov dx, [ds:si] ;Read the FAT entry from memory
test dx, 1 ;See which way to shift
jz read_next_file_even
and dx, 0x0fff
jmp read_next_file_cluster_done
read_next_file_even:
shr dx, 4
read_next_file_cluster_done:
pop ds ;Restore DS to the normal data segment
mov cx, dx ;Store the new FAT entry in CX
cmp cx, 0xff8 ;If the FAT entry is greater or equal
jl read_file_next_sector ;to 0xff8, then we've reached end-of-file
;--------------------------------
; Make es and ds point to segment where 2nd stage was loaded.
mov ax, word LOAD_SEGMENT
mov es, ax
mov ds, ax
; Jump to second stage start of code:
jmp LOAD_SEGMENT:0
;-------------------
;then reboot.
bootFailure:
mov si, diskerror
call WriteString
call Reboot
;=============================
; functions
;============================
WriteString:
lodsb ; load byte at ds:si into al (advancing si)
or al, al ; test if character is 0 (end)
jz WriteString_done ; jump to end if 0.
mov ah, 0xe ; Subfunction 0xe of int 10h (video teletype output).
mov bx, 7 ; Set bh (page number) to 0, and bl (attribute) to white (9).
int 0x10 ; call BIOS interrupt.
jmp WriteString ; Repeat for next character.
WriteString_done:
ret
;----------------------------------------
Reboot:
mov si, rebootmsg ; Load address of reboot message into si
call WriteString ; print the string
xor ax, ax ; subfuction 0
int 0x16 ; call bios to wait for key
db 0xEA ; machine language to jump to FFFF:0000 (reboot)
dw 0x0000
dw 0xFFFF
;---------------------------------------------
ReadSector:
xor cx, cx ; Set try count = 0
readsect:
push ax ; Store logical block
push cx ; Store try number
push bx ; Store data buffer offset
mov bx, [TrackSect] ; Get sectors per track
xor dx, dx
div bx ; Divide (dx:ax/bx to ax,dx)
; Quotient (ax) = LBA / SectorsPerTrack
; Remainder (dx) = LBA mod SectorsPerTrack
inc dx ; Add 1 to remainder, since sector
mov cl, dl ; Store result in cl for int 13h call.
mov bx, [HeadCnt] ; Get number of heads
xor dx, dx
div bx ; Divide (dx:ax/bx to ax,dx)
; Quotient (ax) = Cylinder
; Remainder (dx) = head
mov ch, al ; ch = cylinder
xchg dl, dh ; dh = head number
mov ax, 0x0201 ; Subfunction 2, read 1 sector
pop bx ; Restore data buffer offset.
int 0x13
jc readfail
; On success, return to caller.
pop cx ; Discard try number
pop ax ; Get logical block from stack
ret
readfail:
pop cx ; Get try number
inc cx ; Next try
cmp cx, word 4 ; Stop at 4 tries
je bootFailure
; Reset the disk system:
xor ax, ax
int 0x13
; Get logical block from stack and retry.
pop ax
jmp readsect
;=============================
;PROGRAM DATA
;======================
kernelname: db "KERNEL BIN"
rebootmsg: db "Press any key to reboot.",0
diskerror: db "Disk error. ",0
loadmsg: db "Loading KERNEL...",0
root_strt: db 0 ;hold offset of root directory on disk
root_scts: db 0 ;holds ;sectors in root directory
file_strt: db 0 ;holds offset of bootloader on disk
;====================================
DB 510 - ($ - $$) DUP(0) ; remplir à hauteur de 510 octets
DW $AA55 ; et placer le marqueur de disque bootable |
SVP HELP secteur de boot.
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