; KolibriOS bootloader
; bootsector for loading from FAT32 flash (or hard) drive
; intended for use with mtldr_f file in root folder
; this code has been written by diamond in 2005,2006,2007 specially for KolibriOS
; this code is loaded by BIOS to 0000:7C00
org 0x7C00
jmp @f
nop
; times 57h db 0
file 'bt2.dat':3,57h
@@:
xor eax, eax
mov ds, ax
mov ss, ax
mov sp, 7C00h
mov [boot_drive], dl
cld
sti
push 800h
pop es
movzx ebx, word [7C0Eh] ; reserved_sect
mov [fat_start], ebx
mov al, byte [7C10h] ; num_fats
mul dword [7C24h] ; sect_fat
add eax, ebx
; cluster 2 begins from sector eax
movzx ebx, byte [7C0Dh] ; sects_per_clust
add bx, bx
sub eax, ebx
mov [data_start], eax
mov eax, [7C2Ch] ; root_cluster
and eax, 0FFFFFFFh
fat32_parse_dir:
xor bx, bx
mov di, bx
push eax
call read_cluster
movzx cx, byte [7C0Dh] ; sects_per_clust
shl cx, 4 ; *0x200/0x20
scan_cluster:
cmp byte [es:di], 0
jz file_not_found
push cx di
mov cx, 11
mov si, mtldr_f
repz cmpsb
pop di cx
jz file_found
add di, 20h
loop scan_cluster
pop eax
call next_cluster
jnc file_not_found
jc fat32_parse_dir
file_found:
pop eax
mov ax, [es:di+14h]
and ax, 0FFFh
shl eax, 10h
mov ax, [es:di+1Ah]
; eax contains first cluster
@@:
xor bx, bx
push eax
call read_cluster
mov ax, es
movzx cx, byte [7C0Dh]
shl cx, 5
add ax, cx
mov es, ax
pop eax
call next_cluster
jc @b
jmp 0:8000h
file_not_found:
mov si, file_not_found_msg
sayerr:
call out_string
jmp $
read_cluster:
; in: eax = cluster, bx->buffer
movzx ecx, byte [7C0Dh]
mul ecx
add eax, [data_start]
; read procedure
; in: eax = absolute sector
; cx = number of sectors
; es:bx -> buffer
read:
add eax, [7C1Ch] ; hidden sectors
push es
read_loop:
pushad
; allocate disk address packet on the stack
; qword +8: absolute block number
push 0
push 0 ; dword +C is high dword
push eax ; dword +8 is low dword
; dword +4: buffer address
push es ; word +6 is segment
push bx ; word +4 is offset
; word +2: number of blocks, limited to 7Fh
sub cx, 7Fh
sbb ax, ax
and ax, cx
add ax, 7Fh
push ax
shl ax, 5
mov cx, es
add cx, ax
mov es, cx
; word +0: size of packet = 10h
push 10h
; now pair ss:sp contain address of disk address packet
mov ax, 4200h
mov dl, [boot_drive]
mov si, sp
int 13h
mov si, disk_read_err
jc sayerr
popaw
popad
add eax, 7Fh
sub cx, 7Fh
ja read_loop
pop es
ret
next_cluster:
push es
push ds
pop es
mov bx, 7E00h
; sector is 200h bytes long, one entry in FAT occupies 4 bytes
; => 80h entries in sector
push eax
shr eax, 7 ; div 80h
cmp eax, [fat_cur_sector]
jz @f
mov [fat_cur_sector], eax
add eax, [fat_start]
mov cx, 1
call read
@@:
pop eax
and eax, 7Fh
mov eax, [7E00h+eax*4]
and eax, 0FFFFFFFh
cmp eax, 0FFFFFF7h
mov si, bad_cluster
jz sayerr
pop es
ret
out_string:
lodsb
test al, al
jz .xxx
mov ah, 0Eh
mov bx, 7
int 10h
jmp out_string
.xxx: ret
file_not_found_msg db 'Cannot find file '
mtldr_f db 'MTLD_F32 '
db 13,10,0
disk_read_err db 'Disk read error',13,10,0
bad_cluster db 'Bad cluster',13,10,0
fat_cur_sector dd -1
times (7DFEh - $) db 0
db 55h, 0AAh
virtual at 7A00h
fat_start dd ?
data_start dd ?
boot_drive db ?
end virtual