Rework fat boot sector, load FAT only once instead of partial
This commit is contained in:
parent
31eac279c2
commit
adf67d9723
325
boot/fat.asm
325
boot/fat.asm
@ -19,7 +19,7 @@ params: times bpb_size db 0
|
||||
|
||||
init: xor ax, ax
|
||||
mov ss, ax
|
||||
mov sp, 0x7C00
|
||||
mov sp, ax
|
||||
|
||||
; save potential partition table entry
|
||||
push ds
|
||||
@ -36,232 +36,153 @@ init: xor ax, ax
|
||||
|
||||
%include "inc/booterr.asm"
|
||||
|
||||
; offsets relative to FS
|
||||
readd: ; add offset of cluster data area to DX:AX
|
||||
push cx
|
||||
mov di, [bp+bpb.direntries]
|
||||
mov cl, 4
|
||||
shr di, cl ; 32 bytes per entry
|
||||
add ax, di
|
||||
adc dx, 0
|
||||
pop cx
|
||||
readr: ; add offset to rootdir to DX:AX (FAT12/FAT16 only)
|
||||
push cx
|
||||
xor ch, ch
|
||||
mov cl, byte [bp+bpb.fatnumber]
|
||||
fatlp: add ax, [bp+bpb.fatsectors]
|
||||
adc dx, 0
|
||||
loop fatlp
|
||||
pop cx
|
||||
readf: ; add offset to FAT table to DX:AX
|
||||
add ax, word [bp+bpb.reservedsects]
|
||||
adc dx, 0
|
||||
readp: ; read sector DX:AX from partition
|
||||
add ax, word [bp+bpb.sectoroffset]
|
||||
adc dx, word [bp+bpb.sectoroffset+2]
|
||||
jnc ($+5)
|
||||
call errcll
|
||||
|
||||
read_: ; read sector DX:AX from disk
|
||||
; uses basic CHS disk IO
|
||||
; this cant reliable read over track boundaries
|
||||
; so we read each single sector in loop over CX
|
||||
chslp: push ax
|
||||
; Read sectors from disk
|
||||
; IN ES:DI target buffer
|
||||
; DX:AX absulute 32-bit sector number
|
||||
; CX numeber of sectors
|
||||
read: push ax
|
||||
push cx
|
||||
push dx
|
||||
push bx
|
||||
|
||||
; calculate CHS data
|
||||
div word [bp+bpb.tracksectors] ; ax:temp = (lba / spt)
|
||||
inc dx ; dx:sector = (lba % spt) + 1
|
||||
mov cl, dl ; sector number
|
||||
xor dx, dx
|
||||
div word [bp+bpb.heads] ; ax:cylinder = (tmp / heads)
|
||||
; dx:head = (tmp % heads)
|
||||
mov ch, al ; cylinder number
|
||||
mov dh, dl ; head number
|
||||
mov dl, [bp+bpb.drivenum] ; drive number
|
||||
xchg ax, bx
|
||||
mov al, byte [bp+bpb.heads]
|
||||
mul byte [bp+bpb.tracksectors]
|
||||
xchg ax, bx
|
||||
|
||||
; set up read buffer
|
||||
mov es, bx
|
||||
xor bx, bx
|
||||
|
||||
; call bios
|
||||
; dx:ax = lba
|
||||
div word bx
|
||||
xchg ax, dx
|
||||
; dx = cylinder, ax = head * tracksectors + sector
|
||||
div byte [bp+bpb.tracksectors]
|
||||
; dx = cylinder, al = head, ah = sector
|
||||
xchg dl, dh
|
||||
ror dl, 1
|
||||
ror dl, 1
|
||||
or dl, ah
|
||||
inc dx
|
||||
; al: head number
|
||||
; dh bit 0-7: cylinder bits 0-7
|
||||
; dl bit 0-5: sector bits 0-5
|
||||
; dl bit 6-7: cylinder bits 8-9
|
||||
mov cx, dx
|
||||
mov dh, al
|
||||
mov dl, [bp+bpb.drivenum]
|
||||
mov ax, 0x0201
|
||||
int 0x13
|
||||
jnc ($+5)
|
||||
call errcll
|
||||
mov bx, di
|
||||
|
||||
int 0x13
|
||||
jc errjmp
|
||||
|
||||
pop bx
|
||||
pop dx
|
||||
pop cx
|
||||
pop ax
|
||||
|
||||
; increment sector num + write offset
|
||||
add ax, 1
|
||||
adc dx, 0
|
||||
add bx, 0x20
|
||||
; loop on CX
|
||||
loop chslp
|
||||
add di, 0x200
|
||||
loop read
|
||||
ret
|
||||
|
||||
next: ; Advances DX:AX to next FAT entry
|
||||
push ax
|
||||
push bx
|
||||
; shift 2 left for dword-sized FAT entries
|
||||
; shift 9 right for sector size
|
||||
mov cl, 7
|
||||
shftl: clc
|
||||
rcr dx, 1
|
||||
rcr ax, 1
|
||||
loop shftl
|
||||
|
||||
mov bx, 0xA0
|
||||
%ifdef FAT12
|
||||
; FAT item might span over two sectors
|
||||
mov cx, 2
|
||||
%else
|
||||
mov cx, 1
|
||||
%endif
|
||||
call readf
|
||||
|
||||
pop bx
|
||||
|
||||
; get address of cluster within sector
|
||||
pop si
|
||||
%ifdef FAT12
|
||||
push si
|
||||
; times 1.5
|
||||
mov ax, si
|
||||
shr ax, 1
|
||||
add si, ax
|
||||
add si, buf
|
||||
lodsw
|
||||
pop si
|
||||
test si, 1
|
||||
jz .odd
|
||||
.even: mov cl, 4
|
||||
shr ax, cl
|
||||
jmp .done
|
||||
.odd: and ax, 0x0FFF
|
||||
.done: xor dx, dx
|
||||
%endif
|
||||
|
||||
%ifdef FAT16
|
||||
; double
|
||||
sal si, 1
|
||||
; make sure its within sector range
|
||||
and si, 0x1FF
|
||||
add si, buf
|
||||
; load word
|
||||
lodsw
|
||||
xor dx, dx
|
||||
%endif
|
||||
|
||||
ret
|
||||
|
||||
; reads current cluster into [dest]
|
||||
readc: ; load cluster number
|
||||
push ax
|
||||
push dx
|
||||
|
||||
; subtract the two dummy entries from FAT start
|
||||
sub ax, 2
|
||||
sbb dx, 0
|
||||
|
||||
; convert cluster number to sector number
|
||||
; this is some cheapo multiplication with 2^n
|
||||
mov cl, [bp+bpb.clustersects]
|
||||
l02: shr cl, 1
|
||||
jz l02e
|
||||
clc
|
||||
rcl ax, 1
|
||||
rcl dx, 1
|
||||
jmp l02
|
||||
l02e:
|
||||
; dx:ax is now sector num in data area
|
||||
mov cx, [bp+bpb.clustersects]
|
||||
xor ch, ch
|
||||
|
||||
call readd
|
||||
|
||||
xchg di, cx
|
||||
mov cl, 5
|
||||
sal di, cl
|
||||
add bx, di
|
||||
|
||||
pop dx
|
||||
pop ax
|
||||
|
||||
ret
|
||||
|
||||
; Load root directory
|
||||
loadr: mov ax, [bp+bpb.direntries]
|
||||
rootdirsects: mov bx, [bp+bpb.direntries]
|
||||
mov cl, 4
|
||||
shr ax, cl
|
||||
mov cx, ax
|
||||
xor ax, ax
|
||||
xor dx, dx
|
||||
mov bx, 0x07C0
|
||||
jmp readr
|
||||
|
||||
; Load file identified by fsattr
|
||||
; Directory data is expected at 0x07C00
|
||||
loadf: mov si, (0x7C00-0x20)
|
||||
sloop: add si, 0x20
|
||||
; check if entry is valid
|
||||
mov al, [si]
|
||||
; unallocated direntry
|
||||
test al, al
|
||||
jnz ($+5)
|
||||
call errcll
|
||||
; deleted files
|
||||
cmp al, 0xE2
|
||||
je sloop
|
||||
; check attr
|
||||
mov al, [si+11]
|
||||
and al, 0x5C
|
||||
cmp al, ah
|
||||
jne sloop
|
||||
mov ax, [si+0x1A]
|
||||
mov dx, [si+0x14]
|
||||
|
||||
; Load cluster chain DX:AX to 0x07C00
|
||||
load: mov bx, 0x07C0
|
||||
lloop: call readc
|
||||
call next
|
||||
%ifdef FAT12
|
||||
cmp word ax, 0x0FF8
|
||||
%endif
|
||||
%ifdef FAT16
|
||||
cmp word ax, 0xFFF8
|
||||
%endif
|
||||
jc lloop
|
||||
shr bx, cl ; 32 bytes per entry
|
||||
ret
|
||||
|
||||
main: ; Set up BP to point to BPB
|
||||
; Relative addressing saves one instruction byte per access
|
||||
mov bp, params
|
||||
; Save DL contents
|
||||
offset_cluster: ; add offset of cluster data area to DX:AX
|
||||
call rootdirsects
|
||||
add ax, bx
|
||||
adc dx, 0
|
||||
offset_rootdir: ; add length of fat table times number of fats
|
||||
xor ch, ch
|
||||
mov cl, byte [bp+bpb.fatnumber]
|
||||
.loop: add ax, [bp+bpb.fatsectors]
|
||||
adc dx, 0
|
||||
loop .loop
|
||||
offset_fat: ; add reserved sectors
|
||||
add ax, [bp+bpb.reservedsects]
|
||||
adc dx, 0
|
||||
offset_part: ; add partition offset
|
||||
add ax, [bp+bpb.sectoroffset]
|
||||
add dx, [bp+bpb.sectoroffset+2]
|
||||
ret
|
||||
|
||||
resetbuf: xor ax, ax
|
||||
mov dx, ax
|
||||
mov di, buf
|
||||
ret
|
||||
|
||||
main: mov bp, params
|
||||
mov [bp+bpb.drivenum], dl
|
||||
|
||||
; load root directory
|
||||
call loadr
|
||||
call resetbuf
|
||||
call offset_rootdir
|
||||
call rootdirsects
|
||||
mov cx, bx
|
||||
call read
|
||||
|
||||
; search for first system file
|
||||
mov ah, 0x04
|
||||
call loadf
|
||||
; search for file
|
||||
mov bx, buf
|
||||
mov cx, [bp+bpb.direntries]
|
||||
.loop: push cx
|
||||
mov si, filename
|
||||
mov di, bx
|
||||
add bx, 0x20
|
||||
mov cx, 0x0B
|
||||
repe cmpsb
|
||||
pop cx
|
||||
loopne .loop
|
||||
jne err_not_found
|
||||
push word [bx-0x20+0x1A]
|
||||
|
||||
; restore drive number
|
||||
mov dl, [bp+bpb.drivenum]
|
||||
; load fat table
|
||||
call resetbuf
|
||||
call offset_fat
|
||||
mov cx, [bp+bpb.fatsectors]
|
||||
call read
|
||||
|
||||
; restore potential partition table
|
||||
pop si
|
||||
; fetch cluster numbers (FAT12-specific)
|
||||
mov di, 0x0500
|
||||
pop ax
|
||||
readfat: stosw
|
||||
mov bx, ax
|
||||
shr bx, 1
|
||||
pushf
|
||||
add bx, ax
|
||||
mov ax, [buf+bx]
|
||||
popf ; CF: 0=even, 1=odd entry
|
||||
jc .odd
|
||||
and ax, 0x0FFF
|
||||
jmp .both
|
||||
.odd: mov cl, 4
|
||||
shr ax, cl
|
||||
.both: cmp ax, 0xFF8
|
||||
jc readfat
|
||||
xor ax, ax
|
||||
stosw
|
||||
|
||||
; load clusters
|
||||
mov si, 0x500
|
||||
mov di, 0x7C00
|
||||
loadclus: lodsw
|
||||
sub ax, 2
|
||||
jc .jump
|
||||
mul byte [bp+bpb.clustersects]
|
||||
call offset_cluster
|
||||
xor cx, cx
|
||||
mov cl, [bp+bpb.clustersects]
|
||||
call read
|
||||
jmp loadclus
|
||||
|
||||
.jump: pop si
|
||||
pop ds
|
||||
|
||||
jmp 0x7C00
|
||||
|
||||
; Padding and signature
|
||||
times (0x1FE - ($-$$)) db 0
|
||||
err_not_found:
|
||||
hlt: hlt
|
||||
jmp hlt
|
||||
|
||||
times (0x1F3 - ($-$$)) db 0
|
||||
filename: db "KERNEL ", "BS "
|
||||
dw 0xAA55
|
||||
buf:
|
||||
|
||||
Loading…
Reference in New Issue
Block a user