rdos/boot/fat12vbr.asm

189 lines
3.4 KiB
NASM

org 0x0500
fdc: ; FDC Descriptor as per ECMA-107
jmp _startup
times (0x03 - ($-$$)) nop
.oem:
db "2B" ; creating system identifier
times (0x0B - ($-$$)) db " " ; space-padded
.ss:
dw 0x200 ; sector size
.sc:
db 0 ; sectors per cluster
.rsc:
dw 2 ; reserved sector count
.fn:
db 0 ; number of file allocation tables
.rde:
dw 0 ; number of root directory entries
.ts:
dw 720 ; total number of sectors
.mi: ; medium identifier
db 0xFD ; 5.25-inch Double sided, 40 tracks per side, 9 sectors per track (360 KB)
.sf: ; sectors per fat
dw 0
.spt:
dw 9 ; sectors per track
.nos:
dw 2 ; number of sides (heads)
.po:
dd 0 ; partition offset (in LBA blocks)
.lrgts:
dw 0
.drv:
db 0 ; drive number
db 0
db 0x29 ; efdc signature
.vid:
dd 0 ; volume id
.vlabel:
db "2B"
times (54 - ($-$$)) db " "
.fstype:
db "FAT12"
times (62 - ($-$$)) db " "
_startup:
xor ax, ax
mov ds, ax
mov es, ax
mov ss, ax
xor sp, sp
; relocate
mov si, 0x7C00
mov di, 0x0500
mov cx, 0x0100
rep movsw
; adjust CS
jmp 0x0000:main
main:
mov [fdc.drv], dl ; backup drive number
adjustchs:
mov ah, 0x08
int 0x13
jc loaddir ; skip if function does not exist
inc dh
mov [fdc.nos], dh
mov ax, cx
and ax, 0x003F
mov [fdc.spt], ax ; no adjustment because sectors are 1-indexed
mov ax, cx
xchg al, ah
mov cl,6
shr ah,cl
inc ax ; convert from maximum number (0-based) to total number (1-based) of cylinders
mul word [fdc.nos] ; number of tracks = number of cylinders * heads
mul word [fdc.spt] ; number of sectors = number of tracks * sectors per track
mov [fdc.ts], ax
loaddir:
mov bx, [fdc.ss] ; bytes per sector
mov cl, 5
shr bx, cl ; div by 2^5, 32 bytes per directory entity
mov ax, [fdc.rde] ; number of root directory entities
xor dx, dx
div bx
mov cx, ax ; cx = number of rootdir sectors
mov ax, [fdc.sf] ; sectors per fat
mul byte [fdc.fn] ; number of fats
add ax, [fdc.rsc] ; ax = starting sector of rootdir
mov bx, buffer
.loop:
call loadblk
add bx, buffer
add ax, 1
loop .loop
scandir:
mov cx, [fdc.rde]
mov bx, buffer
.loop:
push cx
mov cx, 0x000B
mov si, bx
mov di, bootfilename
repe cmpsb
pop cx
jz found
add bx, 0x0020 ; 32 bytes per directory entry
loop .loop
call error
db "NOT FOUND", 0
found:
add bx, 26
mov ax, [bx]
call print16
jmp hlt
error:
call print16
pop si
mov bx, 0x0000
mov al, 0x20
.loop:
mov ah, 0x0e
int 0x10
lodsb
cmp al, 0x00
jne .loop
; fall through into htl
hlt:
hlt
jmp hlt
; Load a single FAT cluster into memory
; in ax cluster number
; bx buffer
;
loadsec:
ret
; Load a single block into memory
; Does not return on error
; in ax sector number
; bx buffer
loadblk:
push ax
push cx
push dx
xor dx, dx
div word [fdc.spt] ; ax:temp = (lba / spt)
inc dx ; dx:sector = (lba % spt) + 1
mov cl, dl ; sector number
xor dx, dx
div word [fdc.nos] ; ax:cylinder = (tmp / heads)
; dx:head = (tmp % heads)
mov ch, al ; cylinder number
mov dh, dl ; head number
mov dl, [fdc.drv] ; driver number
mov ax, 0x0201 ; ah=0x02 al=0x01
int 0x13
jc .error
pop dx
pop cx
pop ax
ret
.error:
xchg al, ah
call error
db "DISK ERROR", 0
%include "print.asm"
times (0x1F0 - ($-$$)) db 0
bootfilename:
db "BOOT"
times (0x1F8 - ($-$$)) db " "
db "BIN"
times (0x1FE - ($-$$)) db 0
dw 0xAA55
buffer: