rdos/boot/vbr.asm

154 lines
2.6 KiB
NASM

org 0x7C00
bpb:
jmp _startup ; 2 bytes
nop ; 1 byte
.oem:
db "OEM " ; OEM
.bps:
dw 0x200 ; bytes per sector
.spc:
db 1 ; sectors per cluster
.rsectors:
dw 2 ; number of reserved sectors
.fats:
db 2 ; number of file allocation tables
.dirents:
dw 2 ; Number of directory entries
.sectors:
dw 720 ; total number of sectors
.mtd: ; media descriptor type
db 0xFD ; 5.25-inch Double sided, 40 tracks per side, 9 sectors per track (360 KB)
.spfat: ; sectors per fat
dw 0
.spt:
dw 9 ; sectors per track
.heads:
dw 2 ; number of heads
.hidsecs:
dd 0 ; number of hidden sectors
.lrgsectors:
dd 0 ; large sector count
.drivenum:
db 0 ; drive number
.winntflags:
db 0
.signature:
db 0x28
times (0x5A - ($-$$)) nop
_startup:
call adjust_bpb
mov ax, 0x0500
mov es, ax
mov si, 1
mov di, 0x7D00
mov cx, 0x10
call loadblk
call dump
.end:
hlt
jmp .end
dump:
mov si, 0x0000
mov cx, 0x10
.nextline:
mov ax, si
call print16
mov al, 0x3A
call printc
mov al, 0x20
call printc
push cx
mov cx, 0x0020
.words:
es lodsb
call print8
loop .words
pop cx
mov al, 0x0A
call printc
mov al, 0x0D
call printc
loop .nextline
ret
; Adjust the bdp data from BIOS-supplied data (if)
; No args, no output
adjust_bpb:
push ax
push cx
push dx
mov ah, 0x08
int 0x13 ; required, QEMU detects 360kB as 1200kB with CHS 80 2 15
jc .end ; skip if function does not exist
inc dh
mov [bpb.heads], dh
mov ax, cx
and ax, 0x003F
mov [bpb.spt], ax ; no adjustment because sectors are 1-indexed
mov ax, cx
xchg al, ah
mov cl,6
shr ah,cl
inc ax ; convert from maximum number (0-based) to total number (1-based) of cylinders
mul word [bpb.heads] ; number of tracks = number of cylinders * heads
mul word [bpb.spt] ; number of sectors = number of tracks * sectors per track
mov [bpb.sectors], ax
.end:
pop dx
pop cx
pop ax
ret
; in si sector number,
; dl drive number
; es:0000 buffer
; out al error code,
; carry set if error
loadblk:
push bx
push cx
push dx
xor dx, dx
mov ax, si
div word [bpb.spt] ; ax:temp = (lba / spt)
inc dx ; dx:sector = (lba % spt) + 1
mov cl, dl ; for int 0x13
xor dx, dx
div word [bpb.heads] ; ax:cylinder = (tmp / heads)
; dx:head = (tmp % heads)
mov ch, al ; for int 0x13
mov ax, dx
pop dx ; drive number comes back into dl
mov dh, al ; for int 0x13
mov bx, 0x0000
mov ax, 0x0201 ; ah=0x02 al=0x01
int 0x13
call print16
lahf
xchg ah, al
call print8
mov al, 0x0A
call printc
mov al, 0x0D
call printc
pop cx
pop bx
ret
%include "print.asm"
times (0x1FE - ($-$$)) nop
dw 0xAA55