rdos/boot/vbr.asm

org 0x7C00

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_old:
  dw 0		; legacy: 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)
.ts:
  dw 720
.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:
  mov [fdc.drv], dl ; backup driver number
  mov ah, 0x08
  int 0x13 ; required, QEMU detects 360kB as 1200kB with CHS 80 2 15
  jc .noadjust ; 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
.noadjust:
  mov ax, 0x0500
  mov es, ax
  mov bx, 0x0000
  mov ax, 1
  call loadblk
  call dump

  mov al, [fdc.sc]
  call print8
.end:
  hlt
  jmp .end

; Load a cluster from cluster offset
loadcluster:
  ret

; Load a single cluster into memory
; in    ax      sector number
;       es:bx	buffer
; out   al      error code,
;       carry set if error
loadblk:
  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
  ret

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

%include "print.asm"

times (0x1FE - ($-$$)) nop
dw 0xAA55