rdos/boot/vbr.asm

org 0

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 cx, 0x0100
  mov si, 0x7C00
  mov di, 0x0500
  rep movsw
  jmp 0x0050:.lowstart
.lowstart:
  mov ax, cs
  mov ds, ax
  mov ss, ax
  xor sp, sp

  call print_inline
  db "VBR: Using", 0

  mov [fdc.drv], dl ; backup drive number
.adjust_chs:
  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

  call print_inline
  db " BIOS", 0
.noadjust:
  call print_inline
  db " CHS ", 0

  mov ax, [fdc.ts]
  div word [fdc.nos]
  div word [fdc.spt]
  call print_number

  call print_inline
  db ",", 0

  mov ax, [fdc.nos]
  call print_number

  call print_inline
  db ",", 0

  mov ax, [fdc.spt]
  call print_number

  call print_inline
  db 0x0A, 0x0D, 0

  mov ax, 0x0070
  mov es, ax
  xor bx, bx
  call loadrootdir

  call dump
.end:
  hlt
  jmp .end

; Load root directory into memory
; in	es	data segment
loadrootdir:
  push bx
  push cx
  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

  mov ax, [fdc.sf]
  mul byte [fdc.fn]
  add ax, [fdc.rsc]

  xor bx, bx ; load at beginning of es
.loop:
  call loadblk
  loop .loop

  pop cx
  pop bx
  ret

; Load a cluster from cluster offset
; in    ax      cluster number
;       es:bx   buffer
loadcl:
  mov ax, [fdc.sf]
  mul byte [fdc.fn]
  add ax, [fdc.rsc]
  call print16
  ret

; Load a single block into memory
; in    ax      sector number
;       es:bx	buffer
; out   al      error code,
;       carry set if error
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
  pop dx
  pop cx
  pop ax
  inc ax
  add bx, 0x0200
  ret

print_number:
  mov cx, 0x000A
  xor dx, dx
  div cx ; ax = dx:ax / 10, dx = dx:ax % 10
  and ax, ax
  jz .print
  push dx
  call print_number
  pop dx
.print:
  mov al, dl
  add al, 0x30
  mov ah, 0x0e
  int 0x10
  ret

print_inline:
  pop si
  push bx
.loop:
  lodsb
  cmp al, 0x00
  je .end
  mov bx, 0x0000
  mov ah, 0x0e
  int 0x10
  cmp al, 0x0D
  jmp .loop
.end:
  pop bx
  push si
  ret

dump:
  mov si, bx
  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 (0x1F0 - ($-$$)) db 0

str_bootfile:
  db "BOOT"
  times (0x1F8 - ($-$$)) db " "
  db "BIN"

times (0x1FE - ($-$$)) db 0

dw 0xAA55