rdos/boot/fat.asm

%define segment   0x00100
%define self      (0x7C00-(segment<<4)) ; 1 sector
%define prog      (RAM-0x1000) ; 4K at the end for OS

cpu 8086
org self
jmp short init

times (0x0B - ($-$$)) db 0

fdc:
.ss:
  dw 0x200	; sector size
.sc:
  db 2		; sectors per cluster
.rsc:
  dw 1		; reserved sector count
.fn:
  db 2		; number of file allocation tables
.rde:
  dw 0x70	; 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 2
.spt:
  dw 9		; sectors per track
.nos:
  dw 2		; number of sides (heads)
.po:
  dd 0		; partition offset (in LBA blocks)
.lrgts:
  dd 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 " "

; mformat keeps writing until here
; if we place init earlier, code gets overwritten
times (62 - ($-$$)) nop

init:
  cli
  jmp segment:main

main:
  ; Stack grows down from 64k
  mov ax, cs
  mov ss, ax
  mov sp, prog
  mov ds, ax
  mov es, ax

  mov [fdc.drv], dl ; save drive number in fd

  mov ax, 1
  mov cx, 8
  mov dx, 0
  mov bx, prog

.loop:
  push ax
  push cx
  push dx

  ; add partition offset (required for HDD)
  add ax, [fdc.po]
  adc dx, [fdc.po+2]

  ; calculate CHS data
  div word [cs:fdc.spt]    ; ax:temp     = (lba / spt)
  inc dx                   ; dx:sector   = (lba % spt) + 1
  mov cl, dl ; sector number
  xor dx, dx
  div word [cs:fdc.nos]    ; ax:cylinder = (tmp / heads)
                           ; dx:head     = (tmp % heads)
  mov ch, al ; cylinder number
  mov dh, dl ; head number
  mov dl, [cs:fdc.drv] ; driver number
  mov ax, 0x0201 ; ah=0x02 al=0x01
  int 0x13
  mov bp, 0x3331
  jc error

  pop dx
  pop cx
  pop ax

  ; count up for next sector
  add bx, 0x0200
  add ax, 1
  adc dx, 0

  loop .loop

  mov dl, [fdc.drv]

  ; jump into kernel
  jmp segment:(prog+0x0800)

error:
  mov ax, bp
  mov ah, 0x0e
  mov bx, 7
  int 0x10
  mov al, 0x21
  int 0x10
  xor ax, ax
  int 0x16
  int 0x19

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

dw 0xAA55