org 0x0000

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 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:
  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
  ; setup stack area growing down from 0x10000
  mov ss, ax
  mov sp, ax
  ; backup important values
  push bx
  push cx
  push dx
  ; adjust CS
  jmp 0x07C0:main

; non-fdc variables
rootdir:
.start:
  dw 0
.length:
  dw 0

main:
  ; setup buffer area directly at 4k
  mov ax, 0x0100
  mov ds, ax
  mov es, ax

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

  call fix_chs
  call calc_sectors
  call find_first_cluster
  xor bx, bx
  call load_cluster

  ; restore important variables
  xor ax, ax
  mov bp, ax
  mov si, ax
  mov di, ax
  pop dx
  pop cx
  pop bx
  jmp 0x0100:0000

; Write AX, a string and bail out
; Return pointer is the string start
; String must be 0 suffixed
error:
  call print16
  push cs
  pop ds
  pop si
  mov bx, 0x0000
  mov al, 0x20
.loop:
  mov ah, 0x0e
  int 0x10
  lodsb
  cmp al, 0x00
  jne .loop
.hlt:
  hlt
  jmp .hlt

; Read CHS data from BIOS and fix fdc values
fix_chs:
  mov ah, 0x08
  int 0x13
  jc .end ; skip if function does not exist
  inc dh
  mov [cs:fdc.nos], dh
  mov ax, cx
  and ax, 0x003F
  mov [cs: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 [cs:fdc.nos] ; number of tracks = number of cylinders * heads
  mul word [cs:fdc.spt] ; number of sectors = number of tracks * sectors per track
  mov [cs:fdc.ts], ax
.end:
  ret

calc_sectors:
  mov bx, [cs:fdc.ss]  ; bytes per sector
  mov cl, 5
  shr bx, cl           ; div by 2^5, 32 bytes per entry
  mov ax, [cs:fdc.rde] ; number of entries
  xor dx, dx
  div bx
  mov [cs:rootdir.length], ax
  mov ax, [cs:fdc.sf]  ; sectors per fat
  mul byte [cs:fdc.fn] ; number of fats
  add ax, [cs:fdc.rsc] ; reserved sectors
  mov [cs:rootdir.start], ax
  ret

; Load the root directory
find_first_cluster:
  mov ax, [cs:rootdir.start]
  mov cx, [cs:rootdir.length]
  xor bx, bx
.blkloop:
  call loadblk
  add bx, [cs:fdc.ss]
  add ax, 1
  loop .blkloop
  push cs
  pop es
  mov cx, [cs:fdc.rde]
  xor bx, bx
.dirloop:
  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 .dirloop
  call error
  db "NOT FOUND", 0
.found:
  push ds
  pop es
  add bx, 26
  mov ax, [bx]
  ret

; Load a single FAT cluster into memory
; in    ax      cluster number
;       es:bx   buffer
;
load_cluster:
  xor ch, ch
  mov cl, [cs:fdc.sc] ; sectors per cluster
  xor dx, dx
  dec ax ; somehow 2 is first cluster
  div word cx
  add ax, [cs:rootdir.start]
  add ax, [cs:rootdir.length]
.loop:
  call loadblk
  inc ax
  add bx, [cs:fdc.ss]
  loop .loop
  ret

; Load a single block into memory
; Does not return on error
; in    ax      sector number
;       es:bx	buffer
loadblk:
  push ax
  push cx
  push dx
  xor dx, dx
  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
  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