org 0x0000

jmp 0x07C0:main

times (0x0B - ($-$$)) db 0 ; space-padded
fdc:
.ss:
  dw 0x200	; sector size
.sc:
  db 2		; sectors per cluster
.rsc:
  dw 1		; reserved sector count
.fn:
  db 2		; number of file allocation tables
.rde:
  dw 0x70	; number of root directory entries
.ts:
  dw 720	; total number of sectors
.mi:		; medium identifier
  db 0xFD	; 5.25-inch Double sided, 40 tracks per side, 9 sectors per track (360 KB)
.sf:		; sectors per fat
  dw 2
.spt:
  dw 9		; sectors per track
.nos:
  dw 2		; number of sides (heads)
.po:
  dd 0		; partition offset (in LBA blocks)
.lrgts:
  dd 0
.drv:
  db 0		; drive number
  db 0
  db 0x29	; efdc signature
.vid:
  dd 0		; volume id
.vlabel:
  db "2B"
  times (54 - ($-$$)) db " "
.fstype:
  db "FAT12"
  times (62 - ($-$$)) db " "

main:
  cli
  ; setup stack area growing down from directly below us
  xor ax, ax
  mov ss, ax
  mov sp, 0x7C00
  ; setup base segments
  mov ax, cs
  mov ds, ax
  mov es, ax
  ; save initial environment
  push bx
  push cx
  push dx

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

%include "chs.inc"

  mov di, 0x1000 ; loading address for roms
  call search_rootdir

  mov ax, 0xFFFF
  call error
  db "THING", 0

search_rootdir:
  mov ax, [fdc.sf]
  mul byte [fdc.fn]
  add ax, [fdc.rsc]
  mul word [fdc.ss]
  mov si, ax ; now pointing to start of rootdir
  mov cx, [fdc.rde]
  jmp .start
.next:
  dec cx
  jz .end
  add si, 0x20
.start:
  call lazy_load
  cmp byte [si], 0 ; end of directory
  je .end
  cmp byte [si], 0xe5 ; deleted entry
  je .next
  cmp word [si+8], 0x4F52 ; first 2 chars of extension: "RO"
  jne .next
  mov ax, [si+10] ; 3rd char of extension, file flags
  and ax, 0x14FF ; zero out flags except directory and system
  cmp ax, 0x044D ; "M" , system flag set, directory flag unset
  jne .next
  mov ax, [si+26]
  push si
  push cx
  call loadrom
  pop cx
  pop si
  jmp .next
.end:
  ret

; in	ax	cluster number
loadrom:
  call print16
  ret

; Makes sure the block starting with (or under SI) is
; already fetched from disk
; in	ds:si	data expected to be fetched from disk
; trashes ax, bx and dx
lazy_load:
  cmp si, [max_si]
  jc .end
  ; calculate next sector number
  mov ax, [max_si]
  xor dx, dx
  div word [fdc.ss]
  ; write to es:bx
  xor dx, dx
  push cs
  pop es
  mov bx, [max_si]
  call load_sector
  ; update max_si counter
  mov ax, [max_si]
  add ax, [fdc.ss]
  mov [max_si], ax
  ; loop until requirement is satisfied
  jmp lazy_load
.end:
  ret

error:
  call print16
  pop si
  mov bx, 0x0000
  mov al, 0x20
.loop:
  mov ah, 0x0e
  int 0x10
  lodsb
  cmp al, 0x00
  jne .loop
.hlt:
  hlt
  jmp .hlt

%include "print.asm"

times (0x1FA - ($-$$)) db 0

dw 0x2B2B

max_si:
dw 0x200

dw 0xAA55