; Memory layout:
%define self      0x00600 ; 1 sector
%define fattab    0x01000 ; variable size
%define rootdir   0x06000 ; variable size
%define psp       (prog - 0x100)
%define prog      0x07C00

%define arguments (psp + 0x81)

org self

jmp short init

cluster_offset:
  dw 0

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 " "

init:
  cli

  ; Stack grows down from PSP + 64k
  mov ax, (psp >> 4)
  mov ss, ax
  xor sp, sp
  push sp

  ; Relocate from [prog] to [self]
  xor ax, ax
  mov ds, ax
  mov es, ax
  mov si, prog
  mov di, self
  mov cx, 0x100
  rep movsw

  jmp 0:main

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

  ; load fat table into memory
  mov ax, [fdc.rsc]
  mov cx, [fdc.sf]
  xor dx, dx
  mov bx, fattab
  call load_sectors

  ; calculate length of rootdir
  mov ax, [fdc.rde]
  mov cl, 4
  shr ax, cl ; 32 bytes per entry
  mov cx, ax

  ; load root dir
  xor dx, dx
  mov ax, [fdc.sf]
  mul byte [fdc.fn]
  add ax, [fdc.rsc]
  mov bx, rootdir
  call load_sectors

  ; remember where we left off
  ; clusters start after rootdir
  mov [cluster_offset], ax

  call load_file
  mov bp, 0x3332
  jc error

  ; setup int 19h call if prog returns
  mov WORD [psp], 0x19CD

  ; clean registers
  xor ax, ax
  xor cx, cx
  xor dx, dx
  xor bx, bx

  ; restore boot drive number
  mov dl, [fdc.drv]

  ; setup segment registers
  push ss
  pop ds
  push ss
  pop es

  ; jump into target
  jmp 0x07B0:0x0100

; Read sectors from disk
; Does not return on error
; ax and bx will be incremented, cx decremented
; in    dx:ax   sector number
;       es:bx   buffer
;	cx	number of sectors to read
; out	dx:ax	next sector to read
;	es:bx	next free buffer
;	cx	zero
load_sectors:
  ; fail instantly if reading sectors > 16 bit
  test dx, dx
  mov bp, 0x3330
  jnz error

.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
  inc ax

  loop .loop
  ret

; Load the file in [filename]
; or exit with carry set
load_file:
  mov si, rootdir
  mov cx, [fdc.rde]
.loop:
  call file_match
  je read_clusters
  add si, 0x20
  loop .loop
  stc
  ret

; Compares the FAT dirent against [filename]
; in	ds:si	filename
; out	ZF	zero flag set if equal
file_match:
  push cx
  push si
  mov di, filename
  mov cx, 11
  repe cmpsb
  pop si
  pop cx
  ret

; Read the file given by cluster number
; into the target program area
; in	ax	cluster number
read_clusters:
  add si, 0x1A
  lodsw
  mov bx, prog
.loop:
  ; read cluster into area for target file
  push ax
  sub ax, 2
  mul BYTE [fdc.sc]
  add ax, [cluster_offset]
  xor dx, dx
  mov cx, [fdc.sc]
  xor ch, ch
  call load_sectors
  pop ax

  ; calculate index in FAT
  mov si, ax
  shr si, 1
  add si, ax
  add si, fattab

  ; load entry from FAT, truncate to 12 bit
  mov dx, [si]
  test ax, 1
  jz .noshift
  mov cl, 4
  shr dx, 4
.noshift:
  mov ax, dx

  and ax, 0x0FFF
  cmp ax, 0x0FF8
  jc .loop

  ret

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

filename:
  db "KERNEL  COM"

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

dw 0xAA55