;Copyright 2019 Benji Dial

;Permission to use, copy, modify, and/or distribute this
;software for any purpose with or without fee is hereby
;granted, provided that the above copyright notice and this
;permission notice appear in all copies.

;THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS
;ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL
;IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO
;EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
;INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
;WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
;WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
;TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE
;USE OR PERFORMANCE OF THIS SOFTWARE.

pointers:
.fat equ 0x0500
.root_dir equ 0x0504
.kernel equ 0x0508

fat_header:
.oem_name            equ 0x7c03;mkfs.fat    - ignored
.bytes_per_sector    equ 0x7c0b;0x0200      - assumed
.sectors_per_cluster equ 0x7c0d;0x01        - assumed
.reserved_sectors    equ 0x7c0e;0x0002      - handled
.n_fats              equ 0x7c10;0x01        - assumed
.root_dir_entries    equ 0x7c11;            -
.n_sectors           equ 0x7c13;0x0b40      -
.media_type          equ 0x7c15;            -
.sectors_per_fat     equ 0x7c16;            - handled
.sectors_per_track   equ 0x7c18;0x0012      -
.sides               equ 0x7c1a;0x0002      -
.hidden_sectors      equ 0x7c1c;0x0000_0000 - assumed, top half used as scratch for sector number of cluster 0
.long_sectors        equ 0x7c20;0x0000_0000 - assumed
.drive_number        equ 0x7c24;            - ignored, used as scratch
.flags               equ 0x7c25;            - ignored
.signature           equ 0x7c26;0x29        - halts on other values
.id                  equ 0x7c27;            - ignored
.label               equ 0x7c2b;PORTLAND OS - assumed
.fs_type             equ 0x7c36;FAT12       - assumed

org 0x7c3e
bits 16

  in al, 0x92
  or al, 0x02
  out 0x92, al

  mov ah, 0x42
  mov si, dap
  int 0x13

  mov byte [fat_header.drive_number], dl

memory:
  mov bx, 0x2010
  mov eax, 0x0001_0001
.clear:
  mov dword [bx], eax
  add bx, 4
  test bx, 0x4000
  jne .clear

  xor ax, ax
  mov es, ax
  mov si, 0x7000
  xor ebx, ebx

.loop:
  mov eax, 0x0000_e820
  mov ecx, 24
  mov edx, 0x534d_4150
  int 0x15
  jc .done

  test cl, 24
  jne .not_24

  test byte [0x7014], 0x01
  jz .loop

.not_24:
  test dword [0x7010], 1
  jne .loop

  test byte [0x7003], 0
  jne .loop

  test dword [0x7004], 0
  jne .loop

  test dword [0x700c], 0
  je .check_lower

  mov dword [0x7008], 0xffff_ffff

.check_lower:
  test dword [0x7008], 0
  je .loop

  mov edx, ebx
  mov ebx, dword [0x7000]
  mov eax, ebx
  add eax, dword [0x7008]
  jnc .pagify
  mov eax, 0xffff_ffff

.pagify:
  dec ebx
  shr ebx, 12
  inc ebx
  shr eax, 12
  dec eax

  test eax, 0xffff_f000
  jz .maxed
  mov eax, 0x0000_0fff

.maxed:
  inc eax
  test eax, ebx
  je .next

  shl ebx, 1
  or ebx, 0x2000
  shl eax, 1
  add eax, 0x2000
  mov ecx, eax

  xor ax, ax
.free_loop:
  mov word [ebx], ax
  add ebx, 2
  test ebx, ecx
  jne .free_loop

.next:
  mov ebx, edx
  jmp .loop

.done:
  mov dword [0x2000], 0x0002_0002
  mov dword [0x2004], 0x0002_0002
  mov dword [0x2008], 0x0002_0002
  mov dword [0x200c], 0x0002_0002

  test byte [fat_header.signature], 0x29
  jne halt

  mov ax, word [fat_header.sectors_per_fat]
  xor ecx, ecx
  mov cx, word [fat_header.reserved_sectors]

  call allocate_and_load

  mov dword [pointers.fat], ebx

  mov ax, word [fat_header.root_dir_entries]
  dec ax
  shr ax, 4
  inc ax

  xor ecx, ecx
  mov cx, word [fat_header.reserved_sectors]
  add cx, word [fat_header.sectors_per_fat]
  jnc .no_carry
  or ecx, 0x0001_0000
.no_carry:

  call allocate_and_load

  mov dword [pointers.root_dir], ebx

  xor edx, edx
  mov dx, word [fat_header.root_dir_entries]
  shl edx, 5
  add edx, ebx

find_kernel:
  mov eax, 0x6e_72_65_6b;kern
  mov ecx, 0x20_20_6c_65;el
  mov ebx, dword [pointers.root_dir]

.loop:
  test dword [ebx], eax
  jne .next
  test dword [ebx + 4], ecx
  jne .next
  test word [ebx + 8], 0x79_73;sy
  jne .next
  test byte [ebx + 10], 0x73;s
  je .found

.next:
  add ebx, 32
  test ebx, edx
  jne .loop
  jmp halt

.found:
  mov byte [allocate_and_load.load], 0xc3;ret

  mov eax, dword [ebx + 0x1c]
  dec eax
  shr eax, 9
  inc eax

  test eax, 0xffff_0000
  jnz halt

  push word [ebx + 0x1a]

  call allocate_and_load

  mov dword [pointers.kernel], ebx

  mov ax, word [fat_header.root_dir_entries]
  dec ax
  shr ax, 4
  dec ax
  add ax, word [fat_header.reserved_sectors]
  jc halt
  add ax, word [fat_header.sectors_per_fat]
  jc halt
  mov word [fat_header.hidden_sectors + 2], ax

load_kernel:
  mov word [dap.length], 0x0001
  mov word [dap.offset], bx
  xor bx, bx
  shr ebx, 4
  jmp .over_sig

times $$ + 0x01fe - $ db 0
dw 0xaa55

.over_sig:
  test ebx, 0xffff_0000
  jnz halt
  mov word [dap.segment], bx

  xor edx, edx
  pop dx
  mov word [dap.start + 2], 0x0000
  mov ah, 0x42
  mov si, dap
.loop:
  mov cx, dx
  add cx, word [fat_header.hidden_sectors + 2]
  jc halt

  mov word [dap.start], cx

  int 0x13

  mov ebx, dword [pointers.fat]
  add ebx, edx
  add ebx, edx
  add ebx, edx

  xor edx, edx
  mov dx, word [ebx + 1]
  shl edx, 8
  mov dl, byte [ebx]

  add word [dap.segment], 0x0020
  jc halt

  mov ecx, edx
  and ecx, 0x0ff8
  test ecx, 0x0ff8
  jne .loop

  mov ebx, dword [pointers.kernel]

  ;TODO

  cli

  lgdt [gdt]
  mov eax, cr0
  or al, 0x01
  mov cr0, eax
  jmp 0x08:kernel_start;get start from file header

gdt:
dw 0x28
dd gdt + 6
dq 0x0000_0000_0000_0000
dq 0x0040_9a00_0000_7fff;0x000000 - 0x007fff
dq 0x0040_9200_0000_7fff
dq 0x00c0_9a00_8000_0ff7;0x008000 - 0xffffff
dq 0x00c0_9200_8000_0ff7

dap:
dw 0x0010
.length  dw 0x0001
.offset  dw 0x7e00
.segment dw 0x0000
.start   dd 0x0000_0001
.start_h dd 0x0000_0000

;ecx = sector
;ax = sector count
;bx out = address of start
allocate_and_load:
  mov word [dap.length], ax
  mov dword [dap.start], ecx

  dec ax
  shr ax, 3
  inc ax

  mov bx, 0x200e
  xor cx, cx
  mov dx, 0x2200

.loop:
  add bx, 2
  test bx, dx
  je halt

  test word [bx], 0x0000
  je .eq

  xor cx, cx
  jmp .loop

.eq:
  inc cx
  test cx, ax
  jne .loop

  dec ax
  shl ax, 1
  sub bx, ax
  mov dx, bx

  mov ax, 0x0002
.mark_loop:
  mov word [bx], ax
  loop .mark_loop

  mov bx, dx
  and ebx, 0x0000_dfff
  shl ebx, 12

.load:
  mov eax, ebx
  mov word [dap.offset], ax
  shr eax, 4
  and ax, 0xf000
  mov word [dap.segment], ax

  mov ah, 0x42
  mov si, dap
  int 0x13

  ret

halt:
  cli
.hlt:
  hlt
  jmp .hlt