path: root/src/bootloader.asm

;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.

kload equ 0x0010_0000

global_info:;0x0500 - 0x05ff (0x0515)
.fat      equ 0x0500;dword - pointer
.root_dir equ 0x0504;dword - pointer
.low_mem  equ 0x0508;word  - usable memory from 0 to 1MiB in kiB
.mid_mem  equ 0x050a;word  - usable memory from 1MiB to 16MiB in kiB
.mem_used equ 0x050c;word  - used memory from 0 to 1MiB in kiB
.kernel_s equ 0x050e;word  - kernel image length in kiB
                    ;dword - kernel use
.high_mem equ 0x0514;word  - usable memory from 16MiB up in 64s of kiB

fat_header:;0x7c03 - 0x7c3d
.oem_name            equ 0x7c03;mkfs.fat    -
.bytes_per_sector    equ 0x7c0b;0x0200      -
.sectors_per_cluster equ 0x7c0d;0x01        -
.reserved_sectors    equ 0x7c0e;0x0002      - handled
.n_fats              equ 0x7c10;0x01        -
.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 -
.long_sectors        equ 0x7c20;0x0000_0000 -
.drive_number        equ 0x7c24;            - ignored, used as scratch for real drive number
.flags               equ 0x7c25;            -
.signature           equ 0x7c26;0x29        - errors on other values
.id                  equ 0x7c27;            -
.label               equ 0x7c2b;PORTLAND OS -
.fs_type             equ 0x7c36;FAT12       - errors on other values

error_codes:
.bad_fat_header      equ 0o000
.insufficient_memory equ 0o001
.no_kernel           equ 0o002
.kernel_too_big      equ 0o003

org 0x7c3e
bits 16

  ;Enable A20 line
  in al, 0x92
  or al, 0x02
  out 0x92, al

  ;Load second sector of bootloader
  mov ah, 0x42
  mov si, dap
  int 0x13

  ;Check FAT signature
  test byte [fat_header.signature], 0x29
  jne bad_fat_header
  test word [fat_header.fs_type], 0x4146;FA
  jne bad_fat_header
  test word [fat_header.fs_type + 2], 0x3154;T1
  jne bad_fat_header
  test word [fat_header.fs_type + 4], 0x2032;2
  jne bad_fat_header
  test word [fat_header.fs_type + 6], 0x2020;
  jne bad_fat_header

  ;Store boot drive ID for later
  mov byte [fat_header.drive_number], dl

memory:
  ;Get available low memory
  int 0x12
  mov word [global_info.low_mem], ax
  mov word [global_info.mem_used], 0x0020

  ;Get available mid and high memory
  xor dx, dx
  mov ax, 0xe801
  int 0x15
  test dx, dx
  jnz .got
  mov cx, ax
  mov dx, bx
.got:
  mov word [global_info.mid_mem], cx
  mov word [global_info.high_mem], dx

  ;Load FAT
  mov ax, word [fat_header.reserved_sectors]
  mov word [dap.start], ax
  xor eax, eax
  mov ax, word [fat_header.sectors_per_fat]
  call load_low
  mov dword [global_info.fat], ebx

load_root:
  ;Load root directory
  mov ax, word [fat_header.sectors_per_fat]
  xor dh, dh
  mov dl, word [fat_header.n_fats]
  mul dx
  add ax, [fat_header.reserved_sectors]
  jnc .no_inc
  inc dx
.no_inc:
  mov word [dap.start], ax
  mov word [dap.start + 2], dx
  mov ax, word [fat_header.root_dir_entries]
  shr ax, 4
  call load_low
  mov dword [global_info.root_dir], ebx

  ;Calculate end of root directory
  xor ecx, ecx
  mov cx, word [fat_header.root_dir_entries]
  shl ecx, 5
  add ecx, ebx
  jc insufficient_memory

find_kernel:
  mov eax, 0x6e72_656b;kern
  mov edx, 0x2020_6c65;el
.loop:
  ;Check name
  test dword [ebx], eax
  jne .next
  test dword [ebx + 4], edx
  jne .next
  test word [ebx + 8], 0x7973;sy
  jne .next
  test byte [ebx + 10], 0x73;s
  je .found
.next:
  ;Next record
  add ebx, 32
  test ebx, ecx
  jne .loop

  ;Got to end of directory
  mov dl, error_codes.no_kernel
  jmp error

  ;ebx is pointer to entry
  ;word [ebx + 26] is first cluster
  ;dword [ebx + 28] is size
.found:
  ;Save size for later
  mov eax, dword [ebx + 28]
  shr eax, 10
  test eax, 0x003f_0000
  jz .store_size
  mov dl, error_codes.kernel_too_big
  jmp error
.store_size:
  mov word [global_info.kernel_s], ax

  ;Save sector number for later
  mov cx, [ebx + 26]

  ;find 1kiB buffer
  mov ax, 2
  mov byte [load_low.ret_with_segment], 0xc3;ret
  call load_low
  shl ebx, 4

load_kernel:
  

load_low:
  ;ax in : number of sectors
  ;qword [dap.start] in : starting sector

  ;Store length in DAP
  mov word [dap.length], ax

  ;Calculate kiB used
  dec ax
  shr ax, 1
  inc ax
  mov dx, ax
  add ax, word [global_info.mem_usage]

  ;Fail if not enough is available
  jc insufficient_memory
  cmp ax, word [global_info.low_mem]
  jg insufficient_memory

  ;Check if address will fit in DAP
  test ah, ah
  jnz insufficient_memory

  ;Commit usage
  mov word [global_info.low_mem_usage], ax

  ;Put address into DAP
  xor ebx, ebx
  mov bh, al
  shr bx, 2
  mov word [dap.segment], bx
  mov word [dap.offset], 0x0000
.ret_with_segment:

  ;Load from disk
  mov si, dap
  mov ah, 0x42
  int 0x13
  shl ebx, 4
  ret

bad_fat_header:
  mov dl, error_codes.bad_fat_header
  jmp error

insufficient_memory:
  mov dl, error_codes.insufficient_memory
  jmp error

error:
  ;Print error code (in octal)
  mov ax, 0xb800
  mov ds, ax
  mov word [0], 0x0f72;E
  mov ax, 0x0f00
  mov al, dl
  and al, 0x07
  or al, 0x30
  mov word [6], ax;bottom digit
  shr dl, 3
  mov al, dl
  and al, 0x07
  or al, 0x30
  mov word [4], ax;middle digit
  shr dl, 3
  mov al, dl
  or al, 0x30
  mov word [2], ax;top digit

  cli
.halt:
  hlt
  jmp .halt

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   dq 0x0000_0000_0000_0001

memory_usage:
.low  dw ;word - used memory from 0 to 1MiB in kiB
.mid  dw 0x0000;word - used memory from 1MiB to 16MiB in kiB
.high dw 0x0000;word - used memory from 16MiB up in 64s of kiB