bits 16
org 0x7c3e

kernel_sectors equ 64
kernel_segment equ 0x3000

support_flags equ 0x4000
pci_hw_char   equ 0x4001
pci_ver       equ 0x4002
last_pci_bus  equ 0x4004
bios_mmap_len equ 0x4006
vbe_mode      equ 0x4008

vbe_block     equ 0x4200
vbe_mode_info equ 0x4400

pci_support equ 0x80
pae_support equ 0x40

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

  xor ax, ax
  mov ss, ax
  mov sp, 0x8000

  mov ax, kernel_segment
  mov es, ax
  mov ax, 0x0200 + kernel_sectors
  xor bx, bx
  mov cx, 0x0002
  xor dh, dh
  int 0x13

  mov ax, 0xb101
  int 0x1a

  cmp edx, 0x20494350
  jne no_pci

  test ah, ah
  jnz no_pci

  mov byte [support_flags], pci_support
  mov byte [pci_hw_char], al
  mov word [pci_ver], bx
  mov byte [last_pci_bus], cl

no_pci:
  mov eax, 1
  cpuid

  test dl, 0x40
  jz no_pae

  or byte [support_flags], pae_support

no_pae:
  mov ax, 0x1000
  mov es, ax
  xor di, di

  mov edx, 0x534d4150
  xor ebx, ebx

mmap_loop:
  mov eax, 0xe820
  mov ecx, 24
  int 0x15

  jc mmap_end
  add di, 24
  test ebx, ebx
  jnz mmap_loop
mmap_end:
  mov word [bios_mmap_len], di

  xor ax, ax
  mov es, ax
  mov di, vbe_block
  mov ah, 0x4f

  mov dword [di], 0x32454256;'VBE2'

  int 0x10

  cmp dword [di], 0x41534556;'VESA'
  jne no_vbe

  mov bx, word [di + 0x0e]
  mov ax, word [di + 0x10]
  mov fs, ax
  xor esi, esi
  mov bp, 0xffff
  mov di, 0x5000

vbe_loop:
  max_height equ 800
  ;fs:bx is mode pointer
  ;esi is highest resolution yet (width * height)
  ;bp is that mode's number
  ;uses 0x5000 - 0x50ff as mode info buffer
  ;skips those without byte-aligned pixels

  mov cx, word [fs:bx]
  cmp cx, 0xffff
  je got_highest_vbe

  add bx, 4

  mov ax, 0x4f01
  int 0x10

  mov al, byte [di]

  test al, 0x01
  jz vbe_loop

  test al, 0x10
  jz vbe_loop

  test al, 0x80
  jz vbe_loop

  mov dx, word [di + 0x14];height
  cmp dx, max_height
  jg vbe_loop

  mov ax, word [di + 0x12];width
  mul dx

  shl edx, 16
  mov dx, ax

  xor eax, eax

  mov al, byte [di + 0x19];bpp
  test al, 0x07
  jnz vbe_loop

  mul edx

  cmp eax, esi
  jle vbe_loop

  mov esi, eax
  mov bp, cx
  jmp vbe_loop

got_highest_vbe:
  cmp bp, 0xffff
  je no_vbe

  mov cx, bp
  mov ax, 0x4f01
  mov di, vbe_mode_info
  int 0x10

  or ch, 0x40
  mov word [vbe_mode], cx

  mov ax, 0x4f02
  mov bx, cx
  int 0x10

  cli

  lgdt [gdt]

  mov eax, cr0
  or al, 0x01
  mov cr0, eax

  jmp 0x10:pmode

no_vbe:
  xor ax, ax
  mov es, ax

  mov ax, 0x1300
  mov bx, 0x004f
  mov cx, no_vbe_str.len
  xor dx, dx
  mov bp, no_vbe_str
  int 0x10

real_halt:
  hlt
  jmp real_halt

no_vbe_str:
  db "NO VBE2!"
.len equ $ - no_vbe_str

bits 32

pmode:
  mov ax, 0x18
  mov ds, ax
  mov es, ax
  mov ss, ax
  mov esp, 0x00040000

  xor ebp, ebp

  call kernel_segment * 16

halt:
  hlt
  jmp halt

times $$ + 510 - 0x3e - $ - (gdt.e - gdt) db 0

gdt:
  dw .e - .t
  dd .t
.t:
  dq 0x0000_0000_0000_0000;0x00: null
  dq 0x0040_8900_4f98_0068;0x08: tss, 0x4f98
  dq 0x00cf_9a00_0000_ffff;0x10: kernel code
  dq 0x00cf_9200_0000_ffff;0x18: kernel data
  dq 0x00cf_fa00_0000_ffff;0x20: user code
  dq 0x00cf_f200_0000_ffff;0x28: user data
.e:

dw 0xaa55