global kboot
global register_isr
extern kmain
extern timer_init

; Multiboot header
section .mboothdr
mboot_hdr:
  align 4
  dd 0x1BADB002
  dd 0x00
  dd - (0x1BADB002 + 0x00)

section .text

kboot:
  cli
  call flush_gdt
  call flush_idt
  call timer_init
  call kmain
  cli
  hlt

gdt:
  ; null descriptor
  dq 0
  ; kernel code segment
  dw 0xFFFF ; limit (bits 0-15)
  dw 0x0000 ; base (bits 0-15)
  db 0x00   ; base (bits 16-23)
  db 0x9A   ; access byte
  db 0xCF   ; flags / limit (bits 16-19)
  db 0x00   ; base (bits 24-31)
  ; kernel data segment
  dw 0xFFFF
  dw 0x0000
  db 0x00
  db 0x92
  db 0xCF
  db 0x00
  ; userspace code segment
  dw 0xFFFF
  dw 0x0000
  db 0x00
  db 0xFA
  db 0xCF
  db 0x00
  ; userspace data segment
  dw 0xFFFF
  dw 0x0000
  db 0x00
  db 0xF2
  db 0xCF
  db 0x00

gdtp:
  dw ($-gdt-1)
  dd gdt

flush_gdt:
  push ebp
  mov ebp, esp
  lgdt [gdtp]
  mov ax, 0x10
  mov ds, ax
  mov es, ax
  mov fs, ax
  mov gs, ax
  mov ss, ax
  jmp 0x08:.end
.end:
  pop ebp
  ret

idt:
  times (256*8) db 0

idtp:
  dw ($-idt-1)
  dd idt

int_handler:
  iret

irq_handler:
  iret

flush_idt:
  push ebp
  mov ebp, esp
  mov ecx, 0
  mov edx, idt
.loop:
  mov eax, int_handler
  cmp ecx, 0x20
  jl .skip
  cmp ecx, 0x2f
  jg .skip
  mov eax, irq_handler
.skip:
  mov [edx], ax
  mov word [edx+2], 0x08
  mov byte [edx+4], 0
  mov byte [edx+5], 0x8E
  shr eax, 16
  mov [edx+6], ax
  add edx, 8
  inc ecx
  cmp ecx, 256
  je .end
  jmp .loop
.end:
  lidt [idtp]
  call pic_init
  sti
  pop ebp
  ret

; void register_isr(uint8_t n, void *handler);
register_isr:
  push ebp
  mov ebp, esp
  mov edx, [ebp+8]
  shl edx, 3
  add edx, idt
  mov eax, [ebp+12]
  mov [edx], ax
  add edx, 6
  shr eax, 16
  mov [edx], ax
  pop ebp
  ret

pic_init:
  push ebp
  mov ebp, esp
  mov al, 0x11
  out 0x20, al
  out 0xA0, al
  mov al, 0x20
  out 0x21, al
  out 0xA1, al
  mov al, 0x04
  out 0x21, al
  mov al, 0x02
  out 0xA1, al
  mov al, 0x01
  out 0x21, al
  out 0xA1, al
  mov al, 0xFF
  out 0x21, al
  out 0xA1, al
  pop ebp
  ret