global gdt
global idt
global kboot
global register_isr
global register_trap
extern con_init
extern fs_init
extern kmain
extern paging_init
extern panic
extern printk
extern syscall_init
extern timer_init
extern traps_init
extern tty_init

%define NRTASKS 64

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

section .text

kboot:
  cli
  ; put the stack pointer near the end of conventional memory
  ; WARNING: don't change this without changing the corresponding
  ; entries in the TSS constructor in sched.c
  mov esp, 0x80000
  ; setup descriptor tables and enable paging
  call flush_gdt
  call enable_a20
  call paging_init
  call flush_idt
  ; initialize the console
  call con_init
  ; add this point, we can display *some* output
  push .msg
  call printk
  add esp, 4
  ; initialize secondary subsystems
  call timer_init
  call fs_init
  call tty_init
  ; last minute setup, then transfer to kmain
  call kmain
  ; if kmain decides to return (which it shouldn't),
  ; disable interrupts and halt the system.
  cli
  hlt
.msg: db "Kernel booting...", 10, 0

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
  ; empty TSS slots for our C scheduler
  times NRTASKS dq 0
  ; garbage TSS descriptor
  dq 0
.end:

gdtp:
  dw (gdt.end-gdt-1)
  dd gdt

garbage_tss:
  times 104 db 0

flush_gdt:
  push ebp
  mov ebp, esp
  push ebx
  ; now, load the GDT
  lgdt [gdtp]
  mov ax, 0x10
  mov ds, ax
  mov es, ax
  mov fs, ax
  mov gs, ax
  mov ss, ax
  jmp 0x08:.end
.end:
  ; finally, we load the task register with
  ; a pointer to our dummy TSS. this is
  ; necessary since the CPU *must* save the
  ; previous task state *somewhere* before
  ; switching tasks. to avoid overwriting
  ; any sensitive information, we reserve
  ; 104 bytes for a *garbage* TSS.
  mov eax, 5
  add eax, NRTASKS
  shl eax, 3
  mov ebx, eax
  add ebx, gdt
  mov edx, garbage_tss
  mov dword [ebx], 103 ; limit - 1
  mov [ebx+2], dx
  shr edx, 16
  mov [ebx+4], dl
  mov [ebx+7], dh
  mov byte [ebx+5], 0x89
  mov byte [ebx+6], 0x00
  ltr ax
  pop ebx
  pop ebp
  ret

idt:
  times (256*8) db 0
.end:

idtp:
  dw (idt.end-idt-1)
  dd idt

int_handler:
  iret

flush_idt:
  push ebp
  mov ebp, esp
  ; install the exception handlers (vectors 0-31)
  call traps_init
  ; install the default handlers (vectors 32-255)
  mov ecx, 0
  lea edx, [idt+(32*8)]
.loop:
  mov eax, int_handler
  mov [edx], ax
  shr eax, 16
  mov [edx+6], ax
  mov word [edx+2], 0x08
  mov byte [edx+4], 0x00
  mov byte [edx+5], 0x8E
  inc ecx
  cmp ecx, (256-32)
  je .end
  add edx, 8
  jmp .loop
.end:
  ; install the system call handler
  call syscall_init
  ; load the IDT, initialize the PIC's, and enable interrupts
  lidt [idtp]
  call pic_init
  sti
  pop ebp
  ret

; void register_isr(uint8_t n, uint8_t dpl, void *handler);
register_isr:
  push ebp
  mov ebp, esp
  mov edx, [ebp+8]
  shl edx, 3
  add edx, idt
  mov eax, [ebp+16]
  mov [edx], ax
  shr eax, 16
  mov [edx+6], ax
  mov eax, [ebp+12]
  and al, 3
  shl al, 5
  mov cl, 0x8E
  or cl, al
  mov [edx+5], cl
  pop ebp
  ret

; void register_trap(uint8_t n, uint8_t dpl, void *handler);
register_trap:
  push ebp
  mov ebp, esp
  mov edx, [ebp+8]
  shl edx, 3
  add edx, idt
  mov eax, [ebp+16]
  mov [edx], ax
  shr eax, 16
  mov [edx+6], ax
  mov eax, [ebp+12]
  and al, 3
  shl al, 5
  mov cl, 0x8F
  or cl, al
  mov [edx+5], cl
  pop ebp
  ret

enable_a20:
  push ebp
  mov ebp, esp
  ; test if the A20 gate was already enabled
  call .testgate
  jne .end
  ; send read controller output port command
  call .waitout
  mov al, 0xD0
  out 0x64, al
  ; read controller output port
  call .waitin
  in al, 0x60
  xchg ax, cx
  ; send write controller output port command
  call .waitout
  mov al, 0xD1
  out 0x64, al
  ; write controller output port (enabling gate A20)
  call .waitout
  xchg ax, cx
  or al, 0x02
  out 0x60, al
  ; check if the gate is now enabled
  call .testgate
  jne .end
  ; if the gate is still not working, try the fast-gate method
  in al, 0x92
  and al, 0xFE
  or al, 0x02
  out 0x92, al
  ; make sure the gate is working (panic if not)
  call .testgate
  jne .end
  push .msg
  call printk
  add sp, 4
  call panic
.waitin:
  in al, 0x64
  test al, 0x01
  jz .waitin
  ret
.waitout:
  in al, 0x64
  test al, 0x02
  jnz .waitout
  ret
.testgate:
  ; test if the A20 gate is enabled by depositing
  ; two words 1MB away from each other in memory.
  mov word [.tmp+0x100000], 0xBEEF
  mov cx, [.tmp]
  mov dx, [.tmp+0x100000]
  cmp cx, dx
  ret
.end:
  pop ebp
  ret
.tmp: dw 0xDEAD
.msg: db "Failed to enable the A20 gate!", 10, 0

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