OSDev.org

Code: Select all

%include "grub.inc" ; needed for the multiboot header

[BITS 32]

[global start]
[extern _k_main] ; this is in the c file

gdtr dw 0   ; for limit storage
     dd 0   ; for base storage

setGdt:
   mov eax,[esp+4]
   mov [gdtr+2],eax
   mov ax,[esp+8]
   mov [gdtr],ax
   lgdt [gdtr]
   ret

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; the IDT with it's descriptors
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
start_of_idt:
;interrupt 0
dw 0x0000
dw 0x10
dw 0x8E00
dw 0x20

;interrupt 1
dw 0x0000
dw 0x10
dw 0x8E00
dw 0x20

;interrupt 2, intel reserved, we set the 'present' bit to 0 on this one
dw 0x0000
dw 0x10
dw 0xE00
dw 0x20

;interrupts 3-14 now, since we are making the descriptors
;identical, we are going to loop to get them all(12 total)
%rep 0xC
  dw 0x0000
  dw 0x10
  dw 0x8E00
  dw 0x20
%endrep

;interrupt 15, intel reserved, we set the 'present' bit to 0 on this one
dw 0x0000
dw 0x10
dw 0xE00
dw 0x20

;interrupt 16
dw 0x0000
dw 0x10
dw 0x8E00
dw 0x20
end_of_idt:

idt_pointer:
dw end_of_idt - start_of_idt - 1
dd start_of_idt

start:
 ; stop using bootloader GDT, and load new GDT
lgdt [gdtr]
lidt [idt_pointer]
 push es
 pop ds
 push ds
STI
 xor   eax,eax 
         in    al,0x60
  call _k_main

  jmp $ ; crash
  hlt ; halt the CPU




;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Multiboot header for GRUB bootloader. This must be in the first 8K
; of the kernel file. We use the aout kludge so it works with ELF,
; DJGPP COFF, Win32 PE, or other formats.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

; these are in the linker script file
EXTERN code, bss, end

ALIGN 4
mboot:
	dd MULTIBOOT_HEADER_MAGIC
	dd MULTIBOOT_HEADER_FLAGS
	dd MULTIBOOT_CHECKSUM
; aout kludge. These must be PHYSICAL addresses
	dd mboot
	dd code
	dd bss
	dd end
	dd start

Code: Select all

start:
 ; stop using bootloader GDT, and load new GDT
    lgdt [gdtr]
    lidt [idt_pointer]

;Setup segment registers and stack

    mov ax, <what is the correct GDT entry for data segments???>
    mov ss,ax
    mov ds,ax
    mov es,ax
    mov fs,ax
    mov gs,ax
    jmp far <what is the correct GDT entry for the code segment???>:.here
.here:
    mov esp, <where is your stack meant to be???>

;Mask all IRQs in the PIC, except the timer IRQ

    mov al,11111110b
    out 0x20+1,al
    mov al,11111111b
    out 0x21+1,al

;Set the IDT entry for the timer IRQ

    mov eax, timerIRQhandler
    mov cx, <what is the correct GDT entry for the code segments???>
    mov [start_of_idt + 0x08 * 8], ax
    shr eax,16
    mov [start_of_idt + 0x08 * 8 + 6], ax
    mov [start_of_idt + 0x08 * 8 + 2], cx
    mov word [start_of_idt + 0x08 * 8+4], 0x8E00

;Enable IRQs and stop

    sti
.wait:
    hlt
    jmp .wait



;Timer IRQ handler

timerIRQhandler:
    push eax
    inc dword [0x000B8000]     ;Increase a dword that is (hopefully) in video display memory
    mov al,0x20
    out 0x20,al                           ;Send "end of interrupt" to the PIC chip
    pop eax
    iret