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; setup screen colors
	xor eax,eax
	mov al,LIGHT_GREY
	shl al,4
	or al,BLACK
	push eax
	call k_set_screen_colors
	add esp,4
	
	call k_clear_screen
	
	; setup an ISR:
	; push DPL param
	xor eax,eax
	push eax
	; push selector param
	mov eax,0x00000008
	push eax
	; push offset of isr
	mov eax,((ISR_0-k_main)+KERNEL_OFFSET)   ;This IS the address of the ISR. This is NOT the problem.
	push eax
	; push interrupt #
	xor eax,eax
	push eax
	call k_install_isr
	add esp,0x00000010
	
	; load the idt
	lidt [idt_descriptor]
	
	; call an interrupt
	int 0x00
	
	; change screen colors
	xor eax,eax
	mov al,BLUE
	shl al,4
	or al,LIGHT_CYAN
	push eax
	call k_set_screen_colors
	add esp,4

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; @param1 interrupt # (0-255)
; @param2 offset of ISR
; @param3 selecter (in GDT) for ISR
; @param4 DPL (ISR ring level)

%define NUM DWORD [ebp-16]
%define OFF DWORD [ebp-12]
%define SEL DWORD [ebp-8]
%define DPL DWORD [ebp-4]

k_install_isr:
	
	; setup stack frame
	push ebp
	mov ebp,esp
	
	; create space for local vars
	sub esp,0x00000010
	
	; get interrupt number
	mov eax,DWORD [ebp+8]
	mov NUM,eax
	
	; get offset
	mov eax,DWORD [ebp+12]
	mov OFF,eax
	
	; get selector
	mov eax,DWORD [ebp+16]
	mov SEL,eax
	
	; get DPL
	mov eax,DWORD [ebp+20]
	mov DPL,eax
	
	; calculate base address for interrupt entry
	xor eax,eax
	add eax,NUM
	imul eax,0x00000008
	add eax,idt_entries
	
	; put the lower word of the offset at the address
	xor ebx,ebx
	mov ebx,OFF
	mov WORD [ds:eax],bx
	add eax,0x00000002
	
	; put the selector at updated address
	xor ebx,ebx
	mov ebx,SEL
	mov WORD [ds:eax],bx
	add eax,0x00000002
	
	; put an empty byte at updated address
	xor ebx,ebx
	mov BYTE [ds:eax],bl
	inc eax
	
	; put the access byte at updated address
	xor ebx,ebx
	mov ebx,DPL
	shl bl,0x05
	or bl,10000000b ; bit mask turns on "present" bit
	or bl,00001110b ; bit mask makes int a 32 bit gate
	mov BYTE [ds:eax],bl
	inc eax
	
	; put the upper word of the offset at the updated address
	xor ebx,ebx
	mov ebx,OFF
	shr ebx,0x00000010
	mov WORD [ds:eax],bx
	
	; clean up stack frame
	mov esp,ebp
	pop ebp
	ret

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ISR_0:
	
	; save registers
	pusha
	push ds
	push es
	push fs
	push gs
	
	; setup stack frame
	push ebp
	mov ebp,esp
	
	; print a string
	push DWORD 25
	push DWORD 80
	push DWORD 1
	push DWORD 1
	push DWORD 1
	push DWORD 1
	xor eax,eax
	mov eax,((isr0_exec-k_main)+KERNEL_OFFSET)
	push eax
	call k_put_string_in_bounds
	add esp,0x0000001C
	
	; clean up stack frame
	mov esp,ebp
	pop ebp
	
	; load registers
	pop gs
	pop fs
	pop es
	pop ds
	popa
	
	iret

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idt_entries:
	
	; insert 256 zeroed out interrupt entries
	%rep 0x00000100
	dd 0x00000000
	dd 0x00000000
	%endrep
	
idt_descriptor:
	
	; 2 byte size
	dw (idt_descriptor-idt_entries-1)
	
	; 4 byte offset
	dd idt_entries

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int 0x00

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sti
	xor eax,eax
	xor ebx,ebx
	div bl

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isr0:
	pusha
	push gs
	push fs
	push ds
	push es
	
	mov eax,0x000B8000
	mov BYTE [ds:eax],"A"
	
	pop es
	pop ds
	pop fs
	pop gs
	popa
	iret

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; idt table
idt_entries:
	
	dw 0xB02F
	dw 0x0008
	dw 0x8E00
	dw 0x0000
	
idt_descriptor:
	
	; table size (2 bytes)
	dw (idt_descriptor - idt_entries - 1)
	
	; table offset (4 bytes)
	dd idt_entries

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	; load IDT
	lidt [idt_descriptor]
	
	; try to call int 0 (div by 0 exception) directly
	int 0x00                                                                 ;IF I COMMENT OUT THIS LINE
	
	; try to provoke int 0 by dividing by 0
	xor ebx,ebx
	div bl                                                                 ;AND THIS LINE,
	
	mov eax,0x000B8002
	mov BYTE [ds:eax],"Q"                                       ; THEN Q IS OUTPUT AS EXPECTED

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isr0:
   pusha
   push gs
   push fs
   push ds
   push es
   
   mov eax,0x000B8000
   mov BYTE [ds:eax],"A"
back:
   jmp back