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CPU 386		; Architecture we're using
BITS 16		; MBRs are 16-bit code
org 7C00h	; The location the BIOS loads the MBR

; Offset of int 0x1C - 4 bytes per entry
%define TIMER_IVT_OFFSET 0x1C * 4

main:
	; Perform this jump to make sure cs is set to 0x0000 and ip is set to 0x7c00(ish) - in line with our org.
	; BIOS behaviour on this is unpredictable
	jmp 0x0000:setcs

	print_char db 0		; Flag to see if we need to print or not
	tick_count db 0		; Tick count for 1Ch

setcs:
	; Base setup

	cld			; Set the direction flag forward, used for movs, stos, etc.
				; Setting this forward will increment cx on operations instead of decrementing it.

	xor ax, ax		; Set ES=DS=0 - this can be changed later for rep operations
	mov ds, ax
	mov es, ax

	mov ss, ax		; Stack segment is 0
	mov sp, 0x7c00		; This sets the stack (with segment 0, so ss:sp = 0000:7c00) to grow below the
				; bootloader. Nothing is relocated or loaded there, so this should be fine.


	; Set up the screen for displaying error messages and load the remaining sectors
	; These calls could be defined as callable functions, but since they are only used once, it doesn't
	; make that much sense to add that overhead and complexity.

	; Clear the screen
	mov ah, 0x07	; Function 0x07: scroll window
	mov al, 0x00	; Clear entire window
	mov bh, 0x07	; White on black
	mov cx, 0x00	; Specifies top left of screen as (0,0)
	mov dh, 0x18	; 18h = 24 rows of chars
	mov dl, 0x4f	; 4fh = 79 cols of chars
	int 0x10	; Video interrupt


	; Move cursor to position 0,0
	mov ah, 0x02	; Function 0x02: set cursor position
	mov dh, 0x00	; Row 0
	mov dl, 0x00	; Col 0
	mov bh, 0x00	; Page 0
	int 0x10	; Video interrupt

	; Stop all interrupts while we change an entry
	cli

	; IVT in real mode is at 0x0000:0x0000
	; Each entry is 2 words; offset and address with address first,
	; then offset. We are using offset 0 in this entire MBR and the
	; address is the defined anim_timer_isr below.
	mov word [TIMER_IVT_OFFSET], tick
	mov word [TIMER_IVT_OFFSET+2], 0x0000

	; Enable interrupts again
	sti

main_loop:
	cmp byte [print_char], 1
	je .print_char
	jmp main_loop

.print_char:
	mov byte [print_char], 0
	mov bh, 0
	mov bl, 0
	mov ah, 0Eh
	mov al, 'x'
	int 10h
	jmp main_loop
	

tick:
	; If we're on tick 5
	cmp byte [tick_count], 5
	je .set_print

	; Increase tick count
	inc byte [tick_count]

	jmp .end_tick

.set_print:
	; Set print_char to true and reset tick count
	mov byte [tick_count], 0
	mov byte [print_char], 1

.end_tick:
	iret


; Padding bytes until the partition table.
; $$ = address of start of section, $ = current position, $-$$ = length of section thus far.
; We use 510, so the last 2 bytes can be specifically set (total sector size = 512).
times 510-($-$$) db 0

; MBR signature
db 55h, 0AAh

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nasm -fbin mbr.asm -o mbr.bin

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qemu-system-i386 -drive "format=raw,file=mbr.bin,if=floppy"

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VBoxManage internalcommands createrawvmdk -filename ./mbr.vmdk -rawdisk ./mbr.bin