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[BITS 16]	; generate 16bit real mode code
[ORG 0x700]	; start address of code is 0x700

; jump to start label (without segment prefix -> 0x0:)
jmp 0x00:start

start:
	cli		; clear interrupt flag

	; setup working stack
	mov ax,0x9000
	mov ss,ax	; set stack base address to 0x9000
	mov sp,0	; set stack pointer to 0

	sti			; set interrupt flag

	mov dx,msg
	push dx
	call printstr

	jmp short $

msg db 'Welcome to simpleOS!',10,0	; welcome message

; print string
printstr:
	pop dx
	mov si,dx
_loop:
	mov al,[si]
	mov ah,0x0E			; move attribute byte to 0x0E
	int 0x10

	mov al,[si]			; just in case the BIOS interrupt modifies the register

	inc si

	cmp al,0			; compare al and 0
	jne _done			; if they are equivalent, jump to the 'done' label

	jmp _loop
_done:
	ret

times 512-($-$$)-2 db 0		; $ stands for the current line offset
							; $$ stands for the offset into the section
dw 0xAA55

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push bp
mov bp, sp
mov dx, [bp+4]

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leave
ret

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[BITS 16]	; generate 16bit real mode code
[ORG 0x7C0]	; start address of code is 0x700

; jump to start label (without segment prefix -> 0x0:)
jmp 0x00:start

start:
	; setup working stack
	mov ax,0x9000
	mov ss,ax	; set stack base address to 0x9000
	mov sp,0	; set stack pointer to 0

	sign db 'H'

	; print character to screen
	mov al,[sign]
	mov ah,0x0E
	int 0x10

	jmp $

times 512-($-$$)-2 db 0
dw 0xAA55

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[BITS 16]	; generate 16bit real mode code
[ORG 0x7C00]	; 0x7C00

; jump to start label (without segment prefix -> 0x0:)
jmp 0x00:start

start:
	; setup working stack
	mov ax,0x9000
	mov ss,ax	; set stack base address to 0x9000
	mov sp,0	; set stack pointer to 0
	xor ax, ax
	mov ds, ax
	jmp next_code

	sign db 'H'

next_code:
	; print character to screen
	mov al,[sign]
	mov ah,0x0E
	int 0x10

	jmp $

times 512-($-$$)-2 db 0
dw 0xAA55

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[BITS 16]	; produce 16bit real mode code
[ORG 0x7C00]	; organize code on 0x7C0 where it gets loaded by the BIOS

jmp 0x00:start	; jump to 'start' label (with sr prefix 0x00: -> cs = 0)

start:
	; setup a working stack at address 0x9000 (should be free for own use)
	mov ax,0x9000
	mov ss,ax	; set stack base address to 0x9000
	mov sp,0	; set stack pointer to 0

	; setup working environment
	xor ax,ax	; get zero into AX
	mov ds,ax	; set DS to zero
	mov es,ax	; set ES to zero

	; jump to actual start function
	jmp _start

	; reserve some memory for the welcome string to output
	msg db 'Hello',0

_start:
	mov ax,msg		; move sign address to ax
	push ax			; push address to stack
	call screen_write

	jmp $			; jump into endless loop

; this function writes a string to the screen
screen_write:
	pop ax			; pop return address from stack
	pop ax			; pop string base address from stack
	mov si,ax		; move base address to SI

_char_loop:
	lodsb			; load character currently being located at the address
					; stored in SI to AL

	cmp al,0		; compare AL and 0: 0 is subtracted from AL and if zero
					; comes out, the zero flag is set
	je _end			; if they are equivalent (-> zero flag is set), jump to end
	
	mov ah,0x0E		; move attribute byte to AH
	int 0x10		; raise BIOS interrupt
	jmp _char_loop

_end:
	ret
	

times 512-($-$$)-2 db 0		; $ stands for the current line offset
; $$ stands for the offset into the section
dw 0xAA55

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        cli                  ; disable interrupts
        hlt                 ; halt - will continue execution after an exception or IRQ
                            ; (we disabled interrupts, so this will stop the processor)

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INFINITE_LOOP:
        hlt                 ; halt - will continue execution after an exception or IRQ
        jmp INFINITE_LOOP      ; jump to INFINITE_LOOP and wait for another exception or IRQ

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cli
hlt