name "loader"
; this is a very basic example of a tiny operating system. 

; directive to create boot file: 
   #make_boot#

; this is an os loader only! 
; 
; it can be loaded at the first sector of a floppy disk: 

;   cylinder: 0 
;   sector: 1 
;   head: 0 



;================================================= 
; how to test micro-operating system: 
;   1. compile micro-os_loader.asm 
;   2. compile micro-os_kernel.asm 
;   3. compile writebin.asm 
;   4. insert empty floppy disk to drive a: 
;   5. from command prompt type: 
;        writebin loader.bin 
;        writebin kernel.bin /k 
;================================================= 


; 
; The code in this file is supposed to load 
; the kernel (micro-os_kernel.asm) and to pass control over it. 
; The kernel code should be on floppy at: 

;   cylinder: 0 
;   sector: 2 
;   head: 0 

; memory table (hex): 
; ------------------------------- 
; 07c0:0000 |   boot sector 
; 07c0:01ff |   (512 bytes) 
; ------------------------------- 
; 07c0:0200 |    stack 
; 07c0:03ff |   (255 words) 
; ------------------------------- 
; 0800:0000 |    kernel 
; 0800:1400 |  
;           |   (currently 5 kb, 
;           |    10 sectors are 
;           |    loaded from 
;           |    floppy) 
; ------------------------------- 


; To test this program in real envirinment write it to floppy 
; disk using compiled writebin.asm 
; After sucessfully compilation of both files, 
; type this from command prompt:   writebin loader.bin    

; Note: floppy disk boot record will be overwritten. 
;       the floppy will not be useable under windows/dos until 
;       you reformat it, data on floppy disk may be lost. 
;       use empty floppy disks only. 


; micro-os_loader.asm file produced by this code should be less or 
; equal to 512 bytes, since this is the size of the boot sector. 



; boot record is loaded at 0000:7c00 
org 7c00h

; initialize the stack: 
mov     ax, 07c0h
mov     ss, ax
mov     sp, 03feh ; top of the stack. 


; set data segment: 
xor     ax, ax
mov     ds, ax

; set default video mode 80x25: 
mov     ah, 00h
mov     al, 03h
int     10h

; print welcome message: 
lea     si, msg
call    print_string

;=================================== 
; load the kernel at 0800h:0000h 
; 10 sectors starting at: 
;   cylinder: 0 
;   sector: 2 
;   head: 0 

; BIOS passes drive number in dl, 
; so it's not changed: 

mov     ah, 02h ; read function. 
mov     al, 10  ; sectors to read. 
mov     ch, 0   ; cylinder. 
mov     cl, 2   ; sector. 
mov     dh, 0   ; head. 
; dl not changed! - drive number. 

; es:bx points to receiving 
;  data buffer: 
mov     bx, 0800h
mov     es, bx
mov     bx, 0

; read! 
int     13h
;=================================== 

; integrity check: 
cmp     es:[0000],0E9h  ; first byte of kernel must be 0E9 (jmp). 
je     integrity_check_ok

; integrity check error 
lea     si, err
call    print_string

; wait for any key... 
mov     ah, 0
int     16h

; store magic value at 0040h:0072h: 
;   0000h - cold boot. 
;   1234h - warm boot. 
mov     ax, 0040h
mov     ds, ax
mov     w.[0072h], 0000h ; cold boot. 
jmp	0ffffh:0000h	     ; reboot! 

;=================================== 

integrity_check_ok:
; pass control to kernel: 
jmp     0800h:0000h

;=========================================== 



print_string proc near
push    ax      ; store registers... 
push    si      ; 
next_char:
        mov     al, [si]
        cmp     al, 0
        jz      printed
        inc     si
        mov     ah, 0eh ; teletype function. 
        int     10h
        jmp     next_char
printed:
pop     si      ; re-store registers... 
pop     ax      ; 
ret
print_string endp




;==== data section ===================== 

msg  db "Loading...",0Dh,0Ah, 0

err  db "invalid data at sector: 2, cylinder: 0, head: 0 - integrity check failed.", 0Dh,0Ah
     db "refer to tutorial 11 - making your own operating system.", 0Dh,0Ah
     db "System will reboot now. Press any key...", 0

;======================================