simple boot sector coding: Filling the 512 Byte with 0

Question

I use the Bochs simulator to run a boot sector, which coded in NASM like this:

  org 07c00h                  ;told the compiler to load the code to 0x7c00h
  mov ax, cs                 
  mov ds, ax
  mov es, ax
  call    DispStr             ;call the string process 
  jmp $                       ;infinite loop
 DispStr:
   mov ax, BootMessage        
   mov bp, ax                 ;ES:BP = string address    
   mov cx, 16                 ;CX = length of the string
   mov ax, 01301h             ;AH = 13, AL = 01H
   mov bx, 000ch              ;(BH = 0) 
   mov dl, 0                  
   int 10h                    ;10h interrupt
   ret
 BootMessage:    db  "Hello, OS world!"  ;message printed
 times    510-($-$$)  db  0              ;fill the left of the 510 byte with 0 
 dw  0xaa55

If times 510-($-$$) db 0 if forbidden, is there any alternative way to fill the left of 510 byte area with 0?

I've tried loop command, but can not work properly.

_"If times 510-($-$$) db 0 if forbidden"_ Why would it be forbidden? — Michael, Apr 07 '16 at 07:56
@Michael But the assistant of our OS class asked us to come up with an optional way to fill the blank... — Stone, Apr 07 '16 at 07:59
Huh. Sounds like a waste of time to me. Is this an OS class or a NASM directives class? — Michael, Apr 07 '16 at 08:02
The TA is wasting your time. There are far more important things in an OS dev class than how to use rep vs $-$$. — David Hoelzer, Apr 08 '16 at 17:59

score 2 · Accepted Answer · answered Apr 07 '16 at 08:25

2

An alternative way would be to use a preprocessor loop (%rep):

%rep 510-($-$$)
db 0
%endrep

If your TA still isn't satisfied I'll leave it to you to dig through the NASM manual for other possible ways of achieving the same thing.

answered Apr 07 '16 at 08:25

Michael

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simple boot sector coding: Filling the 512 Byte with 0

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