Nop at start of bootloader

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Qu4Z

Joined: 11 Feb 2007
Posts: 2

Qu4Z

Hi!
I'm reasonably new to this forum, so forgive me if I didn't read any forum rules that weren't jumping in my face.
I've been coding a floppy bootloader in FASM (cliche' I know, but what'm I going to do?), and upon looking through some examples on this forum i noticed that most seem to start with

Code:

jmp start
nop
start: 
;bladibla

I read somewhere that FASM required it, and my bootloader seems to boot fine, even without it. Is this perhaps something to do with FAT12 that doesn't affect me 'cause I'm using unformatted floppies?
Or it a hardware compatibility thing (to be compatible with some types of hardware).
As I said, my computer seems to boot it fine, as does bochs, so I'm a little confused as to why it's there.

Regards,
Qu4Z

11 Feb 2007, 10:31

Octavio

Joined: 21 Jun 2003
Posts: 366
Location: Spain

Octavio

Qu4Z wrote:

Hi!
I'm reasonably new to this forum, so forgive me if I didn't read any forum rules that weren't jumping in my face.
I've been coding a floppy bootloader in FASM (cliche' I know, but what'm I going to do?), and upon looking through some examples on this forum i noticed that most seem to start with
Code:
jmp start
nop
start: 
;bladibla
    
I read somewhere that FASM required it, and my bootloader seems to boot fine, even without it. Is this perhaps something to do with FAT12 that doesn't affect me 'cause I'm using unformatted floppies?
Or it a hardware compatibility thing (to be compatible with some types of hardware).
As I said, my computer seems to boot it fine, as does bochs, so I'm a little confused as to why it's there.

Regards,
Qu4Z

that is because after the 3 first bytes is found the parameters table and the
'jmp' sometimes is just 2 bytes some people add a 'nop' or use a 'jmp word' wich is 3 bytes in size if your boot code do not have a parameter (not a fat system) table you do not neet this jmp at all.

11 Feb 2007, 14:23

Borsuc

Joined: 29 Dec 2005
Posts: 2465
Location: Bucharest, Romania

Borsuc

Isn't FAT something like this:

Code:

jmp start
nop

; data here (format header whatever)
db ...

start:
; code here

But why doesn't it put at the end? it would be much simpler, and without the jump. So if you don't use FAT and need some data, try it at the end Wink

14 Feb 2007, 13:25

Qu4Z

Joined: 11 Feb 2007
Posts: 2

Qu4Z

Thanks

23 Feb 2007, 21:44

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