org $ & bootsectors

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anthill

Joined: 22 Jun 2004
Posts: 2

anthill 23 Jun 2004, 01:34

I have been trying to write a boot sector, and the most confusing problem I have had is how to get the boot signature- 0AA55h
- to offset 510 in the output.
In various forums I found this--

org 7c00h
bootstart:

Some code......
more code..
ditto
ditto

times 510-($-bootstart) db 0h
dw 0AA55h
---Which works, but seemed needlessly complex.
This--times 510-$ db 0h
---did not work, and I could not find a reasonable explanation for it.
And because I like to understand my code, I did some playing and came up with this explanation.....

The org directive simply resets the output location counter- $ to its argument, in this case--
7c00h --which is equivalent to the more self explanatory--
$=7c00h .
This--times 510-$ db 0h-- does not work because $, is at this point, equal to 7c00h plus the length of the already compiled code--
Some code......
more code..
ditto
ditto--
Thus $ is obviously a larger value than 510, 510-$ will be negative, and you cannot repeat someting (times 510-$ db 0h)
a negative number of times.

To sum up, a more self explanatory piece of code would look something like--
$= 7c00h

Some code......
more code..
ditto
ditto

times 510-($-7c00h) db 0h
dw 0AA55h

23 Jun 2004, 01:34

crc

Joined: 21 Jun 2003
Posts: 637
Location: Penndel, PA [USA]

crc 23 Jun 2004, 10:53

Quote:

To sum up, a more self explanatory piece of code would look something like--
$= 7c00h

Some code......
more code..
ditto
ditto

times 510-($-7c00h) db 0h
dw 0AA55h

What about this:

Code:

org 7c00h
. . . . Some code . . . . 
times (7c00h+510)-$ db 0
db 0AA55h

It will fail to compile if the boot sector code is >510 bytes, but otherwise works perfectly.

_________________
Charles Childers, Programmer

23 Jun 2004, 10:53

neonz

Joined: 02 Aug 2003
Posts: 62
Location: Latvia

neonz 26 Jun 2004, 20:31

I'm using:

Code:

org 7C00h
. . . .  some code . . . .
rb 7C00h+512-2-$
dw 0AA55h

26 Jun 2004, 20:31

crc

Joined: 21 Jun 2003
Posts: 637
Location: Penndel, PA [USA]

crc 27 Jun 2004, 00:43

Quote:

I'm using:

org 7C00h
. . . . some code . . . .
rb 7C00h+512-2-$
dw 0AA55h

Why the 512-2? Wouldn't the following code also work?

Code:

    org 7C00h
    . . . .  some code . . . .
    rb 7C00h+510-$
    dw 0AA55h

_________________
Charles Childers, Programmer

27 Jun 2004, 00:43

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 27 Jun 2004, 06:58

Also, if you make the one more step to the simplification:

Code:

    org 7C00h
    . . . .  some code . . . .
    rb 7DFEh-$
    dw 0AA55h

27 Jun 2004, 06:58

neonz

Joined: 02 Aug 2003
Posts: 62
Location: Latvia

neonz 27 Jun 2004, 13:19

crc wrote:

Why the 512-2? Wouldn't the following code also work?

512-2 looks better IMHO Smile

Yes, theoretically it slows down compile time Wink

, but I like to have readable source code.

27 Jun 2004, 13:19

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