Conserving space?

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rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 12 Oct 2006, 04:59

Hey! I was just working on another small OS for fun, and I noticed something:

If I have my 16-bit startup code, then my GDT between that and my 32-bit code, it results in a smaller size than if I had my GDT at the bottom.

Here's an better depiction:

Code:

ORG 7C00h

START16:
...
   jmp 08h:START32

GDT:
...

START32:
...

INFINITE_LOOP:
   jmp INFINITE_LOOP
TIMES 510-($-$$) db 0
dw AA55

Comes out smaller than the following:

Code:

ORG 7C00h

START16:
...
   jmp 08h:START32

START32:
...

INFINITE_LOOP:
   jmp INFINITE_LOOP

GDT:
...

TIMES 510-($-$$) db 0
dw AA55

Does anyone know why?

P.S. - When I say GDT, I mean the GDT itself and the descriptor.

12 Oct 2006, 04:59

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 13 Oct 2006, 22:13

If anyone needs more real code as opposed to pseudocode to understand what I mean, I can show you the code from my TinyOS where I learned of this happening.

13 Oct 2006, 22:13

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 14 Oct 2006, 20:32

Does anyone know why it may be doing this? I mean, if the GDT is like it is in the second bit of code, it gets above 512, and I have to start subtracting zeros from the TIMES 510-($-$$) db 0, but if it's like it is in the first code clip, it doesn't take up nearly as much space.

14 Oct 2006, 20:32

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 15 Oct 2006, 02:07

I just started putting the IDT into my OS, and it's the same idea, that as long as the GDT and IDT are in between the 16-bit and 32-bit code, it stays small.

15 Oct 2006, 02:07

Goplat

Joined: 15 Sep 2006
Posts: 181

Goplat 15 Oct 2006, 02:07

I can't think of any reason, but the easiest way to find out would probably be to just look at the two files side by side in a hex editor.

15 Oct 2006, 02:07

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20513
Location: In your JS exploiting you and your system

revolution 15 Oct 2006, 02:20

We can't help you if you don't post your code.

15 Oct 2006, 02:20

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 15 Oct 2006, 02:52

Okay... Now I'm embarrassed... I tried it again, and it stayed small... The stupid thing I did was I put the GDT after the times 510 and end of bootsector signature when I did it before... Sorry!

15 Oct 2006, 02:52

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