Lost in offsets (int 13h)

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DustWolf

Joined: 26 Jan 2006
Posts: 373
Location: Ljubljana, Slovenia

DustWolf 23 Jun 2008, 22:21

Hello,

I have written a custom MBR and am using it as a 'floppy' in VMWare.

All the code is longer than 512 bytes, so what I have done is locate code inside the first 512 bytes at 7C00h that uses INT 13h to load up sector 2 (which is the next 512 bytes basically) into 7E00h and then jumps to it.

All my code is in one FASM file and I am using:

Code:

times   (7E00h-$)       db ?

...to pad the first half of the code so that when INT 13h loads up the data, it is exactly where it should be if all of the data was loaded up in one go.

Unfortunately something appears to be wrong here. When I boot up my program, the first half exectues without a problem, INT 13h succeeds, the second half of the program also does exactly what it should, but my data pointers for data in the lower half are all wrong.

I assume what has happened is that the padding added more data and there is a misalignment. Am I padding my code wrong?

23 Jun 2008, 22:21

edfed

Joined: 20 Feb 2006
Posts: 4353
Location: Now

edfed 24 Jun 2008, 05:22

yes, you pad your code wrong.
see on some examples like mines to know how to do.

24 Jun 2008, 05:22

DustWolf

Joined: 26 Jan 2006
Posts: 373
Location: Ljubljana, Slovenia

DustWolf 24 Jun 2008, 17:33

Care to provide a handy link? I'm not sure what to look for when you put it like that.

24 Jun 2008, 17:33

TNick

Joined: 29 Jan 2008
Posts: 13
Location: Brasov, Romania

TNick 24 Jun 2008, 17:51

Well... I really do not know what link should I post... maybe this one, but I guess you already know about this. Smile

The problem is that:
- at the start of your file, you should place a

Code:

org 0x7c00

wich will inform the assembler that, even if, in file, you are at offset 0, it should treat the code as being placed at that address.
At this point $ is equal with 0x7c00. As you write code, $ will increase. At the end of what you need to be placed in first sector, you may add something like this:

Code:

if ($ - 0x7c00) > 510 ; if we are above allowed size
 display 'Too much data in Sector 1!!!',13,10; this will alert you
else if ($ - 0x7c00) < 510 ; if we are below that size
  db (510 - ($ - 0x7c00)) dup 1 ; fill the space with 1 ...  or 0 
end if
dd 0x55,0xAA ; and place a signature
Sector2:

After this, you may place data and code that you want in sector 2 ... and 3

... and, if you want to know how manny sectors you need to read, place this at the very end:

Code:

if (512 - (($ - Sector2) and 0x1FF)) > 0
   db (512 - (($ - Sector2) and 0x1FF)) ; fill up to make a full sector
end if
SctToRead = ($ - Sector2) / 0x200 ; or 512 = the size of one sector

HTH

Nick

24 Jun 2008, 17:51

DustWolf

Joined: 26 Jan 2006
Posts: 373
Location: Ljubljana, Slovenia

DustWolf 24 Jun 2008, 23:26

Using this code I get the exact same problem as with my code.

Must be something stupid... here is my code:

Code:

     mov     cx,2            ;sector=2 cylinder=0
        mov     dh,0            ;head=0
     mov     dl,0            ;drive=floppy A (80h=hdd)
   mov     es,cx           ;segment 0

All the code runs fine but in the lower half, si seems to point at offset 0x200 instead of 0x219 (I used a hex editor on binary and compared to the display on screen). If I put my lower half data (bootMsg) with the other data higher up, it works fine.

EDIT: Nevermind! I can't believe I spent two days trying to figure that out! XP

So optimizing code before you finnish it is bad...

ps.: for the record this code works fine:

Code:

        times   (7DFEh-$)       db ?
                db      55h,0AAh

24 Jun 2008, 23:26

edfed

Joined: 20 Feb 2006
Posts: 4353
Location: Now

edfed 26 Jun 2008, 19:40

there are problems:

you don't write the right values.

first:

Code:

mov cx,2
mov dx,0
mov bx,0
mov ax,segment
mov es,ax
mov al,sectorcount
mov ah,diskoperation
int 13h

and second:

Code:

free =  510-(padding-$$)
padding rb free
        dw 0aa55h

this code uses a label to calculate the amount of byte to pad.

this is very usefull in case of modularised boot sector.

you can participate in one of my multiple threads on the subject if you want.
my current researchs on OO disk operations
bootloader created with the help of many coders
this one is exactlly like the current thread
this one is about how to boot, and switch to Protected mode
and this one is about an hardcore project

26 Jun 2008, 19:40

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