Why is INT 0x13 AH=0x02 crashing?

[Isaac]

Hi. I'm trying to write a stage 1 boot loader. I setup all the registers to call int 0x13:

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mov bx,0x1000
mov es,bx
mov bx,0x0
mov dh,0
mov al,4
mov cl,16
mov ah,0x02
mov ch,1

Now I call int 0x13 and try to read stage 2 from the disk:

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int 0x13
jc .error
jmp $
.error:
mov bx,ERROR
call print_string
jmp $

But it fails. .error gets called and it prints an error message. Why?

[Octocontrabass]

How did you get the values that you're using to call int 0x13?

[Isaac]

The values I stored in those registers tell the BIOS where to read from and where to write to.

[Octocontrabass]

Well yes, I know that's what they're supposed to do.

I want to know how you chose those values specifically.

[iansjack]

You don't specify the drive number. It's probably trying to read a non-existant drive.

[Isaac]

ES:BX - I chose where to store the data based on a map that I saw. The map shows where everything is loaded into RAM. I just looked around for a big empty space.
DH,CH,CL - I chose these values based on the structure of an ISO 9660 filesystem. I'm trying to load the primary volume descriptor so that I can find out where the root directory is and where stage 2 is.
AH - This has to be 0x02.
AL - This is the size of the primary volume descriptor.

I can change some of these values if that's the problem.

The drive number is automatically stored in DL when the BIOS loads this code.

[Octocontrabass]

ISO 9660

Are you booting from a CD in no-emulation mode?

[Isaac]

Yes. Is that a problem?

Actually, I tested this code on both qemu and real hardware.

[Octocontrabass]

I think you should look at the El Torito specification (PDF). In particular, read section 5.3.

[Isaac]

I read it. But what are you trying to point out?

[Octocontrabass]

After the boot process has been initiated the INT 13 Extensions (functions 41-48) will access the CD using 800 byte sectors and the LBA address provided to INT 13 is an absolute sector number.

You must use int 0x13 ah=0x42 to read the disk.

[Isaac]

I tried that and it still fails. Is there any way I can know way it fails? Can I watch what goes on while I run the bootsector on qemu?

[Octocontrabass]

The function will return an error code if it fails. Display the error code on the screen, then look up what it means in RBIL or some other source.

[iansjack]

I tried that and it still fails. Is there any way I can know way it fails? Can I watch what goes on while I run the bootsector on qemu?

We have now determined that your original code is never going to work. So what code are you now using to do the Extended Read Sectors call? It's almost certain that your DAP isn't valid.

[Isaac]

My DAP:

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mov byte [bx],0x10
mov byte [bx+1],0x0
mov word [bx+2],4
mov dword [bx+4],0x9000
mov byte [bx+8],2
mov si,bx

BX contains the location of my DAP... Anything wrong with my DAP?