Why is INT 0x13 AH=0x02 crashing?

[Octocontrabass]

Anything wrong with my DAP?

That depends. Where's the code to fill in the rest of it?

[iansjack]

Anything wrong with my DAP?

Yes. Bytes 9-15 of it are undefined.

[Isaac]

I followed what they say on Wikipedia. Look: http://en.wikipedia.org/wiki/INT_13H#IN ... From_Drive
The table shows that the last field in the DAP goes from 0x8-0xF. I filled in that field. Even though I didn't use up ALL the space, isn't it okay?

[Octocontrabass]

Even though I didn't use up ALL the space, isn't it okay?

You're missing some fundamental concepts of assembly programming.

If I do this:

Code: Select all

mov al, 5
mov [bx], al
mov ax, [bx]

What value will be stored to AX?

[iansjack]

Suppose that before you fill those 8 bytes they contain:

0xd 0x4 0x0 0x5 0x8 0x3 0x0 0x1

Now you write 0x2 to the first byte. What number does that 8-bit field (that says what sector you want to start reading at) now contain? And what are the chances that there is a sector of that number on your disk?

You have an uninitialized variable; that's one of the commonest bugs in programming. You need to write all 8 bytes of the number not just the least significant one. With a little simple debugging you would catch errors like this immediately.

[Isaac]

I fixed up my DAP:

Code: Select all

mov byte [bx],0x10
mov byte [bx+1],0x0
mov word [bx+2],4
mov dword [bx+4],0x9000
mov dword [bx+8],2
mov dword [bx+12],0

But it still prints an error message. Why is that?

[iansjack]

I don't know. What error code is the BIOS call returning?

[Isaac]

I don't really know much about error codes. But the number returned in AH is 0x1.

[iansjack]

http://en.wikipedia.org/wiki/INT_13H#IN ... From_Drive

Int 0x13, ah=1.

[Octocontrabass]

01h: invalid function in AH or invalid parameter

It sounds like there's still something wrong with the values being passed to int 0x13.

[iansjack]

Or, possibly, the BIOS doesn't support the call. Less likely but it should be checked first.

[Isaac]

The BIOS supports the call. I just can't figure out what's wrong with my DAP.

[thepowersgang]

Could you please post the code that invokes the system call (not just the code that populates the DAP). Preferably in the form that you're compiling (not just cut+pasted from separate sections of code).

[Octocontrabass]

I just can't figure out what's wrong with my DAP.

The DAP is not the only thing you need to check. Get in a debugger and make sure all of the values you're passing to int 0x13 are correct.

[iansjack]

The BIOS supports the call. I just can't figure out what's wrong with my DAP.

How do you know? You haven't shown any code that tests this.

The DAP business is easy - use a debugger to inspect it just before you make the BIOS call. Is it what you expect it to be? And are your expectations correct?