Weired first sector

stones3000 · Post by **stones3000** » Sun Sep 02, 2007 11:02 pm

I was doing a checksum of all sectors read from a virtual disk using qemu. I used the MINDRVR driver sample code from http://www.ata-atapi.com/products.htm.

I initialised the virtual disk with all '0's (charater 0). when I read first sector, there is a 'P' character appeared in it, located at the 279th byte. I tried both DMA and PIO, both has 'P' in the first sector. I also wrote a very simple PIO driver myself and it doesn't have the 'P' appeared when reading the first sector.

I can't understand why. Can anyone help? Is it something wrong with the driver or qemu?

Thanks!!!

Avarok · Post by **Avarok** » Mon Sep 03, 2007 2:38 am

It's a bad idea to test for a single byte located in the first sector. If you hand program by PIO to do something, you're using different instructions than the bios method or whatnot. One of those instructions may be the 'P' you're looking at, since all ascii characters translate to numbers, as does machine code. The first sector is also your bootloader. The code there may or may not be shifting your sentinel out of place.

My favorite sentinel has always been 0xbabe, 'cause it makes me cooler than 0xaa55.

Seriously though, if that's not the problem, and indeed it's not finding your sentinel I'd guess your PIO driver is broken.

There's almost never something wrong with Qemu at this point.

Combuster · Post by **Combuster** » Mon Sep 03, 2007 3:06 am

Avarok wrote:My favorite sentinel has always been 0xbabe, 'cause it makes me cooler than 0xaa55.

It does make some bioses think you are a femme fatale instead of a coding guru and consequently refuse to boot for that reason

JamesM · Post by **JamesM** » Mon Sep 03, 2007 3:51 am

my favourite sentinel is 0xdeadbaba. Dead baby jokes ftw

stones3000 · Post by **stones3000** » Tue Sep 04, 2007 11:30 pm

So, I should start to read from the second sector on disks for checksum.

What really happens when reading the first sector (the MBR)? Why only the MBR gives me the weired "p" character, even though I memset the whole sector to '0'?

Combuster · Post by **Combuster** » Wed Sep 05, 2007 8:10 am

Seriously though, if that's not the problem, and indeed it's not finding your sentinel I'd guess your PIO driver is broken.

What really happens when reading the first sector (the MBR)? Why only the MBR gives me the weired "p" character, even though I memset the whole sector to '0'?

We have nothing to diagnose your problem, we have barely anything to base guesses on. We need to see code.

stones3000 · Post by **stones3000** » Wed Sep 05, 2007 4:34 pm

Combuster wrote: We have nothing to diagnose your problem, we have barely anything to base guesses on. We need to see code.

I'm just wandering if anyone has the same problem with me. My code is too lengthy to read. Let's see what I can do myself. Thanks for your help