From: nowhere@nevernet.at   
      
   On 31/03/2022 08:37, muta...@gmail.com wrote:   
   > Currently I read the FAT boot sector which contains,   
   > among other things, the number of heads and the   
   > number of sectors per track.   
   >   
   > I use those values in the boot sector code to convert   
   > an LBA into CHS.   
   >   
   > Someone has reported the "University Challenge" disk,   
   > which is only 40 MB instead of the normal 1 GB, not   
   > working on Hyper V under Windows 11.   
   >   
   > The UC disk works fine on real hardware and qemu   
   > and Bochs.   
   >   
   > I haven't proven 100% but it is certainly getting to the   
   > boot sector and I think it has an issue in there, not   
   > loading the 3 sectors of the boot loader properly.   
   >   
   > I suspect the problem is that the 40 MB drive uses   
   > some "real" disk geometry values instead of the   
   > fake ones needed for large disks.   
   >   
   > Hunting around I found INT 13H AH=08H which looks   
   > like it will retrieve the number of heads and sectors   
   > per track actually being used by the BIOS, and I would   
   > have thought that was the definitive values, which   
   > should overwrite the values stored in the BPB of the   
   > boot sector.   
   >   
   > Is that the right approach?   
      
   the BIOS may be able to get correct geometry data   
   by using Identify-Device (you can do that as well).   
      
   CHS<->LBA conversions must be aware of faked head-   
   numbers,it were quite common to abuse heads>>tracks   
   to achieve a larger CHS range. IIRC std-limit=2GB.   
   24 bits addressing 512 byte sectors: 2^24*512=8GB   
      
   If the CHS value in the MBR is wrong then blame the   
   formatting tool. But perhaps you misinterpreted it.   
   __   
   wolfgang   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)