Re: [Qemu-devel] HD Image Size Limits and hdachs

[Fabrice Bellard <fabrice@bellard.org>]

Hi,

Happy new year to all :-)

Jason Brittain wrote:
> Jim C. Brown wrote:
> 
>> One thing is that the maximum number of heads is 256, not 16.
> 
> 
> That's a good catch, if true.  Thanks.  But, the doc (the HTML one from 
> CVS HEAD) is
> very specific about the range being from 1 to 16.  I believe you're 
> probably right
> and the doc is wrong.

No, the doc is correct. One must distinguish between the Physical CHS 
(with 16 heads max and this is what -hdachs specifies) and the Logical 
CHS (with 255 heads max) which is only used by the BIOS to increase the 
number of sectors that can be addressed. THe logical CHS is specified 
indirectly when you tell "lba" or "none" as forth argument to -hdachs.

Anyway, nowadays CHS is no longer used when accessing modern hard disks, 
so it is only a problem for old software using the BIOS to access the 
hard disk. So _never_ use -hdachs, except if you have to use an old DOS 
disk image coming from another emulator.

Fabrice.