Extended BIOS translation (worth disabling?)

[Mike Isely]

On Thu, 5 Feb 1998, Peter Holzer wrote:

> On Wed, Feb 04, 1998 at 05:50:26PM -0600, Rob Browning wrote:
> 
> > I currently have translation enabled, which, as I understand it, means
> > that the drives appear to have a different geometry than they actually
> > do so that DOS can access drives > 1GB.
> 
> No. You always have some translation enabled, and the BIOS never sees
> the real geometry of the drive. The difference between "normal"
> translation and "extended" translation is that the first uses 32 sectors
> and 64 heads, while the second uses 63 sectors and 255 heads. Since the
> BIOS can only handle 1024 cylinders this gives you a maximum disk size
> of 1GB and slightly under 8GB respectively. 
> 
> >   1) Is there any benefit (performance or other) from the unix
> >      perspective to turning the option off?
> 
> No.

Yes.  With translation off, you get finer granualarity of control in
fdisk.  You can partition down to 1MB granualarity instead of about 7MB. 
However, that really isn't very important, and if you're running fdisk
under DOS, it will probably break anyway because it won't be able to see
the whole disk... 

> 
> >   2) If I do turn it off, will I still be able to use the drives from
> >      Win95, as long as I keep the DOS partitions under the 1024K
> >      boundary on each drive?
> 
> I think so (haven't tried it, though). If Win95 uses its own drivers
> instead of the BIOS, you would also be able to access partitions beyond
> the 1024 cylinder boundary, you just cannot boot from them.
> 

At least not without some booting contortions.  For the same reason, you
may still have some problems under Linux.  The problem isn't Linux (or
Win95), it's the PC BIOS and it affects every OS at boot time.  You still
have to boot the system, and any boot loader you use has to go through the
system's BIOS - inherenting all of its limitations - in order to do its
work.  This affects LILO, or whatever boot mechanism you choose for Linux. 
You can get around this problem by creating a small partition that sits
entirely below the 1GB boundary, then boot your kernel from there.  Once
the kernel starts up, you then stop relying on the BIOS and the 1GB limit
goes away. The entire kernel image and everything else that LILO needs to
touch must be in that filesystem.  You can probably do a similar stunt for
Win95 by ensuring that the boot (C:) drive with Win95 installed on it
either isn't SCSI or is at the front of the disk below 1GB.  Of course, if
Win95's own drivers still have this limitation (I really don't know), then
nothing will help. 


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