Text 5828, 66 rader
Skriven 2005-03-25 18:51:00 av MARTIN ATKINS (1:123/140)
     Kommentar till en text av JEFF GUERDAT
Ärende: Root Directory
======================
-=> JEFF GUERDAT wrote to MARTIN ATKINS <=-

JG> On 03-22-05, MARTIN ATKINS said to JAMES BRADLEY:

MA>So it's a 10 gig disk. But you should (i say must but some disagree)
MA>have your root partition in the first 1024 cyls so.

MA>1024*255*63*512 = 8,422,686,700 bytes

MA>Your root directory needs to reside below 8 gig.

JG> I assume you're referring to working with older BIOSes that don't
JG> support booting from >8GB partitions (1024 cylinders), right?  

Yes and no. ;) I think the BIOS only address 1024 cylinders but
each cylinder can address another cylinder via the operating system.
So 1024 can address the next 1024 so you can have 2048 and that
can address another 2048 so you get 4096 and so on. The BIOS can
only understand complete disks so any number that falls over
the 1024 to the power of two is just a part of the next group
of cylinders. If you put the root directory outside the 1024
range the BIOS may (will?) fail to see it. Lets say you have
a disk of 9729 cylinders and use the cylinder*head*sector*512
rule to calculate the disk 

Disk /dev/hda: 
255 heads, 63 sectors/track, 9729 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

9729*255*63*512 = 80,023,746

So the disk is 80gig. Using the 1024 to the power of two principle
you get 8192 cylinders that the BIOS can understand as reported 
by the MBR.

8192*255*63**512 = 67,381,493 or 67gig.

This is where your root directories should (must?) reside.

JG> Newer BIOSes should work regardless of the OS (I've run into this with
JG> Windows many a time on older machines but not new ones)...

You may be right about newer BIOSes but the OS is not the problem
in this instance. Say you have an old WIN95 on a 10gig disk
and you are forced to make 2gig partitions.

Disk /dev/hdb:
255 heads, 63 sectors/track, 1216 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

1216*255*63*512 10,001,940

Win 95 is only using 8gig so there is 2gig spare. 

It is not in my view safe to use that 2gig as a root partition for Linux 
because only 1024 can be directly addressed by the BIOS.

1024*255*63*512 = 8,422,686,700

I am willing to take criticism on this but it is better to delete the
last Win partition and and make the last 4gig Linux or any other OS.

L8r. :)
 
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