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Question on 'map' and 'hide'

From: cr
Subject: Question on 'map' and 'hide'
Date: Fri, 26 Sep 2003 07:01:20 +1200

Not exactly a bug, more like a few points that aren't quite clear in the 

(Incidentally, I did hit a nasty bug - I was trying to boot Win95 on /hdc1 by 
using the 'map' and 'hide' technique, and either Grub or Win95 went and 
screwed with my Linux partition on /hda5 - changed it to type Amoeba and 
lost the end of it where it adjoined my /hda6 /swap  partition - I see 
that's been reported already on this list.Fortunately  Linux   gpart   
(Guess Partition) rescued me.    I now have my DOS/Windoze partitions on 
/hda where they think they belong.  It seems safer.  ;)

Anyway, what the manual doesn't make quite clear is some of the 

map (hd0) (hd1)
map (hd1) (hd0)
swaps over  /hda  and  /hdb

How does it do this?   Write to the BIOS?    And does the mapping 
survive a reboot?   If so, how does one cancel it when wanting to 
boot straight into  /hda ?

I gather, though it may fool DOS/Windows,  Grub itself takes no notice of 
the mapping, since 

map (hd0) (hd1)
map (hd1) (hd0)
rootnoverify (hd1)    still points at /hda.

Does 'hide' take notice of the mapping?   That is, if I want to hide /hdb1
after I've swapped the drives with map, do I do

map (hd0) (hd1)
map (hd1) (hd0)
hide (hd0,0)       (i.e. as re-mapped)


map (hd0) (hd1)
map (hd1) (hd0)
hide (hd1,0)      (i.e. the actual drive)

I gather 'hide' toggles a bit in the partition's boot record.    This 
presumably survives a reboot.  Does Linux etc take notice of this bit - 
i.e., DOS/Win can't 'see' a hidden partition, but can Linux?

One final point that the manual doesn't state - I gather that (like DOS, but 
unlike Linux)  Grub ignores non-used drive 'slots' in its numbering - that 
is, if I have, say, a CD-ROM in /hdb, Grub numbers drives as follows:

/hda     Disk 1       (hd0)
/hdb     CD-ROM     --
/hdc     Disk 2       (hd1)
/hdd     Disk 3       (hd2)

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