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Re: Drivemapping Questions

From: Jake Thomas
Subject: Re: Drivemapping Questions
Date: Sat, 26 May 2012 15:59:08 -0700

> Hi! I've been meaning to ask these questions concerning GRUB2 and 
> drivemappings for a while.
> It's my understanding that the BIOS (and maybe EFI does this too?) scans for 
> hard drives and makes a drivemapping for each in memory.
> When one gets to the Grub command line and runs "ls", those things it lists 
> (like (hd0), (hd0,msdos1), (hd1), (hd1,gpt1), etc.), are those the 
> drivemappings? Or are only the "drive only" ones drivemappings (ones for the 
> whole drive, no partition specified)?
> If the BIOS (or EFI?) doesn't detect a hard drive or for some reason doesn't 
> make a drivemapping for it, can Grub detect the hard drive, period? If it 
> can, can it step in and make the drivemapping?
> As I was beginning to touch on earlier, are drivemappings typically made for 
> each partition, or only for whole drives? Does the BIOS make drivemappings 
> for every partition, provided it knows the partitioning scheme used?
> I know that Grub has a part_msdos module and a part_gpt module. This leads me 
> to believe Grub is detecting partitions itself, thus drivemappings given by 
> the BIOS are whole-disk only? Does Grub go in and make a drivemapping for 
> each partition once it detects them, or does it just know for itself where 
> they are?
> I know that Grub4DOS allows the user to actually create new drivemappings. 
> For example, you could give a partition its own "whole disk" drivemapping, 
> such as "(hd32)". You can even give an iso (optical disc image file) its own 
> "whole-disk" drivemapping, like "(hd32)" or something. The image must be 
> continuous on the disk, of course. These custom drivemappings can then be 
> chainloaded.
> Does Grub2 allow for custom drivemapping creation?
> I'm thinking about FreeDOS. I can make a partition on a GPT schemed disk, and 
> this one partition be like an entire MBR schemed hard drive. What's 
> interesting is that the GPT specification actually includes such a stunt with 
> the partition type code EF01. That's right. An EF01 partition in a GPT 
> schemed hard drive is like an entire MBR schemed hard drive. It's kind of 
> like GPT's version of an extended partition, in that you have partitions 
> within a partition.
> If I installed FreeDOS to a hard drive image, and blew this whole image onto 
> just a partition within a GPT scheme, could I give this one partition its own 
> "whole disk" drivemapping, like "(hd32)" or something, and chainload it as a 
> whole disk?
> I know I could just chainload the partition without giving it a whole disk 
> drivemapping, but DOS needs to have that drivemapping in memory to read and 
> write to its "hard drive", which is really only a partition. It's not 
> GPT-aware, but it could see and use the MBR-schemed partition as a whole, 
> separate hard drive if the partition has it's own whole disk drivemapping.
> If the BIOS makes drivemappings for each partition, then I would also need to 
> make drivemappings for each partition in the EF01 partition for DOS to work 
> out of it, because that'd be what DOS expects. If not, then just the whole 
> disk mapping for the partition would be good, because DOS wouldn't have been 
> made dependant on something that usually doesn't exist, that something being 
> drivemappings for each partition. (I.e., would I need to then make something 
> like "(hd32,msdos1)"?) If DOS expects each partition within the hard drive to 
> have a drivemapping, I would need to make drivemappings for each partition 
> within the EF01 partition, whereas if DOS parsed the partition table for 
> itself or had some interrupt handle partitions in a non-drivemapping fashion, 
> or something like that, I wouldn't need to have drivemappings for each 
> sub-EF01 partition, and just the drivemapping for the whole EF01 partition as 
> a whole disk would suffice.
> Also, how is GRUB's freedos command used?
> I know it's easy to boot a floppy image with DOS on it using memdisk, but 
> it'd be nice to have DOS on an actual partition, for several reasons. One, 
> saving stuff back to disk from within DOS. With memdisk, everything is done 
> to a copy of the image in RAM, and then lost on shutdown. Two, if it was on 
> its own partition, I could put stuff on and take stuff off DOS's partition, 
> from another OS, without having to mount an image. Also, if there's a bunch 
> of stuff I want to use with DOS, I could avoid having all that RAM used up 
> (with memdisk, all those tools would have to be in an image and copied to 
> RAM).
> Cheers,
> Jake

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