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Re: [Bug-grub] Using grub for booting imaged machines

From: OKUJI Yoshinori
Subject: Re: [Bug-grub] Using grub for booting imaged machines
Date: Thu, 05 Oct 2000 22:31:40 +0900

From: Alessandro Rubini <address@hidden>
Subject: Re: [Bug-grub] Using grub for booting imaged machines
Date: Thu, 5 Oct 2000 14:16:56 +0200

> There has been no reply to this question. While I offered to document
> stage-1.5, I can't find any information about it, either. Can someone
> please shed some light to help me getting started?

  Quoted from the original document by Erich Boleyn:

OPTIONAL Stage 1.5

Since the Stage 2 is so large (typically 30K or larger in size), a
special stripped down version of the Stage 2 code, sort of a Stage
1.5, can be used because it might fit into one of the fixed areas of
the disk. The idea is to have the Stage 1.5 look for a certain file
and attempt to execute it as a Stage 2 (this allows changing Stage 2
easily, while leaving it a normal file in the filesystem).

This is actually the case for the BSD FFS filesystem. There is a fixed
area 7K in size at the beginning of the partition which is reserved
for the bootloader. The Stage 1.5 with BSD FFS filesystem support is
about 6.5 K in size when compiled in a.out format, and fits nicely,
allowing the Stage 2 to be a normal file in the "/boot/grub" directory
of the install partition.

This same technique can in principle be applied to other filesystems,
but the problem here is that the fixed area available to bootloaders
is generally quite small (0.5 to 3 K), and a Stage 1.5 wouldn't fit.

It contains embedded data on where to look for the install partition
and the Stage 2 (it uses what would be the "configuration file" entry
in the Stage 2 for this purpose).


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