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Re: Memory Map information: GRUB2 vs. Legacy GRUB (GRUB1)

From: Vladimir 'phcoder' Serbinenko
Subject: Re: Memory Map information: GRUB2 vs. Legacy GRUB (GRUB1)
Date: Fri, 14 Aug 2009 17:21:54 +0200

>> >  2) The Memory Map information under GRUB2 seems to be "zeroed" which
>> > leads me to believe that there must be something different in the way
>> > Legacy GRUB and GRUB2 handle Data and BSS segments (ie: initializing
>> > data by zeroing out).
>> The most common reason is the OS zeroing it out. Legacy put the memory
>> map in lowmem. GRUB2 puts it just after the kernel image. Some kernels
>> may overwrite it during an initialisation phase.
> That will explain it.  If GRUB2 puts the memory map just after the
> kernel image then it means that the OS cannot assume that the memory
> range immediately after the kernel image is free.  In fact, the
> multiboot OS "must" use the provided memory map to decide what ranges of
> memory are available for "dynamic memory" and which of it is free.  I
> mistakenly thought that the memory map will always be in low memory and
> only assume that the first 1 MB of physical memory should be left alone
> at boot time to preserve the information.
> BTW, why the difference?  Was putting multiboot information (eg: memory
> map) in low memory clobbering BIOS information or the VGA/EGA/CGA memory
> mapped areas?  Just curious.
Multiboot specification never promised anything about where mbi is
stored. Read it here: Don't assume
anythin not explicitly stated in this document
> Thanks for clearing that up!
> Peace,
> Frank
>> >
>> > I am using GRUB2 version 1.96 from the Fedora RPM repository; hence, I
>> > wanted to know if a later SVN version has a fix or is there a change in
>> > the specifications.
>> Specifications are the same but we fixed a lot of bugs.
>> >
>> > Peace,
>> > Frank
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Vladimir 'phcoder' Serbinenko

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