Question about multiboot

[Luca Abeni]

Hi all,

I have a question about the multiboot standard:
according to the standard, when a multiboot compliant kernel is loaded
in memory it can read the amount of upper memory from the multiboot
information structure, and the upper memory is assumed to start at 1
megabyte.
Now, how can the kernel understand which part of the upper memory is
free (part of that memory contains the kernel itself and the modules)?
Must the kernel computes the free memory by itself , or does the
multiboot standard provide some way to obtain that information?
In the first case, is the maximum between the _end symbol and all the
mod_end a safe assumption for the first free address in upper memory? 

                                Thanks in advance,
                                        Luca