[syslinux] isolinux-debug: Approaching overflow

[H. Peter Anvin]

Gene Cumm wrote:
> 
> Examining isolinux-debug.map, .bss1 seems to be ending just above the
> 64kiB mark, mostly because .adv gets incremented up (0xB200 to
> 0xB400).
> 
> Am I correct in assuming that it would either require reworking the
> layout or adding another true segment to be able to handle this?
> Doing another true segment would probably require storing a handle in
> a .bss type section and initializing early.  A quick idea on the
> layout might be moving .adv to the top but unfortunately I don't
> understand enough about the thoughts surrounding the layout.
> 

Something like that... rebalancing the various .bss segments might be an
option.  I'm considering doing something off-segment for rarely used
data; in particular the font buffer is 8K in size which is a lot for
segment zero.  Furthermore, the ADV probably has to expand to 2x4K at
some point (because disk drive manufacturers are itching at changing to
4K sectors on disk) so it may need to move anyway.

The concern,of course, is that intersegment access is always painful and
error prone, and pushing the upper limit of the low memory area may
cause failures, especially for PXELINUX.  Now, to be fair, I have
reduced the low memory footprint *a lot*, so perhaps it's not such a big
deal; on the other hand, perhaps we can get away with a smaller cache.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.