Re: [Etherboot-developers] Q: does xstart work?

[Eric W B. <ebi...@ln...>]

ke...@us... (Ken Yap) writes:

> >xstart16 still needs a little work to get the bootp data structure below
> >1MB.  But othewise it should work fine.
> >
> >The open issues are:
> >- How to copy/move bootp low for 16bit code.
> 
> Also the segment descriptor array.

Hmm. I don't quite follow you there.  I only need 6 bytes for the
global descriptor pointer, and then I can use a global descriptor
table > 1MB.  That part is working fine.
 
> Maybe this ties in with the memory allocator routines. Ask it for a
> chunk of memory below 1MB and memcpy the bootp and segment descriptor
> structures before calling xstart. You could also ask the memory
> allocator for a chunk for use as stack.

Probably the fun part is keeping track of which areas have been allocated
to the partially loaded image.
 
> I'm not so concerned about the text segment size of Etherboot when
> Etherboot is converted to run high. In the current version the main
> concern is the 48kB limitation. I think there are few machines that
> can't spare a couple of hundred kB from the top of memory during the
> image loading phase.  One is not supposed to load memory to brim anyway,
> Linux will need room to expand when it gains control.

Except when you have a ramdisk.  But a sane implemenation will load
the ramdisk low, and then push it up higher if required.  Though I seem
to like the insane varieties.

I agree the uncompressed size is not a big deal.  The compressed size
continues to concern me.  Even with LinuxBIOS I am loading it in
a rom chip.  A much bigger one than etherboot traditionally lives in
true, but still a rom.   And the primary reason I am working with
etherboot is that it is small, and my other alternatives are not.  So
I don't want to mess that up. 

As far as stack size.  I really only care there except memory < 1MB is
a limited resource in high demand.  What clicked for me a little while 
ago is that I don't have to keep anything permantnely below 1MB.  I
can just push all of my code on the real mode stack.  And I worry a
little if I am using someone else's stack that I will overflow it.
The code is tight enough now that overflowing stacks is no longer an
issue.  

Though it was quite fun reducing the footprint from 300 bytes to 80 :)

Mostly I did it to combat my small observation that whenever I touch
etherboot in a nontrivial the core gets bigger.  

O.k. I am so tired I am rambling.  After I get some sleep I will check
do a little cleanup and see what parts of my code I can check in.

Eric