[SeaBIOS] Running out of space in e/f-segments

Kevin O'Connor kevin at koconnor.net
Sat Aug 21 20:41:54 CEST 2010

SeaBIOS is coming close to exceeding 128K in size.  Right now, it's
approximately 107K on qemu with default options and 120K with
full debugging and all options enabled.

SeaBIOS currently contends with option roms for space.  Every byte
that SeaBIOS uses is one less byte available for option roms.  It's
technically possible for SeaBIOS to exceed 128K, but it's likely that
we'll start seeing option roms not fitting around that point.

It would also be nice to be able to do malloc_low allocations from the
e-segment.  This too requires freeing up space that SeaBIOS is
currently using to store code.

I've been thinking of a few different ways to free up space.  The most
promising approach seems to be to relocate all or part of SeaBIOS'
32bit "flat" code.  This 32bit code is larger than the segmented code
(67K vs 40K) and it's the area that has been growing the most.

To relocate the code, the build can store a list of code that depends
on a fixed address and then at runtime SeaBIOS can modify the code
with the new address.

Some different permutations of this idea that I've come up with are:

1 - On boot, have SeaBIOS relocate all of it's 32bit code to permanent
    high ram.

    CONS: Reserves memory that then can't be used by the OS (67K

    CONS: SeaBIOS code is still limited to 256K size (c+d+e+f

2 - Have the build separate out the POST code from other 32bit code
    (eg, boot & resume) and store the POST code in a separate
    CBFS/fw_cfg "file".  During boot SeaBIOS would extract the
    separate POST code blob into temporary high ram and run it.
    Currently, the POST code is the bulk of the 32bit "flat" code (57K
    vs 10K).

    CONS: It adds complexity during deployment as both the main
    SeaBIOS blob and the CBFS/fw_cfg POST blob would need to be

3 - On boot, relocate only POST code to temporary high ram, and have
    SeaBIOS reset the machine if the POST code is ever rerun.

    CONS: Requires a reliable way of resetting the machine.

    CONS: SeaBIOS code is still limited to 256K size (c+d+e+f

Any thoughts?  I kinda prefer option 3.  Resetting the machine on a
rerun of POST would be a good thing to do anyway, as trying to re-post
the machine has never really worked well.  It does, however, make the
build a bit more complex.


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