Squeezing out some 70 bytes out of the boot2 loader

[Adrian Penisoara]

Hi,

On Jan 31, 2008 8:58 PM, Eduardo Morras <nec556 at retena.com> wrote:

> At 21:39 20/01/2008, you wrote:
>
> >Hello
> >
> >   I am trying to hack in some symlink support into the
> [sys/boot/i386/]boot2
> >bootloader (for my project [1]) and I seem to fall short of about 69
> bytes:
> >
> >as  -o boot2.o boot2.s
> >ld -static -N --gc-sections -nostdlib -Ttext 0x2000 -o
> >boot2.out/build/obj/build/src/sys/boot/i386/boot2/../btx/lib/crt0.o
> >boot2.o sio.o
> >objcopy -S -O binary boot2.out boot2.bin
> >btxld -v -E 0x2000 -f bin -b
> >/build/obj/build/src/sys/boot/i386/boot2/../btx/btx/btx -l boot2.ldr  -o
> >boot2.ld -P 1 boot2.bin
> >kernel: ver=1.01 size=7b0 load=9000 entry=9010 map=16M pgctl=1:1
> >client: fmt=bin size=1581 text=0 data=0 bss=0 entry=0
> >output: fmt=bin size=1e45 text=114 data=1d31 org=0 entry=0
> >-69 bytes available
> >*** Error code 1
> >
> >   What can I do to get room for about 70-100 bytes for these changes to
> make
> >it into the bootloader ?
> >
> >[1] I'm trying to get support for /boot being mounted as a separate FS
> and
> >as such I would need to have a "self-pointing" symlink (e.g. "boot -> ."
> )
> >to easily mask the fact that the boot stuff is now right in the root of
> that
> >FS. Fortunately the FORTH loader does support symlinks and I do not get
> >problems with it. I know that I can use /boot.kernel as a workaround, but
> >that is not too elegant.
>
> How big is boot2? You can compress it with an arithmetic coder, and
> only put the decoder plus the compressed boot2. There are very tiny
> arith coders, in assm, with 100-120 bytes of code (even less).
>
>
boot2 has a fixed 7680 bytes size and it's made up of the boot2.bin (5505
bytes, stripped off boot2.o) and "cooked" with btxld(8). I see that the
resulting boot2[.ld] file must not grow over 7680 bytes. Currently
boot2.ldis at 7749 bytes...

I guess code compression should be hacked into [sys/boot/i386/btx/]btxldr
but, since I have an easy workaround for now (touching base with
/boot.config), for the moment I'd rather trust the mature and long used
present code. There will be time to get back to this later, unless grub
prevails or another 1st stage bootloader emerges... ;)

Thanks for the tips,
Adrian Penisoara
ROFUG / EnterpriseBSD