I wrote a set of patches out of concern that even if you compile a 386 
kernel a lot of code irrelevent to legacy machines still remains. Things 
like the Pentium TSC register, DMI information, ESCD parsing, and the 
use of CPUID do not apply to these machines, but looking at System.map 
you can see they're still there.

Already with these patches I can compile a zImage kernel that is 450kb 
large (890kb decompressed) with a small initramfs payload, floppy and 
kernel module support, FPU emulation, that can successfully boot on an 
ancient 386 laptop with only 1MB of extended memory. Eventually what I'd 
like to have is the ability to compile a pure 386 kernel with all 
non-386 functions removed (and perhaps the same for 486 machines).

These patches were written against the vanilla 2.6.21.1 kernel. They 
will have no effect UNLESS you make menuconfig and explicitly enable 
them there.