(Many thanks to Philip Hands <p h i l @ h a n d s . c o m> and
Avery Pennarun <a p e n w a r r @ w o r l d v i s i o n s . c a> for this explanation)
Tecras and other notebooks, and some PCs have a problem where they
fail to flush the cache when switching on the a20 gate (IIRC), which
is provoked by bzImage kernels, but not by zimage kernels.
bzImage files are actually "big zImage" not "bzipped Image". bzImage
kernels can be as large as you like, but because they need to decompress
into extended memory, they aggravate this problem. zImage kernels just
compress into conventional memory, so they never need to touch the a20 gate,
but they hit the 640k limit.
This problem is *not* present in late 2.1.* and later (2.2 when
available) kernels, bzImage kernel work just fine (the necessary code
has been added, see the .S files in the kernel sources).
For older kernels there are two solutions that I know of:
1) apply a patch, which flushes the cache. Unfortunately this
causes other machines to crash so is not universally applicable
(hence the tecra disks being segragated from the mainstream)
2) build a zimage, rather than bzimage kernel. This seems to get
round the problem. use the --zimage option to make-kpkg, or even
set this as the default in /etc/kernel-pkg.conf.