Panic on boot. How do I get a kernel dump.

All of the doc's I can find are for getting a kernel dump after it's booted
(i.e., set the dumpdev using dumpon, etc.). I have a panic during boot and
it says it won't dump b/c there's no dump device. I cannot find any doc's
anywhere that mention how to set the dump device before you boot (i.e., from
the loader, patch the kernel, whatever...). Does anyone know how to get an
early dump?

Thanx, tom