Can't use 16GB RAM in G5

[Justin Hibbits]

Just bought some fancy 12GB RAM for my G5 (late 2005, DC 2.3GHz), to
bring the total to 16GB.  But, if I max it out, the boot hangs after:

KDB: current backend: ddb

Sits there, then the fans start spinning up.  Removing the outer two
sticks makes it boot completely, but then I only have 12GB.  I know all
sticks work, and that the outer two slots work, from testing for a bad
stick earlier.

Any known issues with that much memory?  Has anyone tried it on ppc?
Methinks there's something involved with the pmap code, but I know
nothing about that code.

- Justin