V9/sys/conf.old/config_diff

obvious differences between old and new config

you can run config with working directory /usr/sys/sysname;
you don't have to be in sys/conf.
if you're in sys/sysname, you don't even have to give the name explicitly.

config doesn't remind you to make depend any more; it just does it for you.

a few files have moved around:
machine configurations are in sysname/conf, not conf/sysname.
dev/conf.c is no more.  config makes a conf.c for each machine.
because conf.c (hence the device switches) are generated by a program,
several kernel files have changed a little to make names regular:
dev/kdi.c kmc.c
sys/neta.c sysent.c sys3.c nami.c mp.c

the inet sources have also been changed to separate tcp from ip (insofar as
that's possible)

there's a new file, /usr/sys/conf/devices, which lists all the device drivers
(and line disciplines and filesystem handlers).  it is used to generate conf.c.