> Why limit it to two additional names?  Why not accept a (more or less)
> arbitrary number of them, from zero upwards?

In my opinion, the PowerPC architecture needs three levels right now, with
perhaps more in the future:

	machine:	prep, ofppc, macppc, amigappc, mpc8xx, mpc82xx
	
	sub-arch:	60x, 40x/8xx

	arch:		powerpc

Files in the sub-arch would include at least pmap.* and pte.*, which need to
vary dramatically between PowerPC families.  For instance, the Motorola
MPC8xx family uses a completely different MMU from the 60x devices, but many
other parts of the architecture are identical.

While these variations could be handled with #ifdefs, I really believe that
when entire subsystems are involved, the method of configuration should be
by inclusion/exclusion of entire source files.

Overall, my preference would be for a somewhat richer structure in sys/arch
to reduce the amount of duplicated and conditional machine-dependent code.

Regards,

David Querbach
Real-Time Systems Inc.