The case in cpu_switch() where there isn't a higher priority thread
(choosethread() == curthread) uses r4 as the PCB context pointer. However, the
use of r4 after the label L2 is incorrect, since it was probably trashed by
the call to choosethread, and in any case was set up to curthread at the start
of the routine.
This condition will occur when an interrupt thread schedules a netisr, which
is a lower priority thread.
Another (probably unnecessary) difference is that I was paranoid about
register trashing, so I decided to save r2 and r13 as well.
Submitted by: Peter Grehan <peterg@ptree32.com.au>
to working but still needs some work to properly switch the full context
(such as saving the fpu registers, switch stacks, etc.). Also, remove some
dead code that was mixed in.
