> I'm curious about SMP:
> 
> Why the heck doesn't this smash all of the device drivers which assume that
> they will have complete control of the system without having something else
> run at the same time except when they permit it (almost all of them?) to
> itty bitty pieces?
> 
> Surely the other CPU's don't block while a device driver is called!

No, but all tasks that access that device go through that device driver. 
Since the device driver's used to having multiple requests come in from
various different threads anyway (hey, the OS will swap out of a thread
that's in the middle of a device driver just as easily as anything else. 
That's what "re-entrant" means.), the device driver already knows
how to deal with that.  Either it has a mutex semaphore controlling access
to it, or it has a buffer and has its own thread for actual access to the
hardware.

Basically TRUE multitasking and SMP have approximately the same
considerations.  Between any two cpu instructions the OS could swap out of
your process and swap back in, so if you want to protect your sequence from
other processes, you have to use semaphores.

Rob
 
--- Xblat