[Gllug] Beagle Board

[Peter Corlett]

On 12 Jan 2009, at 17:48, Andrew Farnsworth wrote:
> In their defense, early PC Hardware did not include the necessary  
> circuitry to permit preemptive multitasking. [...]

Actually, it did.

To do pre-emptive multitasking, all you need is some means of  
generating a reasonably frequent and hopefully also regular interrupt.  
Ideally this comes from some sort of programmable timer, but if all  
else fails, zero-crossing of the PSU or some sort of timeout on an I/O  
device will have to do.

On interrupt, the CPU can then push registers onto the stack, move the  
stack pointer to a different location, and pop the registers to  
restore the state for that process. Hey presto, a pre-emptive task  
switch.

The great thing about /* you are not expected to understand this */ is  
that it's really quite simple.

Sure, you don't get protection between processes or virtual memory,  
and you may find you need to relink executables at launch time, but  
that's not *necessary* for pre-emptive multitasking to work. That's  
how the Amiga managed to pull off pre-emptive multitasking on a plain  
68000 CPU.


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