Wednesday, April 29, 2015

Some more progress: Simple PPC multitasking.

Now that we can execute PPC code from the 68K side, the next step is to execute 68K code from the PPC side. In WarpOS, this is done with the Run68K() function.

In a standard WarpOS program, the PPC task is setup by a 68K task, as the OS is 68K. When the PPC code is called from the sonnet library, the 68K task enters the WaitPort() function so the Amiga OS can execute other tasks while the 68K tasks waits for the PPC. We can use this same port from the PPC side (and so this waiting 68K task) to execute 68K code by sending a message to this port containing the information needed (called PPStruct within WarpOS). Actually, this port, set up by the sonnet library, waits for two types of messages (at the present). A message signaling that 68K code needs to be executed, or a message stating that the PPC has finished its code.

It's actually a bit more complicated than this, but we'll save the real tech talk for another time.

At the moment, the sonnet library only supports synchronous executing, which means that the 68K task waits for its PPC mirror task to complete before continuing and the other way around. For multitasking on the PPC side, we need to have code in place that puts the PPC task in a wait state when this happens and schedule another 'ready' PPC task. This code is commonly known as a 'Scheduler'. All this is of course already implemented on the 68K side by the Amiga OS.

When the 68K is ready, it signals the PPC that it can continue its code. So the sonnet library scheduler sets the task again in a 'ready' state and decides to put it into running based on priority compared to other 'ready' tasks,

So after putting Run68K() and a simple task scheduler in place, the WarpOS program 'Tabletennis' now works on Sonnet:

Now the less fun work begins as with the more complex programs, the ugly beast of cache coherency and consistency in a dual CPU system rears it head. So next up is to write code which properly handles the flushing and invalidation of caches.

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