[PicForth] Re: multitasker.fs question

[Alex Holden]

David McNab wrote:
> Just one question - given that the return stack on PIC16F chips is 
> totally non-addressible, does this mean that 'yield' instructions may 
> only occur in the very top level of each task?

Yep, it's a pretty annoying limitation of the architecture. And for the 
same reason the task is always reentered at the start of the toplevel 
word, not at the word after the yield. I was doing stuff like this until 
I realised from reading the generated assembly that there's no need for 
the outer loop because the multitasker will just reenter at the 
beginning of the word:

task : fred
	begin
		barney
		yield
	again
;

> In other words, would I crap on the return stack if task 'fred' calls 
> word 'mary', and 'mary' sits in a loop periodically executing a 'yield'?

Yep. I have a task which polls a piece of hardware ten times a second 
and another which talks to a user via a serial terminal. When the user 
executes a command which requires the task to accept a string, the task 
sits around in 'accept' indefinitely and the hardware servicing stops 
for the duration. Calling yield if key? returns false would be ideal but 
that won't work so I've decided to simply document the fact that normal 
operation of the system ceases while user interaction is taking place 
(it only happens when the system is being configured so it's not really 
a big deal in this case).

> If this is the case (I expect it would be), then it might be worth a 
> mention in the doco.

It is, in the second sentence. "It allows you to relinquish the CPU 
whenever you want, provided that you are not in the middle of a call 
(context-switch only occurs during top level calls)."

-- 
------------ Alex Holden - http://www.linuxhacker.org ------------
If it doesn't work, you're not hitting it with a big enough hammer