Re: Policies In Ada

["Samuel A. Mize" <smize@magellan.bgm.link.com>]

scott@tcville.es.hac.com wrote:
> 
> In article 190000c2@Stephen.ers.ie, "Stephen Cox" <stephen@ers.ie> () writes:
> >Also, I have implemented two tasks with the same priority i.e. they both
> >have pragma priority(0); in their declaration yet only the first task seems
> >to execute. Any ideas?
> 
> This could be a feature of your OS.

This depends on whether Stephen is using Ada 83 or 95, and
if Ada 95 on whether it supports Annex D.

** Assuming Stephen is using Ada 95 with support for Annex D **
it's a feature of the default task dispatching policy
("FIFO_Within_Priorities").  Tasks run until they hit a
"task dispatching point" -- a same-priority task CAN'T
interrupt the current task.  To get time-slicing, you must
use a Task_Dispatching_Policy pragma (AND the compiler
must support a time-slicing policy, which is not required).

Task dispatching points include delay statements (including a
delay 0.0), completion of an accept statement, when a
higher-priority task becomes ready -- these are examples,
there are others.  If your tasks are loops, you can put a
"delay 0.0" into both loops, and they should both get to run.

But don't just take this as a cookbook approach -- read and
understand ARM section D.2.

** If it's an Ada 95 compiler but doesn't support Annex D **
you get pot luck for task dispatching policy.  However,
FIFO_Within_Priorities is still likely because it's easier
to build than time-slicing.

** For Ada 83 **
I don't remember the rules for same-priority tasks (if any).

Samuel Mize

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