On Jun 21, 2007, at 9:47 AM, Zhifeng Lai wrote:
Dear Steve,
Thanks a lot for your information!
(1) In TinyOS tutorial, it is said that "Hardware event handlers
are executed in response to a hardware interrupt and also runs to
completion, but may preempt the execution of a task or other
hardware event handler. " (URL: http://www.tinyos.net/tinyos-1.x/
doc/tutorial/lesson1.html). If "no platform implement nested
interrupts", you're certainly right. Can anyone confirm this
information?
Some platforms do implement nested interrupts. Specifically, the UART
stack of the atmega128 declares its transmit interrupts to be
preemptible. However, most interrupts are declared non-preemptible.
(2) To my understanding, the keyword "async" is used to specify a
piece of code is an asynchronous operation, which means that
"operations that occur without a regular or predictable time
relation to other events." (URL: http://en.wikipedia.org/wiki/
Asynchronous_operation). I think this meaning is lost when we call
async command from task. Is it true that: (1) call async command;
(2) call sync command; (3) signal async event; (4) signal sync
event all equal to subroutine call. I also want to know the exact
meaning of "a task post is an async operation", in Philip Levis's
programming guide (I know the mechanism of deferred procedure call).
Asynchrony generally depends on whose perspective you have. From the
perspective of the scheduler, tasks run synchronously. From the
perspective of a component, its tasks run asynchronously. In the
context of nesC, async refers to "from task context." That is
"operations that occur without a regular or predictable time relation
to a task."
The 'async' keyword does not denote that the code will always execute
asynchronously. It denotes that the function can be safely called
from code that is async. Synchronous (task) code can execute the
function as well. In practice, there is a root calling context, which
is either sync or async. The sync root calling context is main; the
async root calling contexts are interrupt handlers.
A task post is an async operation because async code can do it.
Phil
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