Start
• Task
Function •
Scheduling
and Timeslice •
Task
Context •
Task
Control Block •
Global
Variables •
Memory
Management •
API
•
Assembly
Functions •
Give
it a Try •
Enhancements
•
References,
Sources & PDF
SCHEDULING
AND TIMESLICE
The scheduler chooses
which task to run. The period of time a task is allowed
to run is called its timeslice.
Under 畫xec, tasks are
run to completion of their timeslices unless they
are explicitly yielding control to the system or hogging
the processor for another timeslice. Although 畫xec
doesn’t assign priorities, tasks can have different
timeslice lengths, so you can cause a compute-intensive
task to run longer if necessary.
The main purpose of the
畫xec control system, then, is to interrupt the processor
at the end of a task’s timeslice and invoke the
scheduler to choose another task to run. 畫xec uses
a timer to interrupt the CPU at regular intervals.
Almost all microcontrollers
have timer interrupt functions that can be used for
this purpose. For example, on the ’HC11, there
are five timer-registers, each one of which can be
set up to generate an interrupt when the timer-register’s
value matches the value of the free-running timer
counter.
The time period when
such an interrupt occurs is called a tick, and its
timeslice is some multiple of ticks. A tick’s
value should be chosen such that the processor is
not interrupted too frequently, but it shouldn’t
be so long that other tasks do not get to execute
in a timely manner.
For the ’HC11, we
chose a value of 2000 cycles per tick, or about 25
盜 for a ’HC11 running with an 8-MHz clock. The
default timeslice is five ticks, or 125 盜. You may
want different values for your system.
INTERRUPT DRIVER
Sometimes your program
may need to perform some function at a regular interval,
and task scheduling may be too slow and unpredictable.
畫xec gives a hook to the timer-interrupt handler—if
the global variable UEXC_InterruptDiver is nonnull,
then it’s assumed to contain the address of a
function.
The system timer-interrupt
handler then calls this function before it performs
other work. To use this feature, assign the address
of your function to be called to this variable.
