Page 260 - Embedded Microprocessor Systems Real World Design

is set. When a byte is received, the receive interrupt service routine (ISR) sets this
semaphore, and the RTOS activates the receive task. The receive task reads the byte
from the universal asynchronous receiver/transmitter (UART) register, processes
the data, and places it in a buffer for the output task. The receive task has the
highest priority because the system cannot afford to miss a serial input byte. Once
the receive task has finished processing the received byte, it becomes not ready until
the next receive interrupt occurs, by asking the RTOS to suspend it until the
semaphore is set again.
When the buffer is near full, the XOFF task is activated (becomes ready) by the
receive processing. XOFF could be “created” by receive processing, where the
receive processing requests that XOFF processing be activated, or XOFF could have
previously been set up to be activated by a semaphore like receive processing was.
XOFF runs until it has successfully sent the XOFJ? signal to the host or until it is
preempted by the receive task. If receive preempts XOFF, it gets control (from the
RTOS), processes the receive data, and then control is returned to XOFF. Again,
the highest-priority ready task is the one executed.
XON is next in priority. If output processing empties the buffer past a certain
point, it activates XON. Output processing has the lowest priority, which is possi-
ble because the XOFF task prevents the buffer from overflowing, so no data ever
are missed. Of course, if the receive data flow cannot be suspended with XOFF,
then output processing would have to have a higher priority so the buffer does not
overflow.
None of this happens by magic. The RTOS can activate a task, when a sema-
phore is set or a message is received, only if it was previously told to do so. Also, in
an RTOSbased system, the interrupt service routines usually get control via the
RTOS, so an ISR may not need to set a semaphore to start a task. Instead, the RTOS
can schedule the task upon activation of the ISR itself.
A final note about scheduling: Both sequential and preemptive sched-
uling systems allow a task to run until finished. The difference is that, in a
preemptive system, a task runs until finished or until preempted. Between two
ready tasks of different priorities, the higher-priority task always preempts the
lower-priority task and finishes first. If two tasks of equal priority are ready at
the same time, a sophisticated RTOS usually activates the one that has been idle
the longest.
A note about terminology: A task is considered active when it actually is running,
when it has been given control of the CPU by the RTOS. A ready task is in the list
of tasks waiting to run. A task can be not ready, such as the receive processing on
the protocol converter while waiting for the start semaphore.
The RTOS knows about only those tasks it has been told about (those that have
been created). The code for other tasks may reside in memory, but they are
invisible to the RTOS until they are created.

Real-Time Operating Systems 241