Real-Time Operating Systems for DSP, part 2 | Network Systems Designline

Get the latest news, products and how-to information on network systems. Sign up for the Network Systems DesignLine newsletter, a weekly e-mail guide dedicated to the needs of engineers developing networking equipment and components. Here is our RSS feed.

Network Systems DesignLine » How-To » Enterprise Networking

HOW-TO : Enterprise Networking

Real-Time Operating Systems for DSP, part 2

Task states

By Robert Oshana

Page 3 of 3

Courtesy of DSP DesignLine
(04/26/2007 3:00 AM EDT)

Task states
Further refinement comes when we allow tasks to have states other than ready for execution. The major states of a typical RTOS include:

Sleeping – The thread is put into a sleeping state immediately after it is created and initialized. The thread is released and leaves this state upon the occurrence of an event of the specified type(s). Upon the completion of a thread that is to execute again it is reinitialized and put in the sleeping state.
Ready – The thread enters the ready state after it is released or when it is pre-empted. A thread in this state is in the ready queue and eligible for execution. (The RTOS may also keep a number of ready queues. In fixed-priority scheduling, there will be a queue for each priority. The RTOS scheduler rather than simply admitting the tasks to the CPU, makes a decision based on the task state and priority.)
Executing – A thread is in the executing state when it executes.
Suspended (Blocked) – A thread that has been released and is yet to complete enters the suspended or blocked state when its execution cannot proceed for some reason. The kernel puts a suspended thread in the suspended queue. There are various reasons for a multitasking DSP to suspend. A task may be blocked due to resource access control. A task may be waiting to synchronize execution with some other task(s). The task may be held waiting for some reason (I/O completion and jitter control). There may be no budget or aperiodic [2] job to execute (this is a form of bandwidth control). The RTOS maintains different queues for tasks suspended or blocked for different reasons (for example, a queue for tasks waiting for each resource).

Terminated – A thread that will not execute again enters the terminated state when it completes. A terminated thread may be destroyed.

Different RTOS's will have slightly different states. Figure 2 shows the state model for a DSP RTOS called DSP/BIOS from Texas Instruments.

(Click to enlarge)
Figure 2. State model (with preemption) for the Texas Instruments DSP BIOS RTOS.

Most RTOSs provide the user the choice of round-robin or FIFO scheduling of threads of equal priority. A time slice (execution budget) is used for round-robin scheduling. At each clock interrupt the scheduler reduces the budget of the thread by the tick size. Tasks will be pre-empted if necessary (A FIFO scheduling algorithm uses an infinite time slice.) At each clock interrupt, the RTOS scheduler updates the ready queue and either returns control to the current task or preempts the current task if it is no longer the highest priority ready task. Because of the previous actions, some threads may become ready (released upon timer expiration) and the thread executing may need to be pre-empted. The scheduler updates the ready queue accordingly and then gives control to the thread at the head of the highest priority queue.

Part 3 introduces the RTOS kernel and shows how it prioritizes tasks. It also explains dynamic memory allocation, system calls, and hardware interrupts.

Used with the permission of the publisher, Newnes/Elsevier, this series of eight articles is based on chapter eight of "DSP Software Development Techniques for Embedded and Real-Time Systems," by Robert Oshana.

Footnotes
2. Tasks are not always periodic. Tasks that are not periodic are called aperiodic. Examples of aperiodic tasks include operator requests, emergency message arrivals, threshold crossing notifications, keyboard presses, mouse movements, and detection of incoming objects.

Page 1 | 2 | 3