Real TIme Operating Systems - Programming with Micro_C

08/05/2025 RTOS Mucos
Unit-II: RTOS Programming
 Basic Functions and Types of RTOS for Embedded Systems
 RTOS mCOS-II
 RTOSVxWorks
 Programming concepts of mCOS-II &VxWorks with
relevant Examples,
 Programming concepts of RTOSWindows CE
 RTOS Linux 2.6.x
 RTOS RT Linux

MicroC/OS-II (commonly termed as µC/OS-
II or uC/OS-II), is the acronym for Micro-Controller
Operating Systems Version 2.
It is a priority-based real-time multitasking operating
system kernel for microprocessors, written mainly in the
C programming language.
It is intended for use in embedded systems.
INTRODUCTION

It is a very small real-time kernel.
Memory footprint is about 20KB for a fully functional
kernel.
Source code is written mostly in ANSI C.
Highly portable, ROMable, very scalable, preemptive real-
time, deterministic, multitasking kernel.
It can manage up to 64 tasks (56 user tasks available).
It has connectivity with μC/GUI and μC/FS (GUI and File
Systems for μC/OS II).
Features of MicroC/OS-II :

It is ported to more than 100 microprocessors and
microcontrollers.
It is simple to use and simple to implement but very
effective compared to the price/performance ratio.
It supports all type of processors from 8-bit to 64-bit.
Cont…

µC/OS-II is a multitasking operating system. Each task is
an infinite loop and can be in any one of the following five
states:
 Dormant
 Ready
 Running
 Waiting
 ISR (Interrupt Service Routine)
Task states:

 Task Feature
 Task Creation
 Task Stack & Stack Checking
 Task Deletion
 Change a Task’s Priority
 Suspend and Resume a Task
 Get Information about a Task
Task Management (Services):

 μC/OS-II can manage up to 64 tasks.
The four highest priority tasks and the four lowest priority
tasks are reserved for its own use. This leaves 56 tasks for
applications.
The lower the value of the priority, the higher the priority
of the task. (Something on the lines of Rate Monotonic
Scheduling).
The task priority number also serves as the task identifier.
Task Feature :

Rate Monotonic Scheduling :
 In Rate Monotonic Scheduling tasks with the highest rate of
execution are given the highest priority
Assumptions:
 All tasks are periodic
 Tasks do not synchronize with one another, share resources, etc.
 Preemptive scheduling is used (always runs the highest priority
task that is ready)
 Under these assumptions, let n be the number of tasks, Ei be the
execution time of task i, and Ti be the period of task i. Then, all
deadlines will be met if the following inequality is satisfied:
ΣEi / Ti ≤ n(2^1/n – 1)

Rate Monotonic Scheduling :
Suppose we have 3 tasks. Task 1 runs at 100 Hz and takes 2
ms. Task 2 runs at 50 Hz and takes 1 ms. Task 3 runs at 66.7 Hz
and takes 7 ms. Apply RMS theory…
(2/10) + (1/20) + (7/15) = 0.717 ≤ 3(21/3 – 1) = 0.780
Thus, all the deadlines will be met
 General Solution?
 As n →∞, the right-hand side of the inequality goes to
ln(2)=0.6931. Thus, you should design your system to use less
than 60-70% of the CPU

PROCESS CYCLE :

There are two functions for creating a task:
OSTaskCreate()
 OSTaskCreateExt().
Task Creation :

Task Management :

Task Management :
After the task is created, the task has to get a stack in
which it will store its data.
 A stack must consist of contiguous memory locations.
 It is necessary to determine how much stack space a
task actually uses.
 Deleting a task means the task will be returned to its
dormant state and does not mean that the code for the
task will be deleted. The calling task can delete itself.

If another task tries to delete the current task, the
resources are not freed and thus are lost. So the task has to
delete itself after it uses its resources
Priority of the calling task or another task can be changed
at run time.
A task can suspend itself or another task, a suspended task
can resume itself.
A task can obtain information about itself or other tasks.
This information can be used to know what the task is doing
at a particular time.
Task Management (cont…):

The services of Memory management includes:
Initializing the Memory Manager.
Creating a Memory Partition.
Obtaining Status of a Memory Partition.
Obtaining a Memory Block.
Returning a Memory Block.
Waiting for Memory Blocks from a Memory Partition.
Memory Management :

Each memory partition consists of several fixed-sized memory
blocks.
A task obtains memory blocks from the memory partition. A
task must create a memory partition before it can be used.
Allocation and de-allocation of these fixed-sized memory
blocks is done in constant time and is deterministic.
Multiple memory partitions can exist, so a task can obtain
memory blocks of different sizes.
A specific memory block should be returned to its memory
partition from which it came.
Memory Management :

Clock Tick : A clock tick is a periodic time source to
keep track of time delays and time outs.
Here, tick intervals varies from 10 ~ 100 ms.
The faster the tick rate, the higher the overhead
imposed on the system.
Whenever a clock tick occurs μC/OS-II increments
a 32- bit counter, the counter starts at zero, and rolls
over to 4,294,967,295 (2^32-1) ticks.
A task can be delayed and a delayed task can also be
resumed.
Time Management :

It involves five services that includes:
OSTimeDLY()
OSTimeDLYHMSM()
OSTimeDlyResume()
OSTimeGet()
OSTimeSet()
Time Management :

Inter-task or inter process communication in μC/OS
takes place using:
Semaphores
Message mailbox
Message queues
Tasks and Interrupt service routines (ISR). They can
interact with each other through an ECB (event control
block).
Inter-Task Communication :

Single task waiting

Multiple tasks waiting and signaling

Tasks can wait and signal along with an optional time out

μC/OS-II semaphores consist of two elements
16-bit unsigned integer count
list of tasks waiting for semaphore
μC/OSII provides
Create, post, pend accept and query services

μC/OS-II message-mailboxes: an μC/OSII object that allows
a task or ISR to send a pointer sized variable (pointing to a
message) to another task.

 μC/OS-II message-queues
Available services: Create, Post (FIFO), PostFront (LIFO),
Pend, Accept, Query, Flush
 N = #of entries in the queue: Queue full if Post or
PostFront called N times before a Pend or Accept

μC/OS-II message-queues organized as circular buffers.

Tasks running under a multitasking kernel should be written in one of
two ways:
1. A non-returning forever loop. For example:
void Task (void *)
{
DoInitStuff();
while (1)
{ do this;
do that;
do the other thing;
call OS service (); // e.g. OSTimeDelay, OSSemPend, etc.
}
}
Writing Applications Under C/OS -II:
μ

2. A task that deletes itself after running.
For example:
void Task (void *)
{ do this;
do that;
do the other thing;
call OS service (); // e.g. OSTimeDelay, OSSemPend, etc.
OSTaskDelete(); // Ask the OS to delete the task
}

Real TIme Operating Systems - Programming with Micro_C

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