Rtos

Real-Time OS
(RTOS)

Copyright © 2012 Embedded Systems
Committee

Agenda
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Basic Definitions
Introduction to RTOS
Scheduling Algorithms
Reentrancy
Shared Resources
RTOS APIs

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Basic Definitions
• What are the Real-Time systems?
– “Those systems in which the correctness of system
depends not only on the logical result of the
computation, but also on the time at which the results
are produced”

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Basic Definitions
• Specifications of a Real-Time system include
both:
– Logical: Produces correct outputs.
– Temporal: Produces outputs at the right time.

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Basic Definitions
• Types of Real-Time requirements are:
– Hard: Failure to meet constraint is fatal.
– Soft: Late completion degrades software quality.

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Basic Definitions
• Misconceptions:
– “Real-Time computing is equivalent to fast
computing”
– Truth is:“Real-Time computing is equivalent to
predictable computing”

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Basic Definitions
• Misconceptions:
– “Real-Time programming assembly coding”
– Truth is: “It is better to automate (as much as
possible) Real-Time system design, instead of relying
on a clever hand-crafted code”

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Basic Definitions
• Misconceptions:
– “Real-Time means performance engineering”
– Truth is: “In Real-Time computing, timeliness is
always more important than performance.

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Basic Definitions
• Misconceptions:
– RTOS introduce considerable amount of overhead
on CPU
– Truth is: An RTOS typically only require between 1%
to 4% of a CPU time.

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Basic Definitions
Real-Time
Embedded
Systems
Embedded Systems

Real-Time Systems

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• Basic Services provided by OS:
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–
–
–
–

Task Management.
Intertask Communication & synchronization.
Timers.
Device I/O Supervision.
Dynamic Memory Allocation.

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• What are tasks?
- A task─ an independent process.
- No task can call another task. unlike the normal C function
which can call another function.
void task(void)
{
/*Some Initialization Code*/
for(;;)
{
/*Task Code*/
}
}
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• What is multitasking?

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• What is context?
PC

ROM

RAM
SP

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CPU
Registers

ROM

Task 1CPU
Registers

RAM
Task2
Stack
Task1 Control Block
Task1
Stack
Task2 Control Block
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Task 2CPU
Registers


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• Task Life Cycle:

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• What is the difference between RTOS & GPOS?

Task
Switching
Time

GPOS

RTOS
Number of Tasks That Can Be Scheduled
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• Characteristics of an RTOS?
–
–
–
–
–

Reliability
Predictability
Performance
Compactness
Scalability

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• What is a scheduler?
– It is part of the kernel which decides which task can
run when.
– There are many algorithms for scheduling & they can
be categorized into two main categories.

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• Non-preemptive (Suppose it is priority based):
Task1

Time

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• Non-preemptive:
Task1

Time
ISR

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• Non-preemptive:
Task1

Time
ISR
Task2
is
Ready

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• Non-preemptive:
Task1

Time
ISR
Task2
is
Ready

Task1

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• Non-preemptive:
Task1

Time
ISR
Task2
is
Ready

Task1
Although task2 is higher in priority than
task1, task 1 gets to finish before task2
gets to start.
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Task2

• Preemptive (Suppose it is priority based):
Task1

Time

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• Preemptive:
Task1

Time
ISR

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• Preemptive:
Task1

Time
ISR
Task2

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• Preemptive:
Task1

Time
ISR
Task2

Task1

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Reentrancy
C
char x;
void foo1(void)
{
x++;
}

Task1
/*Some Code*/
foo1();
/*Some Code*/

Assembly
char x;
foo1:
mov R1,x;
add R1,1;
mov x,R1;

Task2
/*Some Code*/
foo1();
/*Some Code*/

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Reentrancy
x=1
Task 1: R1=0

Task 2: R1=0

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Reentrancy
x=1
Task 1: R1=1

Task 2: R1=0

mov R1,x;

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Reentrancy
x=1
Task 1: R1=2

Task 2: R1=0

mov R1,x;
add R1,1;

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Reentrancy
x=2
Task 1: R1=2

Task 2: R1=0

mov R1,x;
add R1,1;
mov x,R1;

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Reentrancy
x=2
Task 1: R1=2

Task 2: R1=0

mov R1,x;
add R1,1;
mov x,R1;

Context Switching
Task2 is ready
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Reentrancy
x=2
Task 1: R1=2

Task 2: R1=2

mov R1,x;
add R1,1;
mov x,R1;

mov R1,x;

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Reentrancy
x=2
Task 1: R1=2

Task 2: R1=3

mov R1,x;
add R1,1;
mov x,R1;

mov R1,x;
add R1,1;

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Reentrancy
x=3
Task 1: R1=2

Task 2: R1=3

mov R1,x;
add R1,1;
mov x,R1;

mov R1,x;
add R1,1;
mov x,R1;

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Reentrancy (Another Scenario)
x=1
Task 1: R1=0

Task 2: R1=0

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Reentrancy
x=1
Task 1: R1=1

Task 2: R1=1

mov R1,x;

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Reentrancy
x=1
Task 1: R1=1

Task 2: R1=1

mov R1,x;

Context Switching
Task2 is ready
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Reentrancy
x=1
Task 1: R1=1

Task 2: R1=1

mov R1,x;

mov R1,x;

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Reentrancy
x=1
Task 1: R1=1

Task 2: R1=2

mov R1,x;

mov R1,x;
add R1,1;

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Reentrancy
x=2
Task 1: R1=1

Task 2: R1=2

mov R1,x;

mov R1,x;
add R1,1;
mov x,R1

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Reentrancy
x=2
Task 1: R1=1

Task 2: R1=2

mov R1,x;

mov R1,x;
add R1,1;
mov x,R1;

Context Switching
Task2 is back to waiting

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Reentrancy
x=2
Task 1: R1=2

Task 2: R1=2

mov R1,x;
add R1,1;

mov R1,x;
add R1,1;
mov x,R1;

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Reentrancy
x=2
Task 1: R1=2

Task 2: R1=2

mov R1,x;
add R1,1;
mov x,R1;

mov R1,x;
add R1,1;
mov x,R1;

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Reentrancy
• When to doubt your function reentrancy?
– When your function accesses a global variable while
this variable is accessed in:
• ISR
• Another task
• Hardware module.

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Reentrancy
• How to make a non-reentrant function reentrant?
– Either using critical section or any task
synchronization services provided by the OS.

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Reentrancy
• Critical Section:
– Context Switching always happens after an interrupt.
– If I disabled interrupts during the time I don’t want
the schedule nor any ISR interrupts the running task
then this part of code is reentrant.
char x;
void foo1(void)
{
DI;
x++;
EI;
}
Committee

Shared Resources
• Tasks always race each other for resources.
• Resources could be many thing like HW
modules, memory & even CPU time.
• We have to control the tasks resources access to
avoid data corruption or basically any
undesirable behavior.
• Semaphores are just one methods of controlling
resources access.
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Shared Resources
• Semaphore vs. mutex

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Intertask Communication
• global variable
RAM
Task 1
write X

X

Task 2
read X
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• Mailboxes
- Any task can send a message to a mailbox and any task can receive a message
from a mailbox

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• Message Queues

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RTOS APIs “uCOS-III”
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OSTaskCreate (Task ptr, arg, prio, stack size)
OSSemCreate (Sem ptr, counter)
OSSemPost (Sem ptr)
OSSemPend (Sem ptr,Timeout)
OSTmrCreate (Tmr ptr, period, callback)
OSTmrStart, OSTmrStop (Tmr ptr)
OSMemCreate (mem ptr, block size, n blocks)
OSMemPut (mem ptr, blk ptr)
OSMemGet (mem ptr)
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References
• uC/OS-II, Jean Labrosse
• Operating Systems: Design & Implementation,
Andrew Tanenbaum
• Embedded.com

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