Rtos

RealReal--Time OSTime OS(RTOS)(RTOS)

Agenda

• Basic Definitions

• Introduction to RTOS

• Scheduling Algorithms

• Reentrancy

• Shared Resources

• RTOS APIs

Agenda

• Reentrancy

• RTOS APIs

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”

Basic Definitions

• Specifications of a Real-Time system include both:– Logical: Produces correct outputs.

– Temporal: Produces outputs at the right time.

Basic Definitions

• Types of Real-Time requirements are:– Hard: Failure to meet constraint is fatal.

– Soft: Late completion degrades software quality.

Basic Definitions

• Misconceptions:– “Real-Time computing is equivalent to fast

computing”

– Truth is:“Real-Time computing is equivalent to predictable computing”

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”

Basic Definitions

• Misconceptions:– “Real-Time means performance engineering”

– Truth is:“In Real-Time computing, timeliness is always more important than performance.

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.

Basic Definitions

Embedded SystemsEmbedded Systems

Real-Time Systems

Real-Time Embedded Systems

Agenda

• Reentrancy

• RTOS APIs

Introduction to RTOS

• Basic Services provided by OS:– Task Management.

– Intertask Communication & synchronization.

– Timers.

– Device I/O Supervision.

– Dynamic Memory Allocation.

• 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*/}

• What is multitasking?

• What is context?

CPU Registers

Task1 Stack

Task2 Stack

Task1 Control Block

Task2 Control Block

Task 1CPU Registers

Task 2CPU Registers

• Task Life Cycle:

• What is the difference between RTOS & GPOS?

Number of Tasks That Can Be Scheduled

Task Switching Time

• Characteristics of an RTOS?– Reliability

– Predictability

– Performance

– Compactness

– Scalability

Agenda

• Reentrancy

• RTOS APIs

Scheduling Algorithms

• 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.

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

TimeTask1

• Non-preemptive:

TimeTask1

• Non-preemptive:

TimeTask1

ISRTask2

is Ready

• Non-preemptive:

TimeTask1

Task2 is

• Non-preemptive:

TimeTask1

Task2Although task2 is higher in priority than task1, task 1 gets to finish before task2 gets to start.

Task2 is

• Preemptive (Suppose it is priority based):

TimeTask1

• Preemptive:

TimeTask1

• Preemptive:

TimeTask1

• Preemptive:

TimeTask1

• Preemptive:

TimeTask1

Agenda

• Reentrancy

• RTOS APIs

Reentrancy

char x;void foo1(void){

char x;foo1:

mov R1,x;

add R1,1;

mov x,R1;

C Assembly

/*Some Code*/foo1();

/*Some Code*/

Task1 Task2/*Some Code*/

foo1();

/*Some Code*/

Reentrancyx = 1

Task 1: R1=0 Task 2: R1=0

Reentrancyx = 1

Task 1: R1=1 Task 2: R1=0mov R1,x;

Reentrancyx = 1

add R1,1;

Reentrancyx = 2

add R1,1;

mov x,R1;

Reentrancyx = 2

add R1,1;

mov x,R1;

Context SwitchingTask2 is ready

Reentrancyx = 2

Task 1: R1=2 Task 2: R1=2mov R1,x;mov R1,x;

add R1,1;

mov x,R1;

Reentrancyx = 2

add R1,1;

mov x,R1;

mov R1,x;

add R1,1;

Reentrancyx = 3

add R1,1;

mov x,R1;

mov R1,x;

add R1,1;

mov x,R1;

Reentrancy (Another Scenario)x = 1

Task 1: R1=0 Task 2: R1=0

Reentrancyx = 1

Context SwitchingTask2 is ready

Reentrancyx = 1

Task 1: R1=1 Task 2: R1=1mov R1,x; mov R1,x;

Reentrancyx = 1

add R1,1;

Reentrancyx = 2

add R1,1;

mov x,R1

Reentrancyx = 2

add R1,1;

mov x,R1;

Context SwitchingTask2 is back to waiting

Reentrancyx = 2

add R1,1;

mov R1,x;

add R1,1;

mov x,R1;

Reentrancyx = 2

add R1,1;

mov x,R1;

mov R1,x;

add R1,1;

mov x,R1;

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.

Reentrancy

• How to make a non-reentrant function reentrant?– Either using critical section or any task

synchronization services provided by the OS.

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;

Agenda

• Reentrancy

• RTOS APIs

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.

• Semaphore vs. mutex

Shared Resources

IntertaskCommunication

• global variable

XTask 1 write X

Task 2 read X

• Mailboxes- Any task can send a message to a mailbox and any task can receive a message from a mailbox

• Message Queues

Agenda

• Reentrancy

• RTOS APIs

RTOS APIs “uCOS-III”• 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)

References

• uC/OS-II, Jean Labrosse

• Operating Systems: Design & Implementation, Andrew Tanenbaum

• Embedded.com

Website: www.escommittee.net

Contact Us: info@escommittee.net

FB: Embedded Systems Committee

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