Embd & RTos Overview

8/6/2019 Embd & RTos Overview

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EmbeddedEmbedded andand RealReal--TimeTime

Operating SystemsOperating Systems

Overview


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Embedded SystemsEmbedded Systems

An embedded system is a computer system

that is integrated into a larger system which is

not, itself, a general purpose computer.Basically it is a microprocessor-based system t

hat is built to control a function or a range of

functions.

Examples: Portable phone, washing machine

photocopier, aircraft's navigation

system, the cash dispenser etc.


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ComponentsComponents

ofofEmbedded SystemsEmbedded Systems

CPU (Central Processing Unit)

Microprocessors are general purposeCPUs meant for general-purpose computer

applications .

Microcontroller consists of CPU, Memory

and set of I/O controllers within a single

chip.


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Components ofComponents of

Embedded SystemsEmbedded Systems contd..contd.. Memory

RAM, ROM, Flash Memory, EPROM,

EEPROM.

I/O Controllers

Provide interface to external devices

Keyboard, display, printer, Digital Input,

Digital Output, Analog Input, Analog Output etc.


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RealReal--Time SystemsSystems

A real-time system is a computer system

in which the correctness of the system

behavior depends not only on the logical

results of the computation, but also on the

physical instant at which these results areproduced.


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Examples ofR-T systems Control of production processes / industrial a

utomation

Railway switching systems Automotive applications

Flight control systems

Environmental acquisition and monitoring

Robotics

Military systems

Space missions


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Types ofR

eal Time Systems Soft R-T system :

Meeting its deadline is desirable

(for performance reasons) but missing does not

cause serious damage.

Examles:

Voice over IP, cellular telephony

Keyboard handling.

Display of video in a window where certain frames

missing wont effect the final display.


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Types ofReal Time Systems contd..

HardR-T system:

Missing its deadline may cause catastrop

hicconsequences on the environment under

control.

Examples:

Sensory data acquisition. Auto Pilot Flight control system and Rocket control.

Automotive air-bag control.

Control robots used in various automatic processes.


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Real Time Operating SystemsReal Time Operating Systems

FromPOSIX 1003.1b:

Realtime in operating systems is

the ability of the operating system to provide arequired level of service in a bounded response time.

RTOS is a system, which satisfies the following

conditions: Responds in a predictable way to unpredictable

external events.

Strict timing constraints.


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Features ofanRTOS

Timeliness

Predictability Priority based preemptive scheduling

Task synchronization mechanisms

Interrupt handling ISRs

Priority Inheritance should exist


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Timeliness

OS has to provide mechanisms for:

time management

handling tasks with explicit time constraints

Achieved through proper scheduling algorithms.

Predictability

Provide timing details for execution ofevery system call, Context Switching, Interru

pt Latency.


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Multitasking and Scheduling

Round-robin Scheduling

Shares theCPU fairly among all ready

tasks of the same priority.

Preemptive Priority Scheduling

Each task has a priority.

CPU is fairly allocated to the highest

priority task that is ready to run.


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Interrupt Handling

Handling of the hardware interrupts by the kernel is

the most critical factor contributing to the determinism

and overall responsiveness of real-time system.

Bottom half handlers:

Off-loading interrupt

processing from the

interrupt handler to a

separately scheduled task.

The system is able to respond to subsequent

interrupts sooner, thus reducing the worst case

interrupt latency

.


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Interrupt Handling contd..contd..

Bottom half handlers are scheduled before all user

tasks i.e, all interrupt processing runs at a higher

priority than all user tasks, including real-time processes.

Interrupt Latency:Measure of the ability of RTOS to respond to the

hardware interrupts.

The time from the start of the physical interrupt to theexecution of the first instruction of the interrupt service

routine.


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Latency

The latency (or tardiness) of a task is the difference bet-ween the instant of time on which the task should have

started (or finished) and the instant of time on which it

actually did.

Latencies are due to several factors:

(i) The timing properties of processor, bus, memory

and peripheral devices,

(ii) The scheduling properties of theRTOS,

(iii) The pre-emptive ness of the OS kernel, and

(iv) The load on the system (i.e., the number of tasks

that want to be scheduled concurrently).

The statistical distribution of the latencies in the

subsequent scheduling of tasks is called the

scheduling jitter.


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Thread Switch Latency

An important contribution to the latency is the context

switch time of the processor; i.e., the time needed to

save the local data of the currently running task

(e.g., the contents of the processor registers; the

stack of the task; the current instruction pointer), and to

replace it with the local data of the newly scheduled task

Def: The time from the execution of the last

instruction of one user-level task to the first

instruction of the next user-level task.


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Priority Inversion : Higher-priority task is

forced to wait an indefinite period for a lower-p

riority task to complete.

Priority Inheritance :

Eliminates the risk of unbounded

priorty inversion.

The lower priority task temporarilyinherits the priority of the higher until it

relinquishes the resource, and returns

to its original priority.


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Monolithic kernelMonolithic kernel

vsvs

MicroMicro--kernelkernel A monolithic kernel has all OS services running within t

he privileged mode of the processor. (This doesnt

mean that the whole kernel is one singleC file!).

A micro-kernel, on the other hand, uses theCPUs

privileged mode only for really core services

(task scheduling, interprocess communication and

memory allocation), and has most of the device

drivers and OS services running as normal tasks.


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Kernel spaceKernel spacevsvs

user spaceuser space Most modern processors allow programs

to run in two different privilege(i.e., hardware

protection) levels.

user space have more protection against

erroneous accesses to physical memory of

I/O devices,but access most of the hardware with larger latencies than kernels space

tasks.


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PrePre--emptive kernelemptive kernelvsvs

nonnon--prepre--emptive kernelemptive kernel

In a non-pre-emptive kernel, a kernel space

task cannot be interrupted by other kernelspace tasks, or by user space tasks.

This makes the design of the kernel simpler,but introduces in-deterministic latencies

which are not tolerable in a RTOS.


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Memory managementvs

Shared memory space irtual memory and dynamic allocation and

de-allocation of memory pages induces

overhead,and some simpler processors have no s

upport for this memory management.

In Real-time kernels all tasks share the same memory space, such that developers must

take care of the proper use of that memory.


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RTOS Available

Proprietary

VxWorks by WindRiver

LynxOS by Lynx

Free/Open-source

RedHats eCos

RTLinux QNX

uC/os

RTEM


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RTOS useRTOS use aerospace & defenceaerospace & defence

Aerospace & Defence Sector

RTOS Use Characteristics

(% Respondents: 1348 Total)

Don't Use RTOS

34,2%

RTE S

0,1%

C/OS0,4%

RT-Linux

0,8%

pSOS

6,7%

QNX

4,5%

Tornado/VX-Works

16,7%

WinCE

0,7%

Proprietary

7,1%

WinNT

3,3%Other Commercial

25,4%


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VxWorks vs RTLinux

Context Switch (S)VxWorks 11 (0.04)

RTLinux 13.4 (0.6)

Priority

Inversion (S)VxWorks 123 (1.67) RTLinux 108 (0.41)

Interrupt Latency (S)VxWorks 98 (0.55)

RTL

inux 132 (1.2)

Overall analysis shows that both operating systems are

suitable for real-time application.

In particular, VxWorks is more deterministic and predictable.


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Date post:	07-Apr-2018
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