Introduction to Embedded Systems

Rabie A. Ramadanrabieramadan@gmail.com

http://www.rabieramadan.org/classes/2014/embedded/

6

Real Time Systems

Real Time Systems

Correctness of the system may depend not only on the logical result of the computation but also on the time when these results are produced

Tasks attempt to control events or to react to events that take place in the outside world

These external events occur in real time and processing must be able to keep up

Processing must happen in a timely fashion, neither too late, nor too early

Some examples include, Air Traffic Control, Robotics, Controlling Cars/Trains, Medical Support, Multimedia.

Types of Real Time Systems

Hard real time systems• Must always meet all deadlines• System fails if deadline window is missed

Soft real time systems• Must try to meet all deadlines• System does not fail if a few deadlines are missed

Firm real time systems• Result has no use outside deadline window• Tasks that fail are discarded

Real time tasks

Periodic

- Each task is repeated at a regular interval

- Max execution time is the same each period

- Arrival time is usually the start of the period

- Deadline is usually the end Aperiodic

- Each task can arrive at any time

Real-Time Scheduling

Determines the order of real time task executions

Dynamic vs. Static• Dynamic schedule computed at run-time based on

tasks really executing, priorities computed on the fly• Static schedule done at compile time for all possible

tasks, priorities are already known

Rate Monotonic

Simplest type of real time scheduling

- Tasks are periodic, with hard deadlines

- Tasks are completely independent and do not communicate with each other

- Tasks are scheduled according to priority and task priorities are fixed

- Computation time is known and constant

Rate Monotonic Higher priorities usually assigned to tasks with

smaller periods If t(h) < t(l), then PR(h) > PR(l), where t indicated

period and PR indicates the priority. This is called rate monotonic priority assignment.

Calculates the critical instant for each task. Occurs when task Ti and all higher priority tasks are

scheduled simultaneously If tasks deadline scheduled at critical instant, then

the task can always meet its deadline.

Deadline Monotonic

Tasks with shorter deadlines get higher priority.

Static Scheduling. If D(h) < D(l), then PR(h) > PR(l), where D

indicates the deadline. This is called Deadline Monotonic priority assignment.

EDF: Earliest Deadline First

Dynamic Scheduling Assume a preemptive system with dynamic

priorities Like deadline monotonic, the task with

shortest deadline gets highest priority, but the difference is real time priorities can vary during the system’s execution.

Priorities are reevaluated when events such as task arrivals, completions occur

Real Time Synchronization Required when tasks are not independent and need to share

information and synchronize If two tasks want to access the same data, semaphores are

used to ensure non-simultaneous access.

Potential Issues• Priority Inversion – A situation in which a higher priority job is

blocked by lower priority jobs for an indefinite period of time• Chain Blocking – A situation in which more than two resources

are available and a high priority task is blocked off from both because of two or more lower priority tasks holding locks on one or both of the resources.

Priority Inversions Low priority task holds resource that prevents execution of

higher priority tasks.- Task L acquires lock- Task H needs resource, but is blocked by Task L, so Task L is

allowed to execute- Task M preempts Task L, because it is higher priority- Thus Task M delays Task L, which delays Task H

Chained Blocking: Problem of PIP

In the worst case, the highest priority task T1 can be blocked by N lower priority tasks in the system when T1 has to access N semaphores to finishthe execution!

Source of the figure: Damir Isovic, Malardaren University, Sweden

Priority Inheritance Protocol

PIP eliminates priority inversion problem• The algorithm will increase the priority of a task to the

maximum priority of any task waiting for any resource the task has a resource lock on

- i.e. if a lower priority task L has a lock on a resource required by a higher priority task H, then the priority of the lower task is increased to the priority of the higher task

- Once the lock is released the task resumes back its original priority.

Priority Ceiling Protocol• Each resource is assigned a priority ceiling, which is a priority equal to

the highest priority of any task which may lock the resource.• PCP eliminates chain blocking which considers the use of more than one

resource or semaphore A task can acquire a lock on resource S only if no other task holds a lock

on resource R. Thus higher priority tasks will not be blocked through both S and R

If a high priority task is blocked through a resource, then the task holding that resource gets the priority of the high priority task. Once the resource is released, the priority is reset back to its original

Timers and Interrupts

16

Synchronization

17

The tight-coupling between the computer and external world distinguishes an embedded system from a regular computer system.

synchronization between the executing software and its external environment is critical for the success of an embedded system. 

18

Definition of ‘Interrupt’

Event that disrupts the normal execution of a program and causes the

execution of special instructions

19

Interrupts

Interrupt

Program

time t

20

Interrupts

Program

Interrupt Service Routine

Interrupt

Program

time t

21

Interrupts

ProgramSave

Context Interrupt Service Routine

Restore Context

Interrupt

Program

time t

mov R1, cent mul R1, 9

eg push R1 eg pop R1

Trigger

22

Hardware event is called a trigger. The hardware event can either be a busy to ready transition

in an external I/O device (like the UART input/output) or an internal event (like bus fault, memory fault, or a periodic timer).

The execution of the interrupt service routine is called a background thread.

The thread is killed when the interrupt service routine returns from interrupt 

A new thread is created for each interrupt request. 

Threads and Processes

23

Threads share:• Access to I/O devices, • System resources, • Global variables,

Processes have separate:• global variables and system resources.

Processes do not share I/O devices.

Exception and Interrupt Handling in ARM as a Case Study

24

Interrupts vs. Polling

25

Polling mechanism wastes the CPU time in looping forever checking some flags to know that the event occurred.

We have nowadays systems with more than one interrupt source.

We may need priorities to be assigned to different Events

Interrupt is the best way to handle events

ARM modes of operation

26

Internally has 7 different modes of operation• User mode: used for normal program execution state• Fast Interrupt Request (FIQ) mode: This mode is used for interrupts

requiring fast response and low latency.• like for example data transfer with DMA

• Interrupt Request (IRQ) mode: used for general interrupt services• Abort mode: selected when data or instruction fetch is aborted• System mode: Operating system privilege mode for users• Undefined mode: When undefined instruction is fetched• Supervisor mode: used when operating system support is needed where it

works as protected mode

Protected Vs. Original Mode

27

Protected mode differed from the original mode: • Later dubbed “real mode”,• Areas of memory could be physically isolated by the processor itself to

prevent illegal writes to other programs running in memory at the same time.

• Prior to protected mode, multiple programs could be running in memory at the same time, but any program could access any area of memory and, therefore, if malicious or errant, for example, could take down the entire system.

• Isolates that possibility by allowing the operating system (OS) to dictate where each program should run.

ARM Modes of Operation

28

ARM Register set

29

Register structure in ARM depends on the mode of operation. There are 16 (32-bit) registers named from R0 to R15 in ARM

mode (usr).• Registers R0 to R12 are general purpose registers,• R13 is stack pointer (SP), • R14 is subroutine link register • R15 is program counter (PC). • R16 is the current program status register (CPSR) and it plays a main role

in the process of switching between modes – eg. the current processor mode

Banked Registers

30

Some registers are available with the same name but as another physical register in memory which is called (banked)

• Decreases the effort needed when context switching is required

• The new mode has its own physical register space and no need to

store the old mode’s register values

Register Organization in ARM

31

ARM Exceptions

32

Exception : any condition that needs to halt normal execution of the instructions.• State of resetting ARM core,• Failure of fetching instructions or memory access

There is always software associated with each exception, • Exception handler.

Each of the ARM exceptions causes the ARM core to enter a certain mode automatically.

Ex.

Vector Table

33

A table of instructions that the ARM core branches to when an exception is raised.

These instructions are places in a specific part in memory and its address is related to the exception type

Exception priorities

34

Since exceptions can occur simultaneously

We may have more than one exception raised at the same time,

The processor has to have a priority for each exception so it can decide which of the currently raised exceptions is more important

Exception priorities

35

Various exceptions that occur on the ARM and their associated priorities.

Both are caused by an instruction entering the

execution stage of the ARM instruction

pipeline

Link Register

36

This register is used to return the PC to the appropriate place in the interrupted task • not always the old PC value. ????????• It is modified depending on the type of exception.

In the case of IRQ exception, the link register is pointing initially to the last executed instruction

In data abort exception , the exception is handled and the PC should point to the same instruction again to retry accessing the same memory location again.

Data Exception : processor can fetch and store data or they can refuse to do either.

Entering and exiting an exception handler

37

Save the address of the next instruction in the appropriate Link Register (LR).

Copy Current Program Status Register (CPSR) to the Saved Program Status Register (SPSR) of new mode.

Change the mode by modifying bits in CPSR.

Fetch next instruction from the vector table.

Leaving exception handler

38

Move the Link Register LR to the PC.

Copy SPSR back to CPSR, this will automatically changes the mode back to the previous one.

Clear the interrupt disable flags (if they were set).

Interrupts

39

Interrupts

40

Interrupts

41

Interrupt handling schemes

42

Interrupt handling schemes

43

Interrupt handling schemes

44

Interrupt handling schemes

45

In class work

46

47

48

Assignment- Thursday 24th

Write a 4 pages report on Embedded System Software profiling Methods or embedded system testing methodology?

Prepare a presentation for about 20 minutes .

49