Embedded Operating Systems

Embedded Software Design 熊博安 國立中正大學資訊工程研究所 [email protected]

Textbook: Programming Embedded Systems in C and C++, 1 Michael Barr, O’Reilly Contents

„ History and Purpose

„ A Decent Embedded Operating System

„ Real-Time Characteristics

„ Selection Process

2 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University History and Purpose

„ Originally, no operating system (OS) „ Users controlled everything „ Later, a loose collection of routines (like modern software library) „ resetting hardware to a known state „ reading state of inputs „ changing state of outputs „ Finally, modern OS „ set of tasks

3 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Tasks

„ Task: a piece of software that can be separated from and run independently of the rest „ A set of embedded software requirements can be DECOMPOSED into a small number of independent pieces (tasks). „ Example: Printer-sharing device „ Task 1: receive data from serial port A „ Task 2: receive data from serial port B „ Task 3: format & send waiting data to printer 4 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Tasks

„ Software abstraction

„ Software design easier

„ Software implementation easier

„ Designer can concentrate on unique features of system under development

„ Need an OS

5 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Embedded OS

„ Small „ No need of filesystem „ No need of GUI „ Single user „ No need of security features of multiuser OS

6 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University ADEOS (A Decent Embedded OS)

„ Developed by Michael Barr „ Less than 1000 lines of source code „ ¾ platform independent, in C++ „ ¼ platform dependent, in assembly „ For learning embedded OS

7 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Tasks

„ Pseudoparallel: take turns using processor „ Processor decides: „ which task gets the processor „ context of a task (like a bookmark for a book reader) „ Instruction Pointer: Pointer to next instruction (80x86 Eg: CS and IP)

„ Stack Pointer: Address of current top of stack (80x86 Eg: SS and SP)

„ Process flag and general-purpose register contents (80x86 Eg: Flags, DS, ES, SI, DI, AX, BX, CX, DX)

8 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Task Class in ADEOS class Task { public: Task( void (*function)(), Priority p, int stackSize ); TaskID id; Context context; TaskState state; Priority priority; int * pStack; Task * pNext; void (*entryPoint)(); private: static TaskId nextId; };

9 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Task States

„ enum TaskState { Ready, Running, Waiting };

10 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Task Mechanics

„ How to create a task for OS to run? „ How to use tasks? „ How to assign priority? „ How to assign stack size? „ etc.

11 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Task Constructor

12 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Task Constructor

13 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Scheduler

„ Decides which ready task gets to execute on the processor „ Common scheduling algorithms for non- embedded systems: „ First-In-First-Out (FIFO) „ Shortest Job First (SJF) „ Round Robin (RR): preemptive „ Not suitable for real-time embedded systems „ requires task priorities

14 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Scheduler

„ Priority-based scheduling needs a backup policy

„ required when tasks have equal priorities

„ Mostly, round-robin

„ In ADEOS: FIFO „ ADEOS supports:

„ 255 tasks, and

„ 255 levels of priorities

15 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Scheduler in ADEOS

„ extern Sched os; 16 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Scheduling Points

„ Set of operating system events that result in scheduler invocation (os.schedule())

„ Task Creation (see constructor)

„ Task Deletion

„ Clock Tick (awakes tasks waiting on timers) (Timer class from Chapter 7 Peripherals: disable Æ enterCS, enable Æ exitCS, 1 ms Æ 10 ms )

17 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Ready List

„ Fast dispatching „ Slow insertion

18 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Idle Task

„ An infinite loop „ Lowest priority „ At the end of ready list „ When no task is ready for execution, scheduler runs the idle task „ Other tasks are called user tasks

19 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Scheduler

20 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Scheduler

21 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Scheduler

„ 2 situations when context switch is not done:

„ Multitasking not enabled (state != Started)

„ to create all tasks first and then start scheduler

„ During interrupt processing (interruptLevel != 0)

„ to speed up interrupt response time

22 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Create all tasks first

23 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University then, start scheduler!

24 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Context Switch

„ Hardware specific, in assembly „ contextSwitch() executed twice!

• Returns non-zero when going to sleep • Returns zero when awaking

25 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Context Switch

26 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Task Synchronization

„ Tasks are not necessarily independent „ Tasks must communicate for solving a larger problem „ To coordinate access to shared data: variables, buffers, device registers, OS must provide one of the following: „ mutex or semaphore, „ message queues, „ monitors

27 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Task Synchronization

„ Mutex

„ a multitasking-aware binary flag

„ grants exclusive access to shared data!

„ setting / clearing are ATOMIC! (guaranteed by OS by disabling interrupts)

28 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Mutex Class

wait for mutex to be cleared and set it

clear a previously set mutex

list of tasks waiting to take mutex

29 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Mutex Constructor

30 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Mutex::take()

31 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Mutex::take() continued

32 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Mutex:: release()

33 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Mutex:: release() continued

34 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Critical Section

„ Mutexes can be used to access shared data without disabling interrupts „ Thus, to enter critical sections we do not need to disable interrupts „ Just use mutexes!!!

35 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Dangers of using Mutex: Deadlock

„ 2 Tasks, each of which require two mutexes: A and B

„ Task 1 has mutex A and requires mutex B,

„ Task 2 has mutex B and requires mutex A, „ Both will wait for each other and a deadlock occurs! „ System will crawl to a standstill „ Need reboot!

36 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Dangers of using Mutex: Priority Inversion „ Low priority task is running and holds a shared resource „ High priority task need that shared resource „ High priority task waits for low priority task to release resource „ Middle priority task activates, preempts low priority task, and runs!!! „ High priority task is waiting for a long time, even forever! „ Space shuttle sent to Mars was lost like this!

37 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University 38 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Priority Inversion

t0 t1 t2 t3 t4 t5 t6

39 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Real-Time Characteristics

„ A late answer is as bad as a wrong one! „ A missed ABS deadline Æ you are in an accident OR you are in a coffin! „ For an embedded OS to be a RTOS (Real- Time OS), it must satisfy 3 goals: „ Deterministic „ Guaranteed Worst-Case Interrupt Latency „ Guaranteed Worst-Case Context Switch Times

40 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University 1. Deterministic RTOS

„ Worst-case execution time of each system call is calculable „ RTOS vendor must publish a data sheet providing minimum, average, & maximum #clock cycles required for each system call „ Different for different processors „ Deterministic on 1 processor Î deterministic on any other processor

41 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University 2. Guaranteed Worst-Case Interrupt Latency in an RTOS „ Interrupt Latency = from interrupt signal arrival to start of ISR „ Finish executing current instruction „ Recognize interrupt type „ Start interrupt (if enabled) „ If interrupts are disabled, the duration must also be considered in the interrupt latency

42 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University 3. Guaranteed Worst-Case Context Switch Time in an RTOS „ Important because it is an overhead of entire system

„ Processor-specific

„ Must be published by RTOS vendor

43 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Selection Process

„ Buy a commercial RTOS, if you can afford to! „ Continuum of functionality, performance, and price! „ Lower end of spectrum:

„ Accelerated Technology’s Nucleus

„ Kadak’s AMX

„ scheduler + a few system calls

44 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Selection Process

„ Higher end of spectrum: „ more functionalities „ stronger guarantees about real-time performance „ costly: $10,000 ~ $50,000 „ royalties: on every copy shipped in ROM „ free technical support, training, development tools „ Wind River Systems’ VxWorks „ Integrated Systems’ pSOS „ Microtec’s VRTX

45 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Selection Process

„ Between these 2 extremes „ more functionalities than scheduler „ reasonable guarantees about real-time performance „ reasonable costs and royalties „ no source code „ extra cost for technical support „ Examples: all other commercial RTOS

46 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University Selection Process

„ Processor Main „ Real-Time Performance Selection „ Budget Criteria „ Compatibility with „ cross-compilers „ debuggers „ development tools „ Differences: processors supported, memory requirements, add-on software modules (network protocol stacks, device drivers, Flash filesystems, …) 47 Embedded Software Design, ©2005, Pao-Ann Hsiung, National Chung Cheng University