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