Real Time & Embedded Systems. Final Exam - Review

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Catherine Webb
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1 Real Time & Embedded Systems Final Exam - Review

2 Final Exam Review Topics Finite State Machines RTOS Context switching Process states Mutex - purpose and application Blocking versus non-blocking Synchronous vs. Asynchronous events C Language signed versus unsigned bit manipulation

3 Finite State Machines States: a set of conditions for a machine that exists for a non-trivial amount of time. The meaning of nontrivial is project dependent. Events: a set of actions that occur effectively instantly in the context of the project. A state may have Entry actions executed when entering a state Exit actions executed when exiting the state, Other actions executed each time the state is run An event triggers the FSM to transition to a new state. 3

4 State Machine Example Elevator Door State Transition Diagram Open button pressed Event State Open Closed Close button pressed The states are a set of conditions that exists for a non-trivial amount of time. Are these the only 2 states for the elevator doors?

5 Elevator Door FSM #2 Closing Close sensor Close button pressed Open Open button pressed Person detected Closed Open button pressed Open sensor Opening

6 Event State Elevator Door State Transition Table State Transition Table close button open button opened sensor closed sensor Open Closing Closed Opening Opening Closing Open Closing Opening Closed

7 Elevator Door FSM cont. You could add more events and states Close timer An open elevator closes its doors after a while Floor buttons probably important in the operation of an elevator Safety why did the close sensor fire when opening the door? If the elevator is moving, can you open the doors?

8 RTOS Topics

9 Multitasking Executive (RTOS) An RTOS is used in embedded systems to allow multiple tasks to run at the same time. An RTOS schedules these tasks in real-time based on their current priority chosen based on a scheduling strategy or metric. The primary function of a multitasking executive is to schedule the tasks for execution in a manner that ensures the meeting of system operational requirements. 9

10 Multitasking Executive (RTOS) 10

11 Context Switching Task synchronization involves suspension and resumption of a task in accordance with the status of other tasks known as context switching The context is the minimal information about a task that must be saved before suspension of the task so that it may be resumed at a later instant (register contents, I/O, memory management, etc.) 11

12 Context Switching Stored in protected memory in a data structure called the Task Control Block (TCB) Context switching takes time that is overhead from the viewpoint of the application program. The amount of time required for context switching is one of important benchmarks used to judge the efficiency of a multitasking executive 12

13 RTOS Process States Task (or tasks if there are multiple processors) with the highest priority will be scheduled for execution unless the execution of the task is blocked. A task is said to be blocked if the current state of program execution requires that it not be allowed to execute. For example, the task may be waiting for some lower priority task to provide it with needed data. 13

14 Process States A process state diagram as a partially defined finite state machine Executing Ready Preempted Task with Highest Priority Resource Missing Resource Released Suspended Delete Task Schedule Task No Longer Needed Aborted Dormant Terminated 14

15 Mutex Threads operating in the same memory space have access to the same data structures & resources. A mutex (mutual exclusion object) is an software object used to control access to shared resources. A mutex is created for each resource requiring exclusive access. pthread_mutex_init() A thread requests access to the shared resource before using it (it may block) thread_mutex_lock() The thread releases the mutex before moving on pthread_mutex_unlock() A mutex is non-recursive 17

16 Events An event is any occurrence that changes the PC nonsequentially it changes the flow of control. Synchronous events: - (synced with code execution) Occur at predictable times. Asynchronous events: - (not synced with execution) Occur at unpredictable times. Periodic events: Repeat at precise, regular times. Aperiodic events: Repeat, but do not occur at regular periods.

17 Logical and Bitwise Operators

18 Logical Operators A logical operator is used to combine 2 or more conditions in an expression. Logical AND - && Operator && returns true when both the conditions in consideration are true; else false Logical OR - Operator returns true when either or both the conditions in consideration are true; else false Logical NOT -! Operator! returns true when either or both the conditions in consideration are true; else false Logical XOR In the Boolean sense, this is just!= (not equal)

19 Logical examples int a = 10, b = 4, c = 10, d = 20; // logical AND example if (a > b && c == d) printf("a is greater than b AND c is equal to d\n"); // doesn t print because c!= d // logical OR example if (a > b c == d) printf("a is greater than b OR c is equal to d\n"); // NOTE: because a>b, the clause c==d is not evaluated // logical NOT example if (!a) // a is true because it is not 0 printf("a is zero\n"); // doesn t print because a!= 0

20 C Bitwise Operators C has 6 operators for performing bitwise operations on integers Operator Meaning & Bitwise AND Result is 1 if both bits are 1 Bitwise OR Result is 1 if either bit is 1 ^ Bitwise XOR Result is 1 if both bits are different >> Right shift LSB is lost. If signed data, MSB is sign extended! If unsigned, filled with 0 s. << Left shift MSB is lost. 0 s shifted into LSB. ~ Ones complement The logical invert, same as NOT

21 Bitwise Boolean examples char j = 11; // = 11 char k =-14; // = -14 Bitwise Boolean Operators char m = j & k; // = 2 char n = j k; // = -5 uchar p = j ^ k; // = 249 NOTE: This is a logical (not Boolean) operation bool q = j && k; // true == 1 bool q = 0 && k; // false == 0

22 Shifting and Inversion

23 Shifting Shifting char j = 11; // = 11 char k = j<<1; // = 22 (j*2) char m = j>>1; // = 5 (j/2) char x = -4; // = -4 char y = x>>1; // = -2 (x/2) Note the sign extensions because char is signed!

24 Shifting char s1, s2, s3, s4; s1=-11; // s2=s1>>1; // s3=117; // s4=s3>>1; // // sign extension! unsigned char u1, u2; u1=245; // u2=u1>>1; // // no sign extension!

25 Inversion Logical invert char j = 11; // j = = 11 char k = ~j; // k = = 244 // Note: j + k = 255

Basic operators, Arithmetic, Relational, Bitwise, Logical, Assignment, Conditional operators. JAVA Standard Edition

Basic operators, Arithmetic, Relational, Bitwise, Logical, Assignment, Conditional operators JAVA Standard Edition Java - Basic Operators Java provides a rich set of operators to manipulate variables.