Chapter 2 Real-Time Systems Concepts Foreground/Background ...

More documents

Recommendations

Info

Assigning Task Priorities• Rate Monotonic Scheduling (RMS)– The highest rate of execution are given the highest priority• RMS makes a number of assumptions:– All tasks are periodic (they occur at regular intervals).– Tasks do not synchronize with one another, share resources, orexchange data.– The CPU must always execute the highest priority task that is ready torun. In other words, preemptive scheduling must be used.• If meet the following inequality equation, all task HARD real-timedeadlines will be met∑iETii⎛≤ n × ⎜2⎝1n⎟ ⎞−1⎠CPU utilization of all timecriticaltasks should be less than70%Other 30% can be used by nontimecritical tasks2-23Mutual Exclusion• Multiple tasks access same area (critical section) mustensure that each task has exclusive access to the data toavoid contention and data corruption• The method of exclusive access– Disabling interrupts– Performing test-and-set operations– Disabling scheduling– Using semaphores2-24
• Disabling and enabling interruptsDisable interrupts;Access the resource (read/write from/to variables);Reenable interrupts;• uc/OS-II provides two macros to disable/enable interruptvoid Function (void){OS_ENTER_CRITICAL();.. /* You can access shared data in here */.OS_EXIT_CRITICAL();}• Not to disable interrupts for too long– Affect the response of your system to interrupts (interrupt latency)2-25• Test-and-Set (TAS)Disable interrupts;if (‘Access Variable ’ is 0) {Set variable to 1;Reenable interrupts;Access the resource;Disable interrupts;Set the ‘Access Variable’back to 0;Reenable interrupts;} else {Reenable interrupts;/* You don’t have access to the resource, try back later; */}• Some processor actually implement a TAS operation inhardware (e.g., 68000 family)2-26
Page 1 and 2: Chapter 2Real-Time Systems Concepts
Page 3 and 4: • Resource- EX: I/O, CPU, Memory,
Page 5 and 6: • Foreground/background programmi
Page 7 and 8: Scheduler• Also called the dispat
Page 9 and 10: Figure 2.5 Preemptive kernelLow Pri
Page 11: Figure 2.7 Priority Inversion probl
Page 15 and 16: • Accessing shared data by obtain
Page 17 and 18: BUF * BufReq(void){BUF *ptr;Acquire
Page 19 and 20: Event Flags (uCOS-II does not suppo
Page 21 and 22: Message queues• Is used to send o
Page 23 and 24: Figure 2.21 non-preemptive kernelIn
Page 25 and 26: Clock Tick• A special interrupt t
Page 27: • Reduce the execution jitter of

Chapter 2 Real-Time Systems Concepts Foreground/Background ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?