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

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Intertask Communication• A task or an ISR to communicate information to anothertask• There are two ways of intertask communication– Through global data (disable/enable interrupts or using semaphore)• Task can only communicate information to an ISR by usingglobal variables• Task can not aware any global variable is changed (unlessusing semaphore or task periodical polling)– Sending messages• Message mailbox or message queue (task can be aware of thedata change)2-39Message MailboxesMailboxTASKPOST PEND10Waiting listTASK• A task desiring a message from an empty mailbox is suspendedand placed on the waiting list until a message is received• Kernel allows the task waiting for a message to specify a timeout• When a message is deposited into the mailbox, how to select atask from the waiting list– Priority based– FIFO2-40
Message queues• Is used to send one or more messages to a task• Is basically an array of mailboxes• The first message inserted in the queue will be the firstmessage extracted from the queue (FIFO) or Last-In_first-Out (LIFO)InterruptQueueISRPOST10PEND0TASK2-41Interrupts• When an interrupt is recognized, the CPU saves– Return address (interrupted task)– Flags• Jump to Interrupt Service Routine (ISR)• Upon completion of the ISR, the program returns to– <strong>Background</strong> for a <strong>Foreground</strong>/background system– The interrupted task for a non-preemptive kernel– The highest priority task ready to run for a preemptive kernelTIMETASKISR #1ISR #2ISR #3Interrupt #1Interrupt #2Interrupt #3Interrupt Nesting2-42
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 and 12: Figure 2.7 Priority Inversion probl
Page 13 and 14: • Disabling and enabling interrup
Page 15 and 16: • Accessing shared data by obtain
Page 17 and 18: BUF * BufReq(void){BUF *ptr;Acquire
Page 19: Event Flags (uCOS-II does not suppo
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 ...

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