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Anonymous gCmmWV3A. Vimal Raj. Syed Rizwan Hussain. Anonymous eWXehp. Deoram Jadhav. Donny Aryobowo. Mohamed Hejazi. If you use the semaphore to access a shared resource then you would initialize the semaphore to 1 i. Semaphores Finally, if the semaphore allows your application to obtain any one of n identical resources then, you would initialize the semaphore to n. The semaphore would then be used as a counting semaphore. Deadlock or Deadly Embrace A deadlock, also called a deadly embrace, is a situation in which two tasks are each unknowingly waiting for resources held by each other.
If task T1 has exclusive access to resource R1 and task T2 has exclusive access to resource R2, then if T1 needs exclusive access to R2 and T2 needs exclusive access to R1, neither task can continue. They are deadlocked. The simplest way to avoid a deadlock is for tasks to: acquire all resources before proceeding, acquire the resources in the same order, and release the resources in the reverse order.
Message Mailboxes Messages can be sent to a task through kernel services. A Message Mailbox, also called a message exchange, is typically a pointer size variable. Through a service provided by the kernel, a task or an ISR can deposit a message the pointer into this mailbox. Similarly, one or more tasks can receive messages through a service provided by the kernel. Both the sending task and receiving task will agree as to what the pointer is actually pointing to. Message Mailboxes A waiting list is associated with each mailbox in case more than one task desires to receive messages through the mailbox.
A task desiring to receive a message from an empty mailbox will be suspended and placed on the waiting list until a message is received. Typically, the kernel will allow the task waiting for a message to specify a timeout. Message Mailboxes If a message is not received before the timeout expires, the requesting task is made ready-torun and an error code indicating that a timeout has occurred is returned to it. When a message is deposited into the mailbox, either the highest priority task waiting for the message is given the message called prioritybased or the first task to request a message is given the message called First-In-First-Out, or FIFO.
Message Mailboxes Kernel services are typically provided to: Initialize the contents of a mailbox. The mailbox may or may not initially contain a message. Deposit a message into the mailbox POST. Wait for a message to be deposited into the mailbox PEND.
If the mailbox contains a message, the message is extracted from the mailbox. A return code is used to notify the caller about the outcome of the call. Message Mailboxes Message mailboxes can also be used to simulate binary semaphores.
A message in the mailbox indicates that the resource is available while an empty mailbox indicates that the resource is already in use by another task. The pointer would typically be initialized to point to some application specific data structure containing a message.
Creating a mailbox is accomplished by calling OSMboxCreate. Message Queues A message queue is used to send one or more messages to a task. A message mailboxes. Through a service provided by the kernel, a task or an ISR can deposit a message the pointer into a message queue.
Message Queues Generally, the first message inserted in the queue will be the first message extracted from the queue FIFO.
As with the mailbox, a waiting list is associated with each message queue in case more than one task is to receive messages through the queue. A task desiring to receive a message from an empty queue will be suspended and placed on the waiting list until a message is received.
Message Queues Typically, the kernel will allow the task waiting for a message to specify a timeout. If a message is not received before the timeout expires, the requesting task is made ready-to-run and an error code indicating a timeout occurred is returned to it.
When a message is deposited into the queue, either the highest priority task or the first task to wait for the message will be given the message. Initialize the queue. The queue is always assumed to be empty after initialization. Deposit a message into the queue POST. Wait for a message to be deposited into the queue PEND. If the queue contains a message, the message is extracted from the queue.
Each pointer would typically be initialized to point to some application specific data structure containing a message. Tasks may be created before multitasking starts or dynamically by a running task. Task Management When created by a task, if the created task has a higher priority than its creator, the created task is immediately given control of the CPU. A task can return itself or another task to the dormant state by calling OSTaskDel.
Multitasking is started by calling OSStart. Task Management Only one task can be running at any given time. A ready task will not run until all higher priority tasks are either placed in the wait state or are deleted.
When a task pends on an event, the next highest priority task is immediately given control of the CPU. The task is made ready when the event occurs. The occurrence of an event may be signaled by either another task or an ISR. The task thus enters the ISR state. The ISR may make one or more tasks ready to run by signaling one or more events. If a higher priority task is made ready to run by the ISR then the new highest priority task is resumed.
Task Management Otherwise, the interrupted task is resumed. When the task regains control of the CPU the task control block allows the task to resume execution exactly where it left off.
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Flag for inappropriate content. Download now. Related titles. Carousel Previous Carousel Next. Jump to Page. Search inside document. To ensure this, interrupts are typically disabled before the critical code is executed and enabled when the critical code is finished Resource A resource is any entity used by a task. Concepts Shared Resource A shared resource is a resource that can be used by more than one task. Multitasking Multitasking is the process of scheduling and switching the CPU between several tasks.
Preemptive Kernel A preemptive kernel is used responsiveness is important. There are two types of semaphores: binary semaphores counting semaphores As its name implies, a binary semaphore can only take two values: 0 or 1. Message Queues Kernel services are typically provided to: Initialize the queue.
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