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Basic services:
Task management (create, delete, initial activation, state).
Time management (activation, policing, measurement of time intervals).
Task scheduling (decide what jobs to execute in every instant).
Task dispatching (putting jobs in execution).
Resource management (mutexes, semaphores, etc.).
This is a fundamental structure of a kernel. It stores all the relevant information about tasks, which is then used by the kernel to manage their execution.
Common data (not exhaustive):
Task identifier.
Pointer to the code to be executed.
Pointer to the private stack (for context saving, local variables, ...).
Periodic activation attributes (task type (periodic/sporadic), period, initial phase, etc).
Criticality (hard, soft, non real-time).
Other attributes (deadline, priority).
Dynamic execution state and other variables for activation control, e.g. SW timers, absolute deadline, ...
TCBs are often defined in a static array, but are normally structured as linked lists to facilitate operations and searches over the task set.
E.g., the ready queue (list of ready tasks sorted by a given criteria) is maintained as a linked list. These linked lists may be implemented e.g. through indexes. Multiple lists may (and usually do) coexist!
Similarly, the information concerning a semaphore is stored in a Semaphore Control Block (SCB), which contains at least the following three fields:
A counter, which represents the value of the semaphore.
A queue, for enqueueing the tasks blocked on the semaphore.
A pointer to the next SCB, to form a list.
