Real Time Systems Design And Analysis

Chapter 3 - Real-Time Operating Systems

3.1   REAL-TIME KERNELS

A process1 is an abstraction of a running program and is the logical unit of work
scheduled by the operating system. It is typically represented by a data structure
that contains at least a state of execution, an identity (real-time), attributes (e.g.,
execution time), and the resources associated with it. A thread is a lightweight
process that shares resources with other processes or threads. Each thread must
“reside” within some process and make use of the resources of that process.
Threads that reside within the same process share that processes’ resources.

Real-time operating systems must provide three specific functions with respect
to tasks: scheduling, dispatching, and intercommunication and synchronization.
The kernel of the operating system is the smallest portion that provides for these
functions. A scheduler determines which task will run next in a multitasking
system, while a dispatcher performs the necessary bookkeeping to start that task.
Intertask communication and synchronization assures that the tasks cooperate.
Various layers of operating system functionality and an associated taxonomy are
given in Figure 3.1.

A nanokernel provides simple thread (lightweight process) management. It
essentially provides only one of the three services provided by a kernel, whereas
a microkernel in addition provides for task scheduling. A kernel also provides
for intertask synchronization and communication via semaphores, mailboxes, and
other methods. A real-time executive is a kernel that includes privatized memory
blocks, I/O services, and other complex features. Most commercial real-time
kernels are executives. Finally, an operating system is an executive that provides
for a generalized user interface, security, and a file-management system.

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1Synonymously called “task.”

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