Page 61 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Exposure to Computer Disciplines

Notes
(i) Ready: The process needs the CPU and will be running as soon as it is permitted to do
so. However, another process of higher or equal priority is RUNNING and the READY
process must wait until all READY process of higher priority are waiting.
(j) Waiting: The process is waiting for something, for example a message, and has no need
of the CPU.
(k) Process types and scheduling in OS68: The most frequently used task/process scheduling
paradigm of the currently available commercial real-time operating system kernels is the
preemptive priority-based scheme. The OS68 executive is also based on this paradigm
/ 2 / . The scheduling principle is simple: Always executes the highest-priority
process of the ready processes. Preemption in this context means that the scheduler
may (virtually) at any time suspend a running low-priority process for a higher-
priority process.
The OS68 operating system provides four different types of processes: prioritized process,
background processes, interrupt processes and timer processes.
Prioritized processes operate on different priority levels. Processes on higher priority
level always run before processes of lower priority. A prioritized process will run as
long as it desires unless a process of higher priority becomes ready. If processes of equal
priority are ready, one is choose by random. A process can wait for one or more specified
signals to arrive. If none of them is present in its queue the process is swapped out and
other process is allowed to run.
A process is swapped in if all processes on higher priority levels are not ready and the process
itself is or become ready, for example when another process sends a message to it.
Background Processes: These are almost the same as the prioritized processes with one
exception: if there is more than one ready background processes at a priority level. The
running process will be interrupted by the OS after a time and the next background process
will be allowed to run.

Interrupt Processes: An interrupt process is immediately called whenever the corresponding
interrupt occurs, provided that no other interrupt of higher or equal hardware priority is
running. An OS68 interrupt process will never become waiting; it is run from beginning to
the end each time it is activated. It is impossible for an interrupt process to wait for a signal
or make a delay.
OS68 timer interrupt processes are identical to interrupt processes for the manner in
which they are called. For each time-interrupt process, the user selects a time to elapse
between each call of the process. Timer-interrupt process should be short (e.g., execute
during one system tick). Timer-interrupt processes have the same hardware priority as
system timer-interrupts.
Questions:

1. Give a brief introduction about OS68.
2. What are the processes and states involved in OS68?

3.8 Summary

• Computer system can be divided into four components—hardware, operating system, the
application program and the user.

• System call is the mechanism used by an application program to request services from the
operating system.

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