An operating system is a computer's
main program
that handles all I/O ports, controls all interrupts and
timers, implements a file system and so on,
and can run
sub-programs.
These sub-programs use defined communication
protocols to use the operating system's services,
such as all the above mentioned (I/O, interrupts,
file system and so on) and a standardized
user interface ("windows", user I/O, etc.).
They each get their own restricted portion of
RAM
and they are not standalone programs that handle
all the hardware ports and registers themselves,
they are rather like
extensions for the
operating-system main program that can be
added (started) and removed (exited) during runtime.
These sub-programs may have a
user interface
and then are called "executables" (*.
exe),
"applications", or just simply "programs",
or they run in the
background as auxiliary tools
for either the operating system itself or for
other programs, and then may be called
"dynamically linked libraries" (*.
dll),
"auxiliary processes", or other such names,
depending on the operating system.
Operating systems allow various such processes,
auxiliary and applications, to run "simultaneously",
which is called
multi-tasking.
Of course, the program pointer only moves
every clock tick and nothing is really simultaneous,
but the processes proceed in small
bursts in a
roundabout fashion so that all can be started
at the same time and progress in their tasks
seemingly parallel to each other.
The operating system either calls modules
of the processes like functions and needs to
wait until they return, which is called
cooperative multi-tasking, or it uses
a timer to switch between the processes,
each time saving and restoring the
program pointer, which is called
preemptive multi-tasking.
Especially in preemptive multi-tasking,
the operating system gives the processes
each different times to run the next burst,
balanced for their
priority, as some
processes may be doing intensive
calculations while others just idly wait
for some user input to happen.
For processing-intensive times, each process may
actively
request a higher or allow a lower
priority when weighted against others,
and/or the user can make this
setting in
the operating system for each application.
Here's an illustration of
coperative multi-tasking:
OS: Okay, process A, your turn!
P-A: Dumdidumdidumdidum. Done!
OS: Thanks. Now, process B, your turn!
P-B: Diddldididdlidi.Done!
OS: Thanks. Now again process A!
P-A: Dudumdidumdidumdidumdumdum. Done!
OS: Thanks. Now again process B!
...
And now
preemptive multitasking:
OS: Process A, proceed!
P-A: Dumdidumd—
OS: STOP! That's enough. Process B!
P-B: Diddldididdli—
OS: STOP! That's enough. Process A!
P-A: —idumdi—
OS: STOP! That's enough. Process B!
...
Learning Programming with Eas — the Tutorial by Molaskes
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