Hard to define precisely, because
operating systems arose historically as people needed to solve
problems associated with using computers.
Much of operating system history driven by relative cost factors
of hardware and people. Hardware started out fantastically expensive
relative to people and the relative cost has been decreasing ever
since.
Relative costs drive the goals of the operating system.
In the beginning: Expensive Hardware, Cheap People
Goal: maximize hardware utilization.
Now: Cheap Hardware, Expensive People
Goal: make it easy for people to use computer.
In the early days of computer use, computers were
huge machines that are expensive to buy, run and maintain. Computer
used in single user, interactive mode. Programmers interact with the
machine at a very low level - flick console switches, dump cards into
card reader, etc. The interface is basically the raw hardware.
Problem: Code to manipulate external I/O devices. Is very complex, and
is a major source of programming difficulty.
Solution: Build a subroutine library (device
drivers) to manage the interaction with the I/O devices. The
library is loaded into the top of memory and stays there.
This is the first example of something that would grow into
an operating system.
Because the machine is so expensive, it is important to
keep it busy.
Problem: computer idles while programmer sets things up.
Poor utilization of huge investment.
Solution: Hire a specialized person to do setup. Faster
than programmer, but still a lot slower than the machine.
Solution: Build a batch monitor. Store jobs on a disk (spooling), have
computer read them in one at a time and execute them.
Big change in computer usage: debugging
now done offline from print outs and memory dumps.
No more instant feedback.
Problem: At any given time, job is actively using either
the CPU or an I/O device, and the rest of the machine is idle and
therefore unutilized.
Solution: Allow the job to overlap computation and I/O. Buffering
and interrupt handling added to subroutine library.
Problem: one job can't keep both CPU and I/O devices busy.
(Have compute-bound jobs that tend to use only the CPU and
I/O-bound jobs that tend to use only the I/O devices.)
Get poor utilization either of CPU or I/O devices.
Solution: multiprogramming - several jobs share system.
Dynamically switch from one job to another when the running job
does I/O.
Big issue: protection. Don't want one job to affect the results of
another. Memory protection and relocation added to hardware, OS must
manage new hardware functionality. OS starts to become a significant
software system. OS also starts to take up significant resources on
its own.
Phase shift: Computers become much cheaper. People costs become
significant.
Issue:
It becomes important to make computers easier to use and
to improve the productivity of the people. One big productivity
sink: having to wait for batch output (but is this really true?).
So, it is important to run interactively. But computers are still
so expensive that you can't buy one for every person. Solution:
interactive timesharing.
Problem: Old batch schedulers were designed to run a job
for as long as it was utilizing the CPU effectively (in practice,
until it tried to do some I/O). But now, people need reasonable
response time from the computer.
Solution: Preemptive scheduling.
Problem: People need to have their data and programs around while
they use the computer.
Solution: Add file systems for quick access to data.
Computer becomes a repository for data, and people don't have to
use card decks or tapes to store their data.
Problem: The boss logs in and gets terrible response time
because the machine is overloaded.
Solution: Prioritized scheduling. The boss gets more of the
machine than the peons. But, CPU scheduling is just an example
of resource allocation problems. The timeshared machine was full
of limited resources (CPU time, disk space, physical memory space, etc.)
and it became the responsibility of the OS to mediate the allocation
of the resources. So, developed things like disk and physical memory
quotas, etc.
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