The operating system defines our
computing experience. It's the first software we see when we turn on the
computer, and the last software we see when the computer is turned off. It's
the software that enables all the programs we use. The operating system
organizes and controls the hardware on our desks and in our hands, yet most
users can't say with any certainty precisely what it is that the operating
In this edition of
HowStuffWorks, we'll tell you what a piece of software must do to
be called an operating system, and show you how the operating system works
to turn a collection of hardware into a powerful computing tool!
The Bare Bones
It's important to realize that not all computers have operating systems. The
computer that controls the
in your kitchen, for example, doesn't need an operating system. It has one
set of relatively simple tasks to perform, very simple input and output
methods (a keypad and an
LCD screen), and simple, never-changing hardware to control. For a
computer like this, an operating system would be unnecessary baggage, adding
complexity where none is required. Instead, the computer in a microwave oven
simply runs a single program all the time.
For computer systems that go beyond the complexity of the microwave,
however, an operating system can be the key to greater operating efficiency
and easier application development. All
have operating systems. The most common are the
Windows family of operating systems, the
UNIX family of operating systems and the
Macintosh operating systems. There are hundreds of other operating
systems available for special-purpose applications, including
specializations for mainframes, robotics, manufacturing, real-time control
systems and so on.
At the simplest level, an operating system does two things:
- It manages the hardware and software resources of the computer system.
These resources include such things as the processor, memory, disk space,
- It provides a stable, consistent way for applications to deal with the
hardware without having to know all the details of the hardware.
The first task, managing the hardware and software resources, is
very important, as various
input methods compete for the attention of the
processing unit (CPU) and demand memory, storage and input/output (I/O)
bandwidth for their own purposes. In this capacity, the operating system
plays the role of the good parent, making sure that each application gets
the necessary resources while playing nicely with all the other
applications, as well as husbanding the limited capacity of the system to
the greatest good of all the users and applications.
The second task, providing a consistent application interface, is
especially important if there is to be more than one of a particular type of
computer using the operating system, or if the hardware making up the
computer is ever open to change. A consistent application program interface
(API) allows a software developer to write an application on one
computer and have a high level of confidence that it will run on another
computer of the same type, even if the amount of memory or the quantity of
storage is different on the two machines. Even if a particular computer is
unique, an operating system can ensure that applications continue to run
when hardware upgrades and updates occur, because the operating system and
not the application is charged with managing the hardware and the
distribution of its resources. Windows 98 is a great example of the
flexibility an operating system provides. Windows 98 runs on hardware from
thousands of vendors. It can accommodate thousands of different
and special peripherals in any possible combination.
Within the broad family of operating systems, there are generally four
types, categorized based on the types of computers they control and the sort
of applications they support. The broad categories are:
- Real-time operating system (RTOS) - Real-time operating systems
are used to control machinery, scientific instruments and industrial
systems. An RTOS typically has very little user-interface capability, and
no end-user utilities, since the system will be a "sealed box" when
delivered for use. A very important part of an RTOS is managing the
resources of the computer so that a particular operation executes in
precisely the same amount of time every time it occurs. In a complex
machine, having a part move more quickly just because system resources are
available may be just as catastrophic as having it not move at all because
the system is busy.
- Single-user, single task - As the name implies, this operating
system is designed to manage the computer so that one user can effectively
do one thing at a time. The Palm OS for Palm
is a good example of a modern single-user, single-task operating system.
- Single-user, multi-tasking - This is the type of operating
system most people use on their
computers today. Windows 98 and the MacOS are both examples of an
operating system that will let a single user have several programs in
operation at the same time. For example, it's entirely possible for a
Windows user to be writing a note in a word processor while downloading a
file from the Internet while printing the text of an
- Multi-user - A multi-user operating system allows many
different users to take advantage of the computer's resources
simultaneously. The operating system must make sure that the requirements
of the various users are balanced, and that each of the programs they are
using has sufficient and separate resources so that a problem with one
user doesn't affect the entire community of users. Unix, VMS, and
mainframe operating systems, such as MVS, are examples of multi-user
It's important to differentiate here between multi-user operating systems
and single-user operating systems that support
Windows 2000 and Novell Netware can each support hundreds or thousands of
networked users, but the operating systems themselves aren't true multi-user
operating systems. The system administrator is the only "user" for Windows
2000 or Netware. The network support and all of the remote user logins the
network enables are, in the overall plan of the operating system, a program
being run by the administrative user.
With the different types of operating systems in mind, it's time to look
at the basic functions provided by an operating system.
When the power
to a computer is turned on, the first program that runs is usually a set
of instructions kept in the computer's read-only memory (ROM)
that examines the system hardware to make sure everything is functioning
properly. This power-on self test (POST) checks the
and basic input-output
systems (BIOS) for errors and stores the result in a special memory
location. Once the POST has successfully completed, the software loaded in
ROM (sometimes called firmware) will begin to activate the computer's
disk drives. In most modern computers, when the computer activates the hard
disk drive, it finds the first piece of the operating system: the bootstrap
The bootstrap loader is a small program that has a single
function: It loads the operating system into memory and allows it to begin
operation. In the most basic form, the bootstrap loader sets up the small
driver programs that interface with and control the various hardware
subsystems of the computer. It sets up the divisions of memory that hold the
operating system, user information and applications. It establishes the data
structures that will hold the myriad signals, flags and semaphores that are
used to communicate within and between the subsystems and applications of
the computer. Then it turns control of the computer over to the operating
The operating system's tasks, in the most general sense, fall into six
- Processor management
- Memory management
- Device management
- Storage management
- Application interface
- User interface
While there are some who argue that an operating system should do more
than these six tasks, and some operating-system vendors do build many more
utility programs and auxiliary functions into their operating systems, these
six tasks define the core of nearly all operating systems. Let's look at the
tools the operating system uses to perform each of these functions.
The heart of managing the processor comes down to two related issues:
- Ensuring that each process and application receives enough of the
processor's time to function properly
- Using as many processor cycles for real work as is possible
The basic unit of software that the operating system deals with in
scheduling the work done by the processor is either a process or a
thread, depending on the operating system.
It's tempting to think of a process as an application, but that gives an
incomplete picture of how processes relate to the operating system and
hardware. The application you see (word processor or spreadsheet or game)
is, indeed, a process, but that application may cause several other
processes to begin, for tasks like communications with other devices or
other computers. There are also numerous processes that run without giving
you direct evidence that they ever exist. A process, then, is software that
performs some action and can be controlled -- by a user, by other
applications or by the operating system.
It is processes, rather than applications, that the operating system
controls and schedules for execution by the CPU. In a single-tasking system,
the schedule is straightforward. The operating system allows the application
to begin running, suspending the execution only long enough to deal with
interrupts and user input. Interrupts are special signals sent by
hardware or software to the CPU. It's as if some part of the computer
suddenly raised its hand to ask for the CPU's attention in a lively meeting.
Sometimes the operating system will schedule the priority of processes so
that interrupts are masked -- that is, the operating system will
ignore the interrupts from some sources so that a particular job can be
finished as quickly as possible. There are some interrupts (such as those
from error conditions or problems with memory) that are so important that
they can't be ignored. These non-maskable interrupts (NMIs) must be
dealt with immediately, regardless of the other tasks at hand.
While interrupts add some complication to the execution of processes in a
single-tasking system, the job of the operating system becomes much more
complicated in a multi-tasking system. Now, the operating system must
arrange the execution of applications so that you believe that there are
several things happening at once. This is complicated because the CPU can
only do one thing at a time. In order to give the appearance of lots of
things happening at the same time, the operating system has to switch
between different processes thousands of times a second. Here's how it
- A process occupies a certain amount of RAM. It also makes use of
registers, stacks and queues within the CPU and operating-system memory
- When two processes are multi-tasking, the operating system allots a
certain number of CPU execution cycles to one program.
- After that number of cycles, the operating system makes copies of all
the registers, stacks and queues used by the processes, and notes the
point at which the process paused in its execution.
- It then loads all the registers, stacks and queues used by the second
process and allows it a certain number of CPU cycles.
- When those are complete, it makes copies of all the registers, stacks
and queues used by the second program, and loads the first program.
All of the information needed to keep track of a process when switching
is kept in a data package called a process control block. The process
control block typically contains:
- An ID number that identifies the process
- Pointers to the locations in the program and its data where processing
- Register contents
- States of various flags and switches
- Pointers to the upper and lower bounds of the memory required for the
- A list of files opened by the process
- The priority of the process
- The status of all I/O devices needed by the process
When the status of the process changes, from pending to active, for
example, or from suspended to running, the information in the process
control block must be used like the data in any other program to direct
execution of the task-switching portion of the operating system.
This process swapping happens without direct user interference, and each
process gets enough CPU cycles to accomplish its task in a reasonable amount
of time. Trouble can come, though, if the user tries to have too many
processes functioning at the same time. The operating system itself requires
some CPU cycles to perform the saving and swapping of all the registers,
queues and stacks of the application processes. If enough processes are
started, and if the operating system hasn't been carefully designed, the
system can begin to use the vast majority of its available CPU cycles to
swap between processes rather than run processes. When this happens, it's
called thrashing, and it usually requires some sort of direct user
intervention to stop processes and bring order back to the system.
One way that operating-system designers reduce the chance of thrashing is
by reducing the need for new processes to perform various tasks. Some
operating systems allow for a "process-lite," called a thread, that
can deal with all the CPU-intensive work of a normal process, but generally
does not deal with the various types of I/O and does not establish
structures requiring the extensive process control block of a regular
process. A process may start many threads or other processes, but a thread
cannot start a process.
So far, all the scheduling we've discussed has concerned a single CPU. In
a system with two or more CPUs, the operating system must divide the
workload among the CPUs, trying to balance the demands of the required
processes with the available cycles on the different CPUs. Asymmetric
operating systems use one CPU for their own needs and divide application
processes among the remaining CPUs. Symmetric operating systems
divide themselves among the various CPUs, balancing demand versus CPU
availability even when the operating system itself is all that's running.
Even if the operating system is the only software with execution needs,
the CPU is not the only resource to be scheduled. Memory management
is the next crucial step in making sure that all processes run smoothly.
Memory and Storage
When an operating system manages the
memory, there are two broad tasks to be accomplished:
- Each process must have enough memory in which to execute, and it can
neither run into the memory space of another process nor be run into by
- The different types of memory in the system must be used properly so
that each process can run most effectively.
The first task requires the operating system to set up memory
boundaries for types of software and for individual applications.
As an example, let's look at an imaginary system with 1
kilobytes) of RAM. During the boot process, the operating system of our
imaginary computer is designed to go to the top of available memory and then
"back up" far enough to meet the needs of the operating system itself. Let's
say that the operating system needs 300 kilobytes to run. Now, the operating
system goes to the bottom of the pool of RAM and starts building up with the
various driver software required to control the hardware subsystems of the
computer. In our imaginary computer, the drivers take up 200 kilobytes. So
after getting the operating system completely loaded, there are 500
kilobytes remaining for application processes.
When applications begin to be loaded into memory, they are loaded in
block sizes determined by the operating system. If the block size is 2
every process that is loaded will be given a chunk of memory that is a
multiple of 2 kilobytes in size. Applications will be loaded in these fixed
block sizes, with the blocks starting and ending on boundaries established
by words of 4 or 8 bytes. These blocks and boundaries help to ensure that
applications won't be loaded on top of one another's space by a poorly
calculated bit or two. With that ensured, the larger question is what to do
when the 500-kilobyte application space is filled.
In most computers, it's possible to add memory beyond the original
capacity. For example, you might expand RAM from 1 to 2 megabytes. This
works fine, but tends to be relatively expensive. It also ignores a
fundamental fact of computing -- most of the information that an application
stores in memory is not being used at any given moment. A processor can only
access memory one location at a time, so the vast majority of RAM is unused
at any moment. Since disk space is cheap compared to RAM, then moving
information in RAM to hard disk can greatly expand RAM space at no cost.
This technique is called virtual memory management.
Disk storage is only one of the memory types that must be managed by the
operating system, and is the slowest. Ranked in order of speed, the types of
memory in a computer system are:
- High-speed cache - This is fast, relatively small amounts of
memory that are available to the CPU through the fastest connections.
Cache controllers predict which pieces of data the CPU will need next and
pull it from main memory into high-speed
cache to speed
up system performance.
- Main memory - This is the
RAM that you see
measured in megabytes when you buy a computer.
- Secondary memory - This is most often some sort of rotating
magnetic storage that keeps applications and data available to be used,
and serves as
under the control of the operating system.
The operating system must balance the needs of the various processes with
the availability of the different types of memory, moving data in blocks
(called pages) between available memory as the schedule of processes
The path between the operating system and virtually all hardware not on the
motherboard goes through a special program called a driver. Much
of a driver's function is to be the translator between the electrical
signals of the hardware subsystems and the high-level programming languages
of the operating system and application programs. Drivers take data that the
operating system has defined as a file and translate them into streams of
bits placed in specific locations on storage devices, or a series of
laser pulses in a
Because there are such wide differences in the hardware controlled
through drivers, there are differences in the way that the driver programs
function, but most are run when the device is required, and function much
the same as any other process. The operating system will frequently assign
high-priority blocks to drivers so that the hardware resource can be
released and readied for further use as quickly as possible.
One reason that drivers are separate from the operating system is so that
new functions can be added to the driver -- and thus to the hardware
subsystems -- without requiring the operating system itself to be modified,
recompiled and redistributed. Through the development of new hardware device
drivers, development often performed or paid for by the manufacturer of the
subsystems rather than the publisher of the operating system, input/output
capabilities of the overall system can be greatly enhanced.
Managing input and output is largely a matter of managing queues
and buffers, special storage facilities that take a stream of bits
from a device, perhaps a
keyboard or a
hold those bits, and release them to the CPU at a rate slow enough for the
CPU to cope with. This function is especially important when a number of
processes are running and taking up processor time. The operating system
will instruct a buffer to continue taking input from the device, but to stop
sending data to the CPU while the process using the input is suspended.
Then, when the process needing input is made active once again, the
operating system will command the buffer to send data. This process allows a
keyboard or a modem
to deal with external users or computers at a high speed even though there
are times when the CPU can't use input from those sources.
Managing all the resources of the computer system is a large part of the
operating system's function and, in the case of real-time operating systems,
may be virtually all the functionality required. For other operating
systems, though, providing a relatively simple, consistent way for
applications and humans to use the power of the hardware is a crucial part
of their reason for existing.
Interface to the World
Just as drivers provide a way for applications to make use of hardware
subsystems without having to know every detail of the hardware's operation,
application program interfaces (APIs) let application programmers use
functions of the computer and operating system without having to directly
keep track of all the details in the CPU's operation. Let's look at the
example of creating a hard disk file for holding data to see why this can be
A programmer writing an application to record data from a scientific
instrument might want to allow the scientist to specify the name of the file
created. The operating system might provide an API function named
MakeFile for creating files. When writing the program, the programmer
would insert a line that looks like this:
In this example, the instruction tells the operating system to create a
file that will allow random access to its data (1), will have a name typed
in by the user (%Name), and will be a size that varies depending on how much
data is stored in the file (2). Now, let's look at what the operating system
does to turn the instruction into action.
- The operating system sends a query to the disk drive to get the
location of the first available free storage location.
- With that information, the operating system creates an entry in the
file system showing the beginning and ending locations of the file, the
name of the file, the file type, whether the file has been archived, which
users have permission to look at or modify the file, and the date and time
of the file's creation.
- The operating system writes information at the beginning of the file
that identifies the file, sets up the type of access possible and includes
other information that ties the file to the application.
In all of this information, the queries to the disk drive and addresses
of the beginning and ending point of the file are in formats heavily
dependent on the manufacturer and model of the disk drive.
Because the programmer has written her program to use the API for disk
storage, she doesn't have to keep up with the instruction codes, data types,
and response codes for every possible hard disk and tape drive. The
operating system, connected to drivers for the various hardware subsystems,
deals with the changing details of the hardware -- the programmer must
simply write code for the API and trust the operating system to do the rest.
APIs have become one of the most hotly contested areas of the computer
industry in recent years. Companies realize that programmers using their API
will ultimately translate into the ability to control and profit from a
particular part of the industry. This is one of the reasons that so many
companies have been willing to provide applications like readers or viewers
to the public at no charge. They know consumers will request that programs
take advantage of the free readers, and application companies will be ready
to pay royalties to allow their software to provide the functions requested
by the consumers.
Just as the API provides a consistent way for applications to use the
resources of the computer system, a user interface (UI) brings structure to
the interaction between a user and the computer. In the last decade, almost
all development in user interfaces has been in the area of the graphical
user interface (GUI), with two models, Apple's Macintosh and Microsoft's
Windows, receiving most of the attention and gaining most of the market
share. There are other user interfaces, some graphical and some not, for
other operating systems.
Unix, for example, has user interfaces called shells that present
a user interface more flexible and powerful than the standard operating
system text-based interface. Programs such as the Korn Shell and the C Shell
are text-based interfaces that add important utilities, but their main
purpose is to make it easier for the user to manipulate the functions of the
operating system. There are also graphical user interfaces, such as
X-Windows and Gnome, that make Unix and Linux more like Windows and
Macintosh computers from the user's point of view.
It's important to remember that in all of these examples, the user
interface is a program or set of programs that sits as a layer above the
operating system itself. The same thing is true, with somewhat different
mechanisms, of both Windows and Macintosh operating systems. The core
operating-system functions, the management of the computer system, lie in
the kernel of the operating system. The display manager is
separate, though it may be tied tightly to the kernel beneath. The ties
between the operating-system kernel and the user interface, utilities and
other software define many of the differences in operating systems today,
and will further define them in the future.
One question concerning the future of operating systems revolves around the
ability of a particular philosophy of software distribution to create an
operating system useable by corporations and consumers together.
Linux, the operating system created and distributed according to the
open source, could have a significant impact on the operating
system in general. Most operating systems, drivers and utility programs are
written by commercial organizations that distribute executable versions of
their software -- versions that can't be studied or altered. Open source
requires the distribution of original source materials that can be studied,
altered and built upon, with the results once again freely distributed.
The continuing growth of the Internet and the proliferation of computers
that aren't standard desktop or laptop machines means that operating systems
will change to keep pace, but the core management and interface functions
will continue, even as they evolve.