OPS201 - Introduction to Operating Systems

What is an Operating System?

An operating system (OS) does exactly what the name states: It is software that operates a computer system!

Not all computers have had or required an operating system. Particularly simple computers, including the earliest models of both business computers and microcomputers, as well as very simple embedded computers, do not require an operating system. The computer embedded into a simple mass-market microwave oven, for example, only ever runs one program, does not have communications capabilities, does not multitask, and is nevery updated, and therefore has no operating system.

However, virtually all contemporary computer systems, including embedded devices, are sufficiently complex that they are equipped with an operating system.

What does an Operating System do?

An operating system performs four main functions:

Management and Separation of Resources

Think of the specifications that were advertised when you bought your last computer (or smartphone):

• multiple CPU cores
• several gigabytes of memory (RAM)
• storage in the gigabytes-to-terabytes range
• a display with a particular resolution
• various peripherals, such as cameras, speakers, and so forth

All of these are system resources. The operating system manages these resources to ensure that they are used effectively, and to ensure that there are no conflicts over their use.

As requested by the user(s), or as triggered by other factors such as the time of day or operational requirements, the OS will create processes -- running copies of computer programs. Each process is allocated compute resources by being permitted to run on one or more computer cores. To run a large number of processes, the OS will switch between them, stopping one process and starting another as needed to ensure that all of the processes get a fair share of system resources (which may not always be an equal share).

The operating system will allocate the available memory to processes and to the operating system's internal operations. The OS will program the system hardware to ensure that no process can overwrite memory allocated to another process.

The OS will also allocate storage space to various files as they are created and extended, and deallocate space when those files are truncated (shortened) or deleted. The space allocated to one file will be protected from use in other files.

Display space is similarly shared; the operating system will ensure that multiple programs can each display in their own windows, but prevent them from destroying graphics being displayed by other windows.

Peripheral devices are managed in different ways according to the characteristics of the device. For example, sound output from multiple processes will be combined for simultaneous output to the speakers (or other output devices). However, when a process accesses a printer, output from that processes is collected into a document, and the documents are queued for sequential printing, because simultaneous access would result in garbled output.

Security Enforcement

It’s important to keep information private in some contexts, and to share information in other contexts. The operating system is responsible for enforcing security rules. For example, on a smartphone, a social media app shouldn’t be able to access data from a banking app, and on a cloud server, one customer shouldn’t be able to access another customer’s data. However, multiple smartphone apps might be permitted to access a photo album, and a company employee might need to view a report generated on a server from multiple customers’ data.

The operating system is responsible for enforcing the security policy.

Hardware Abstraction

There are many different types of devices that perform similar functions, and multiple ways that these devices can be connected. For example:

• A keyboard may be connected via a USB connection, or a wireless Bluetooth connection.
• A mouse may be connected via a wired USB, wireless USB, or wireless Bluetooth connection. But there are also other types of devices which can provide the same capability of allowing the user to interact with the display, including trackpads, trackballs, and touchscreens.
• Sound may be played over built-in speakers, an external analog speaker, an analog headset, a digital speaker or headset with a USB connection, a Bluetooth speaker, earbuds, or headset, or played through a television connected via HDMI.

The operating system also abstracts hardware details. This means that programs can access devices in a general way without having to be programmed to individually deal with each type of device that may be used. This enables a program to request keyboard input or play sounds without regard to the details of the specific hardware available.

Maintaining the Programming Model

The operating system, computer hardware, and development tools (compiler, linker, and so forth) work together to present the “programming model” – a conceptual framework which software developers use when creating software. For example, several different application program may be designed to occupy the same area of memory; obviously, this presents a conflict when these applications are used at the same time, so the operating system works with the computer’s hardware to load the applications into different areas of physical memory, and then use the computer’s virtual memory capabilities to make each program appear to be loaded into the region of memory for which it was written.