INVITATION TO CYBERSECURITY 26 running at once. The OS switches between them so quickly that they all appear to be running at the same time, but in reality, they all must share the CPU, so technically only one can be run at a time. Managing processes in an OS is kind of like the plate spinning acts in a circus, where the juggler keeps multiple plates spinning by tending to each plate for a moment and then moving from plate to plate to plate. OSs are constantly switching between processes to keep them all going. There is a limit to how many processes can be kept spinning by the OS—if too many programs are started, the computer will start to become sluggish. Operating systems also manage I/O devices, mediating their access to the CPU. They provide device drivers so that peripheral devices (i.e., outside devices that are plugged into a computer) can have efficient access to the CPU. A device driver is a low-level program that manages communication between a peripheral device and the CPU. For example, every time a character is typed on a keyboard, the keyboard device driver triggers a processor interrupt. A processor interrupt is a signal sent to the CPU to prompt it to handle a new action. In this case, the CPU needs to read the character and take the appropriate action, such as outputting it to the monitor. It is mind-boggling that a computer can do so many operations so quickly amidst such constant interruptions! Operating systems also make available system calls (syscalls) to programs. System calls are OS-defined procedures that allow user programs to exercise some control over OS functionality. They may prompt the OS to carry out an action on processes, files, devices, etc., and they are typically executed with special privileges, so they have well-defined boundaries. Operating systems also perform many basic cybersecurity functions, including managing user logons, protecting user data, and isolating processes from one another. Section 8.2.2 covers access control in OSs. 2.3.4 The Web Browser The web browser, next to the operating system, is the second most important user-facing program that runs on a modern day computer. Web browsers are the program through which we run all the other programs we use online such as email, social media, and apps like Google’s Docs, Sheets, and Slides. It is the main program we use to gain access to the cloud. The cloud is an expression for the computer servers we access online. It is a fitting expression because we do not actually need to know the details of where we are connecting and how it works—we can just consider it a black box. The cloud is yet another example of abstraction that hides enormous complexity. A server is a specialized computer optimized to rapidly process requests for data. Cloud servers are meant to be accessible at all times from any Internet-connected computer. We spend most of our time online using web browsers to access applications and data on servers. A few popular web browsers are Microsoft’s Edge, Mozilla’s Firefox, Google’s Chrome, and Apple’s Safari browsers.
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