My previous CS101 post explained what operating systems are, and what services they provide. This post offers a quick tour through some basic operating systems concepts, and explains in more detail how the OS provides certain services.

I’ll describe in turn how the OS manages each of the four basic resources: CPU, disk, memory and network. You’ll probably have heard of some of the concepts before, but not known exactly what they referred to. So now you’ll know!

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It’s been a while, but CS101 now resumes… 

In a previous post I mentioned that modern computer systems consist of layer upon layer of increasingly complex building blocks. In this post I’ll talk briefly about the most basic of these building blocks: the operating system. 

An operating system (OS) is a piece of software that provides a set of common services to all the other software running on a computer. These services primarily involve managing shared resources, notably CPU, disk, memory and network. 

Two classes of OS dominate the desktop world: Microsoft Windows, and UNIX-like OSes. UNIX was an OS originally developed over 40 years ago at Bell Labs. It inspired a host of descendants, and its design lives on to this day, including in popular OSes such as Linux, FreeBSD and Mac OS X.

Why are OS services important? For two main reasons: interface and coordination.

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You suffered admirably through my necessary but dense preliminary discussions of boolean logic, binary arithmetic and memory hierarchy. Now comes the payoff - a series of posts about things you’ve actually heard of. First up: software.

I’m sure you have at least a rough idea of what hardware and software are. In fact, if you’re reading this, you probably know a lot of people who write software for a living. But you may be wondering what it means to “write software” or “run a program”. Or you may still marvel at how it is that we can make a pile of electronic circuits into some magical device that can show us pictures of kittens on skateboards.  Read on to find out more!

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In my first two CS101 posts (here and here) we discussed the basic electronic circuits used to compute logic conditions and basic arithmetic. At the end of my last post I alluded to a third element we need before we can construct something worth calling a ‘computer’. That element is memory.

Memory gives us the ability to have the current computation be influenced by the result of past computation. It’s what allows us to compose sequences of basic operations to produce ad-hoc, complex computations. These sequences of instructions are called programs.

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In my debut CS101 post I discussed Boolean algebra, and how any Boolean function can be modeled by a real-life electronic circuit. In this post we’ll explore how this fact provides computers with the ability to do arithmetic.

Natural Numbers

To do arithmetic, you first need to be able to represent numbers. The most basic numbers are the natural numbers, the whole numbers we use for counting things.  To write down a counting number we can simply represent it as a list of ‘things’ of that size. Say we pick the hash symbol # to represent a ‘thing’, then we can write down the counting numbers as #, ##, ###, ####, ##### and so on. Of course we’d also need some symbol, like 0, to represent “no things”. 

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Several people have suggested that I write a series of “CS101” posts explaining computer science and software engineering fundamentals, unrelated to anything in the news cycle. This idea appealed to my inner didact, so I’ll try a few posts along those lines and see what responses I get. Feel free to comment and/or suggest topics.

Hopefully, reading these posts will provide non-engineers in the tech industry with a better sense of what goes on under the hood, of what their engineering colleagues do, and of why software engineering is such a fascinating, intricate discipline. To any engineers reading this: you’ll have to forgive my simplification of many concepts, in the interest of clarity and brevity.

I’ll kick off, appropriately, by discussing the lowest level concept in Computer Science…

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