Computer Science Essentials

1. What computers actually do

Everything a computer does — every app, every game, every video call — reduces to the same small loop: take input, store it, transform it, produce output, and repeat. This lesson explains why that four-step loop is enough to build anything, what hardware and software each contribute to it, and how to look at any piece of software and see the loop underneath. It is the foundation for every other lesson in this course, because you cannot reason about programs until you can see what a program is for.

2. Bits, bytes & data

A computer stores numbers, text, images, sound and video as the same substance: patterns of two states. Nothing is stored "as a picture" — it is stored as bits, and only the interpretation decides what they mean. This lesson builds that representation from the ground up: binary arithmetic, text encodings, Unicode, images, colour, audio, files and compression, with worked conversions you can check by hand. Representation is where most real-world bugs live — off-by-one in an index, an emoji that shatters a name field, a colour that shifts between screens — so this lesson is a debugging tool as much as theory.

3. Logic & algorithms

An algorithm is a finite sequence of unambiguous steps that transforms inputs into outputs. This lesson builds that definition carefully — finite, unambiguous, and with a defined input and output — then teaches the tools for writing them: Boolean logic, the three control structures (sequence, selection, iteration), functions as named sub-procedures, pseudocode as a design language, and tracing as a way to test your thinking before you test your code. You will also learn to measure an algorithm's cost, which is the skill that separates a working solution from a good one.

1. Data structures

The way data is organised decides which operations are fast, which are slow and which are even possible. This lesson teaches the fundamental structures — arrays, linked lists, stacks, queues, sets, maps, trees and graphs — as answers to questions you will actually face: do I need order, do I need lookup by key, do I need last-in-first-out, do I need a hierarchy? Each structure gets its definition, its costs, a worked use case, and the mistakes people make choosing it. Choosing well is most of what "algorithm design" means in practice.

2. Memory, programs & processes

A program sitting on a disk is inert; a process is that program loaded into memory and running with resources the operating system has granted it. This lesson explains what changes at that moment: how virtual memory gives each process its own address space, what the stack and the heap are for, what happens on a function call, why memory leaks and segmentation faults occur, and how to read the resource numbers in your task manager. It closes the gap between "the code I wrote" and "the thing that is running".

3. Networks & the internet

The internet is thousands of independently owned networks that agreed on a common way to move packets, so that any machine can reach any other. This lesson builds that system from the bottom: the layered model, IP addressing and routing, TCP's reliability, DNS, and the application protocols on top — then puts it together with a hands-on traceroute and the reading you will need whenever a request fails. Networking is where the abstraction stack becomes visible, because every layer is a real machine doing real work somewhere in the world.