Sefism CS Core Subjects Cheat Sheet

What this cheat sheet is

The CS core subjects cheat sheet is the revision companion to the CS core subjects roadmap. It covers how computers run programs, OOP, operating systems, networks, databases, software engineering and theory, in 72 concepts across eight chapters. Each concept is a short definition with why it matters, often with an example or a pitfall.

The sheet's own instruction is the right way to use it: check you can explain each idea in your own words, then go back to the roadmap topic for anything you cannot.

One edition

Unlike the language-based sheets, this one has a single edition. The subjects are the same whatever language you write in.

Chapter by chapter

1. How a computer runs a program (10 concepts). Binary and hexadecimal, two's complement, character encoding, floating point, logic gates, CPU, registers and ALU, fetch, decode, execute, the memory hierarchy, stack and heap, and compiler, interpreter and JIT.
2. Object-oriented programming and design (9 concepts). Class and object, coupling and cohesion, encapsulation, inheritance, polymorphism, abstraction and interfaces, composition, SOLID and what a design pattern is.
3. Operating systems (11 concepts). Kernel and system calls, process, thread, context switch, CPU scheduling, virtual memory, paging and page faults, race condition, mutex and semaphore, deadlock and file system.
4. Computer networks (9 concepts). OSI and TCP/IP models, IP address and subnet, routing and NAT, TCP, UDP, port and socket, DNS, HTTP, and TLS and HTTPS.
5. Databases: theory and internals (9 concepts). Relational model, primary and foreign keys, the ER model, normalisation, transaction, ACID, isolation levels, index and query plan.
6. Software engineering (9 concepts). Software development life cycle, Agile, Scrum and Kanban, requirements, UML diagrams, version control, testing levels, test case design, code review and refactoring, and technical debt.
7. Theory worth knowing (9 concepts). Big O, logic and proof, sets, functions and relations, graphs, counting and probability, finite automata, context-free grammars, the compiler pipeline, and the halting problem and NP.
8. Tie it together with projects (6 concepts). Learning by building, scoping a systems project, an end-to-end trace, primary sources, explaining it back, and a README as a design document.

Rules of thumb from the sheet

The sheet closes with short tips that apply to every subject:

• Run every idea you study: fork a process, race two threads, capture a DNS lookup, time a query before and after an index.
• If you cannot explain a topic aloud in two minutes without notes, you do not know it yet.
• Draw it first: memory layouts, network layers, ER diagrams and commit graphs are easier on paper than in prose.
• Do calculations by hand until they are routine: conversions, scheduling tables, subnets and complexity.
• Use past papers to find weak concepts, then study the concept, not the model answer.

Alongside the 72 concepts there is a glossary of 147 shorter terms, so a word you meet in a lecture or a man page can be looked up in a line.

How to use it with the roadmap and your courses

• Alongside a course, read the matching chapter the week the topic is taught, and again after the roadmap stage's exercises. The exercises are where the definitions start to mean something.
• Before an exam, go chapter by chapter and say each concept aloud without looking. Mark the ones you cannot explain and go back to the roadmap topic for those, not to the whole chapter.
• Before a technical conversation or round, chapters 3, 4 and 5 are the ones most often asked about: processes and threads, what happens when you load a page, and transactions and indexes.
• While building the capstone project, chapter 8 is a short checklist for scoping it and writing a README that doubles as a design document.

How it works on Sefism

Everything on Sefism is free to learn from. The public page at CS core subjects cheat sheet shows every chapter title, with the first two chapters open in full. Later chapters show their title, a short introduction and how many concepts they hold.

Keeping the sheet uses credits, and it is yours for good after that. Adding the CS core subjects roadmap to your dashboard also opens this sheet, so if you plan to follow the roadmap you do not need to keep the sheet separately. New members get starting credits when they join, and the first roadmap is free. Create a free account to get started, and if a concept still does not make sense after reading it, ask Tauseef a question.

Related on Sefism

• Fundamentals worth learning as a software engineer
• How to read and understand a large codebase
• Use The Index, Luke, for the database chapter in depth
• The University to job-ready cheat sheet, which pairs these subjects with projects and applications

Frequently asked questions

Is this enough to pass my OS, networks and DBMS exams? It covers the core ideas each subject is built on, at revision depth. Use it to find your gaps, then fill them from the roadmap and your course book.

Why is there a chapter on projects in a theory sheet? Because building a shell, a server or a tiny database is the quickest way to turn definitions into understanding. The roadmap ends with one for the same reason.

What is the difference between this and the University to job-ready sheet? The job-ready sheet covers these subjects more briefly and adds projects, experience and the hiring process. This one goes deeper into the subjects themselves.