CS core subjects cheat sheet

1. 1How a computer runs a programBits, logic, the CPU, memory and the path from source code to running instructions. Every later chapter sits on this picture.10 concepts

   1. 1Binary and hexadecimalNumber systems with base 2 and base 16.
   2. 2Two's complementThe standard way to store signed integers: to negate a number, invert every bit and add one.
   3. 3Character encodingThe mapping from characters to numbers and bytes.
   4. 4Floating pointThe binary format for real numbers: a sign, an exponent and a fraction.
   5. 5Logic gatesTiny circuits that compute a boolean function of their inputs.
   6. 6CPU, registers and ALUThe CPU executes instructions.
   7. 7Fetch, decode, executeThe loop every CPU runs: fetch the instruction at the program counter, decode it, execute it, then move to the next instruction or jump.
   8. 8Memory hierarchyStorage layers from registers and CPU caches to RAM, SSD and network, each much larger and much slower than the one above.
   9. 9Stack and heapTwo memory regions of a running program.
   10. 10Compiler, interpreter and JITA compiler translates a whole program to machine code ahead of time; an interpreter executes it as it reads.

   10 in this chapterOpen this chapter
2. 2Object-oriented programming and designClasses, the four pillars, composition and SOLID. The aim is code that stays easy to change, not a list of definitions.9 concepts

   1. 1Class and objectA class defines the data and behaviour of a kind of thing; an object is one instance of it with its own state.
   2. 2Coupling and cohesionCohesion is how closely the parts of one class belong together; coupling is how much one class depends on the internals of others.
   3. 3EncapsulationKeeping an object's data private and exposing only operations that keep it valid.
   4. 4InheritanceA subclass reuses and extends a parent class, and can be used wherever the parent is expected.
   5. 5PolymorphismOne call working on many types, each responding in its own way.
   6. 6Abstraction and interfacesDepending on what something does rather than how.
   7. 7CompositionBuilding an object from other objects it holds and delegates to, such as a Car that has an Engine.
   8. 8SOLID principlesFive design guidelines: single responsibility, open closed, Liskov substitution, interface segregation and dependency inversion.
   9. 9Design patternA named, reusable solution to a common design problem, such as Strategy for swappable algorithms or Observer for notifying listeners.

   9 in this chapterOpen this chapter
3. Operating systemsHow one machine runs many programs safely: processes, threads, scheduling, virtual memory, synchronisation and file systems.11 conceptsOpens with credits
4. Computer networksLayers, addressing, transport, DNS, HTTP and TLS: everything between typing a URL and seeing the page.9 conceptsOpens with credits
5. Databases: theory and internalsThe relational model, keys, normalisation, transactions, isolation and indexes. Queries stay light here; the SQL roadmap goes deeper.9 conceptsOpens with credits
6. Software engineeringHow teams build software: life cycles, requirements, UML, version control, testing and code quality.9 conceptsOpens with credits
7. Theory worth knowingComplexity, discrete maths, automata, grammars and the limits of computation, at the level of intuition.9 conceptsOpens with credits
8. Tie it together with projectsBuild small systems that force the subjects to connect, and use exams to check understanding rather than memory.6 conceptsOpens with credits
9. 9Glossary, A to ZEvery concept in the sheet and the smaller terms it uses, in one list.219 terms

OS, networks, databases, OOP.