Computer Science Cheat Sheet

hey all, notes in order of the exam spec (the syllabus), nothing extra, concise and handy, covers key definitions of ALL topics, including some helpful formulas. Good Luck :))

Topic 1: Fundamentals of Algorithms

What is an ==algorithm==? :: set of instructions used to solve a problem or complete a task

What is ==decomposition==? :: breaking larger problems into smaller, sub-problems so that each accomplish an identifiable task, which might itself be further subdivided

What is ==abstraction==? :: Abstraction is the process of removing unnecessary detail from a problem

Topic 2: Fundamentals of Data Representation

@@Data structure@@ :: way of storing and organizing a set of data, that can easily be edited/changed @@Subroutine@@ :: named block of code which can be run by calling its name in a program

@@Function@@ :: a procedure which has a return value Parameter – input variable for a subroutine @@Assembler@@ :: converts assembly language (low level) into machine code

@@Interpreter@@ :: converts high level programming into machine code (line by line)

@@Compiler@@ :: converts high level programming into machine code (whole program at once)

@@Scope@@ :: which part of a program can see variables/constants/data structures

here’s a neat little comparison table between assembler, interpreter and compiler:

Feature	Interpreter	Compiler	Assembler
Input	A High-Level Language	A High-Level Language	Assembly Language (Low Level)
Output	No output, program runs straight away	Machine Code	Machine code
How it works	Translates source code and immediately runs it	Compiles source code so it can be run later	Assemble source code so it can be run later
Speed of execution	Slow as it needs to be translated each time it is run	Fast, as CPU runs the machine code	Fast, as the CPU runs the machine code
What users need to run program	End users need the interpreter and the program	Users need just the compiled program	Users need just the compiled program
Source Code	End users can see the source code	End users cannot see the source code	End users cannot see the source code

Bit :: one unit of data

Character set :: defined list of characters recognized by computer’s hardware and software

Pixel :: single dot of colour in an image

Bitmap :: graphical image made up of pixels

Pixel resolution :: number of pixels in an image

Colour depth :: bits used per pixel

Sample :: measure of amplitude at a point in time

Sampling rate :: number of samples taken per second

Sample resolution :: number of bits used per sample

here are the important formulas for this topic:

File size of image (in bits) = width (pixels) x height (pixels) x colour depth (bits)

think of this one as a formula triangle, with file size at the top, and the three others below

File size of sound (bits) = sample rate (Hz) x time (sec) x sample resolution (bits)

again, file size is the top of the triangle, with the three others below

Topic 4: Computer Systems

%%Computer system%% :: combination of hardware and software used to perform tasks

%%Hardware%% :: physical part of a computer

%%Software%% :: programs that are downloaded/installed onto a computer

%%System software%% :: programs which help run and maintain a computer system

%%Application software%% :: programs which help the user perform day to day tasks

%%Operating system%% :: platform for hardware to use the software

%%Volatile memory%% :: temporary storage that is lost when power turns OFF (RAM)

%%Non-volatile memory%% :: permanent storage that is kept when the power turns OFF (ROM, HDD)

%%Cloud storage%% :: secondary storage, at a remote location which is accessible via the internet

%%Embedded System%% :: computer system with a specific purpose, built into another system/machine

%%RAM%% :: stores running applications while the computer is ON

%%ROM%% :: stores the BIOS (instructions needed to turn on the computer and cannot be edited) %%Cache%% :: stores the most frequently accessed instructions from the RAM (Level 1, Level 2, L3)

here is the memory pyramid

level one cache closest to CPU so at the top, and so on Level 1 cache is used around 50% of the time, Level 2 90%, so its no doubt that cache makes a big difference in the computer’s performance speed, as it is fast access memory location that is quicker to access than RAM and slower than registers.

Topic 5: Computer Networks

==Computer Network== :: a group of computers connected together to share resources

==Network Protocol== :: a set of rules which allow devices to communicate

==Network Security== :: methods which protect networks from unauthorised access and harm

==Authentication== :: verifying the user’s identity when accessing data/network, to prevent unauthorised access

==Firewall== :: protects network from unauthorised access by controlling incoming/outgoing network connections

==Encryption== :: scrambling data to change how it looks to prevent unauthorised access (only people with decryption key can access original)

Topic 6: Cyber Security

^^Cyber security^^ :: methods which prevent data/programs from Attack, Damage and Unauthorised access

^^Penetration Testing^^ :: testing the security of a system/network to find any vulnerabilities.

^^Black-box testing^^ :: stimulates external hacking or cyber warfare attack

^^White-box testing^^ :: stimulates malicious insider (spy) who knows something about the target system

^^Social engineering^^ :: art of manipulating people to give up personal information/details ^^Malware^^ :: malicious software/code