CDS Physics · Formula sheet

Electronics and Computer Fundamentals formulas

2 formulas, 6 reference tables and 17 common traps for CDS Physics Electronics and Computer Fundamentals, grouped by subtopic.

Full notes with worked examples

Electronics, Hardware and Data

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Memory, storage and file sizes

Size of a sound or video file

\text{size in bytes} = \dfrac{\text{bit rate (bits per second)} \times \text{time (seconds)}}{8}

bit ratebits stored for each second of playing time
$8$ bits in one byte

Binary numbers and logic operators

Subtraction by 2's complement

A - B = A + (\overline{B} + 1) \quad \text{(keep only } n \text{ bits)}

$\overline{B}$ B with every bit flipped (1's complement)
$n$ number of bits the machine holds

Semiconductors, diodes and transistors

Region of a transistor	Doping	Size
Emitter	Heavy (the most impurity): it supplies the charge carriers	Medium
Base	Light (the least impurity)	Very thin, so carriers cross it without being lostQ CDS 2016 (II): in a bipolar junction transistor the base has the least impurity.
Collector	Moderate	The largest, so it can get rid of heat

Computer generations and new kinds of computing

Generation	Main hardware	Roughly when
First	Vacuum tubes	1940s to mid 1950s
Second	Transistors	Late 1950s to mid 1960s
Third	Integrated circuits (ICs)	Mid 1960s to early 1970s
Fourth	Microprocessors (very large scale integration)	From 1971Q CDS 2026 (I): the fourth generation used microprocessors.
Fifth	Artificial intelligence and parallel processing	Still developing

Common traps

The base is thin and lightly doped

In a transistor the emitter is the most heavily doped, the collector is the largest, and the base is the thinnest and the least doped.

A hot semiconductor conducts better

Heating a metal raises its resistance. Heating a semiconductor frees more charge carriers, so its resistance falls.

Integrated circuits are the third generation, microprocessors the fourth

The order is vacuum tubes, transistors, integrated circuits, then microprocessors. Do not move the microprocessor back to the third generation.

A qubit belongs to quantum computers

The qubit is the two-valued unit of a quantum computer, not of an ordinary (classical) computer or of a laser.

Flash is erased electrically, not by ultraviolet light

EPROM is wiped by ultraviolet light through a window in the chip. Flash memory is a kind of EEPROM: it is wiped electrically and keeps its data without power.

Bits are not bytes

A bit rate in kbps counts bits. Divide by 8 to get bytes before you compare with a size in MB.

Two sizes of megabyte

Memory counts 1 MB = 1024 × 1024 bytes; drive makers and bit-rate sums often use 1,000,000 bytes. The answers differ by about 5%, so pick the option nearest to yours.

AND needs both, OR needs one

AND gives 1 only when both inputs are 1. OR gives 1 when at least one is 1. Mixing them up reverses a truth table.

Flip, then add 1

The 2's complement is the flipped number plus 1. Flipping alone gives the 1's complement, which is one less.

Software and Networks

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System software, application software and the operating system

Software	Kind	What it does
Windows, Linux, UNIX	System (operating system)	Manages memory, files, devices and the user interface
Linker	System	Joins compiled code with the subroutines it callsQ CDS 2026 (II): linking a program with its subroutines is not a job of the operating system.
Compiler, interpreter, assembler	System	Turn a program into machine code
Spreadsheet, MS PowerPoint, AutoCAD	Application	Do a task for the user
UNIX	System (operating system)	Written in C; portable, multi-user and time-sharedQ CDS 2026 (I): UNIX can be time-shared by several users.
AUTOEXEC.BAT	Start-up file (MS-DOS)	A list of commands run automatically when the computer startsQ CDS 2026 (II): AUTOEXEC.BAT holds the commands run at start-up.

Machine, assembly, high-level and markup languages

Level	What it looks like	Examples
Machine language	0s and 1s run directly by the processor	The processor's own instruction codes
Assembly language	Short words (mnemonics), one for each machine code	x86 assembly, MIPS assemblyQ CDS 2026 (I): of Python, Java, x86 and MIPS, the assembly languages are x86 and MIPS.
High-level language	Close to English; runs on many processors	Python, Java, C
Markup language	Tags that label the parts of a document	SGML, HTML, XMLQ CDS 2026 (II): SGML is an open standard that does not depend on a computer's hardware.

Networks, protocols and mobile communication

TCP/IP layer	What it does	Examples
Application	The services users' programs call on	HTTP, FTP, SMTP, TELNET, DNS
Transport	Delivers data between programs on two machines	TCP, UDP
Internet	Gives addresses and routes packets	IP
Link	Sends bits over the local connection	Ethernet, Wi-Fi

Firewalls and malware

Term	What it is
Firewall	Checks every packet against rules and blocks what is not allowedQ CDS 2025 (II): a firewall examines packets, prevents unauthorised access and blocks suspicious activity.
Virus	Malware that attaches to a file and spreads when that file is run
Worm	Malware that spreads by itself across a network
Trojan horse	Malware that looks like useful software but carries a harmful partQ CDS 2025 (I): the Trojan horse is the malicious software among hacking, password cracking and a firewall.
Ransomware	Malware that locks the user's files and demands money
Spyware	Malware that secretly records what the user does
Phishing	A fake message or website that tricks a user into giving passwords

Common traps

Linking is the linker's job

A linker is system software, but it is a separate program. Joining a program with its subroutines is not one of the operating system's own jobs, and the linker is not application software either.

AUTOEXEC.BAT runs commands; CONFIG.SYS loads drivers

At start-up MS-DOS reads CONFIG.SYS for drivers and settings, then runs the commands listed in AUTOEXEC.BAT. COMMAND.COM is the interpreter, not a list of commands.

Assembly belongs to one processor family

Each assembly language matches one processor's instruction set, so it is not portable. Python and Java are high-level and run on many processors.

A markup language does not depend on the machine

SGML, HTML and XML only label the parts of a document. They are independent of the hardware and of the operating system, which is the whole point of them.

TCP and IP are not application protocols

TCP works at the transport layer and IP at the internet layer. The application layer holds the user services: HTTP, FTP, SMTP, TELNET.

Software ports are numbers, not sockets

A software port is a number that picks out a service (25 for email, 21 for file transfer). The sockets on the side of a computer, such as USB, are hardware ports.

A firewall is not antivirus

A firewall blocks unauthorised network traffic. Finding and removing viruses already on the computer is the job of antivirus software.

Hacking is not software

Hacking and password cracking are things people do. Malware is a program: a virus, worm, Trojan horse, ransomware or spyware.

Electronics and Computer Fundamentals formulas

Electronics, Hardware and Data

Memory, storage and file sizes

Binary numbers and logic operators

Semiconductors, diodes and transistors

Computer generations and new kinds of computing

Common traps

Software and Networks

System software, application software and the operating system

Machine, assembly, high-level and markup languages

Networks, protocols and mobile communication

Firewalls and malware

Common traps

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