CDS Physics · Electronics and Computer Fundamentals
Software and Networks
System and application software and the jobs of an operating system, machine, assembly, high-level and markup languages, the layers and ports of the internet, and firewalls and malware.
Why this matters
Fourteen CDS questions, all from 2025 (I) to 2026 (II): six from 2025 and eight from 2026. Five are about software and the operating system, three about programming and markup languages, three about networks and three about firewalls and malware.
Concept 1 of 4: System software, application software and the operating system
Software comes in two kinds. System software runs the computer itself: the operating system, and the tools that turn a program into something the machine can run. Application software does a job for the user, such as a spreadsheet or a drawing program. The operating system is the manager: it shares out memory, moves data to and from devices, keeps the files and gives the user a screen to work with.
Definition
What to know:
- System software: the operating system, compilers, interpreters, assemblers, linkers, loaders and device drivers.
- Application software: word processors, spreadsheets, presentation programs (MS PowerPoint), AutoCAD, web browsers, games.
- Jobs of the operating system: allocating memory, managing devices and data transfer to and from them, managing files, running programs in turn, giving a user interface.
- A linker joins a program's compiled code with the library routines (subroutines) it calls, making a file that can run. That is the linker's job, not the operating system's.
- UNIX (Bell Labs, 1970s): written mostly in C, so it can be moved to many kinds of computer; multi-user and time-sharing.
- MS-DOS (and Windows up to the late 1990s) read CONFIG.SYS at start-up to load drivers and settings, then ran AUTOEXEC.BAT, a list of commands. COMMAND.COM is the program that reads and runs typed commands.
| 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. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (I) 2025 — General Knowledge · Q102Moderate
Example 1 · Electronics and Computer Fundamentals · Software and Networks
Which one of the following is not an application software ?
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.
Concept 2 of 4: Machine, assembly, high-level and markup languages
A processor understands only its own codes of 0s and 1s, its machine language. Assembly language writes each of those codes as a short word such as ADD or MOV, so it is still tied to one family of processors. High-level languages read almost like English and run on any computer that has a translator for them. Markup languages are different again: they do not give orders, they label the parts of a document.
Definition
What to know:
- Machine language: binary codes the processor runs directly.
- Assembly language: a symbol (mnemonic) for each machine code, so it belongs to one processor family (x86, MIPS, ARM). An assembler turns it into machine code.
- High-level languages (Python, Java, C): independent of the processor. A compiler translates the whole program first; an interpreter translates and runs it a line at a time.
- Markup languages describe a document's structure with tags. SGML (an ISO standard of 1986) is an open standard that does not depend on the hardware or the operating system and defines document types; HTML and XML grew out of it.
| 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. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (II) 2025 — General Knowledge · Q81Easy
Example 2 · Electronics and Computer Fundamentals · Software and Networks
Which language uses the symbolic representation of machine codes needed to program a particular processor or processor family ?
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.
Concept 3 of 4: Networks, protocols and mobile communication
The internet is built in layers. The bottom layers move bits and packets from machine to machine; the top layer holds the services people use, such as web pages, email and file transfer. Many services can run on one computer at the same time, so each is given a port number, the way flats in one building have door numbers.
Definition
What to know:
- TCP/IP layers, top down: application (HTTP, FTP, SMTP, TELNET, DNS), transport (TCP, UDP), internet (IP) and link (Ethernet, Wi-Fi).
- Go-back-N is a method of resending lost frames (error control), not an application protocol.
- Software (logical) ports are numbers that point a connection to the right service: HTTP 80, HTTPS 443, FTP 21, SMTP 25, TELNET 23, SSH 22.
- Hardware ports are the sockets that join devices such as cameras and scanners: USB, HDMI.
- CDMA (Code Division Multiple Access) lets many mobile callers share one band of frequencies by giving each a different code. It ran many 2G and 3G mobile phone networks; 4G and 5G networks use a different method (OFDMA).
| 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 |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (II) 2026 — General Knowledge · Q61Moderate
Example 3 · Electronics and Computer Fundamentals · Software and Networks
Which one among the following protocols works at application layer in a TCP/IP protocol suit ?
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.
Concept 4 of 4: Firewalls and malware
A firewall is a guard at the gate: it looks at every packet coming in from the internet or going out, and lets through only what its rules allow. Antivirus software is the cleaner inside: it finds and removes harmful programs already on the computer. Malware is any program written to do harm.
Definition
What to know:
- Firewall (hardware or software): examines each packet, prevents unauthorised access and blocks suspicious activity. It does not connect telephones and does not remove viruses.
- Antivirus: scans for malware and removes it.
- Malware is software. Hacking and password cracking are activities, and phishing is a trick, not a program.
| 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 |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (I) 2025 — General Knowledge · Q103Easy
Example 4 · Electronics and Computer Fundamentals · Software and Networks
Which among the following is a malicious software ?
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.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (4)
System software, application software and the operating system6 rows
| 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 languages4 rows
| 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 communication4 rows
| 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 malware7 rows
| 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 |
Watch out for (8)
- Linking is the linker's job→ System software, application software and the operating system
- AUTOEXEC.BAT runs commands; CONFIG.SYS loads drivers→ System software, application software and the operating system
- Assembly belongs to one processor family→ Machine, assembly, high-level and markup languages
- A markup language does not depend on the machine→ Machine, assembly, high-level and markup languages
- TCP and IP are not application protocols→ Networks, protocols and mobile communication
- Software ports are numbers, not sockets→ Networks, protocols and mobile communication
- A firewall is not antivirus→ Firewalls and malware
- Hacking is not software→ Firewalls and malware
Test yourself on Electronics and Computer Fundamentals
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