Computer Systems, Networks, and Security
Core concepts
Understand the layered machine: CPU (fetch–decode–execute cycle, registers, clock), memory hierarchy (registers, cache, RAM, secondary storage) and why locality matters, binary representation (two's complement, ASCII/Unicode, sound and image sampling), and Boolean logic with truth tables. Know operating-system roles: process scheduling, memory management, file systems, and interrupts. For networks, know the TCP/IP model layers, IP addressing basics, DNS, HTTP, packet switching, and the difference between TCP (reliable, ordered) and UDP (fast, connectionless). Security: authentication vs authorisation, encryption (symmetric vs asymmetric), hashing, firewalls, malware types, phishing, and the CIA triad (confidentiality, integrity, availability).
Traps
- Two's complement: the most negative value has no positive counterpart; range for n bits is −2^(n−1) to 2^(n−1)−1.
- Cache is faster but smaller than RAM-questions reverse the hierarchy.
- TCP guarantees delivery; UDP does not. Streaming and gaming favour UDP.
- Encryption protects confidentiality; hashing verifies integrity but is not reversible.
- A firewall filters traffic; it does not remove malware already inside.
Worked example
Represent −5 in 8-bit two's complement: +5 = 00000101; invert = 11111010; add 1 = 11111011. Check: 11111011 = 128+64+32+16+8+2+1 interpreted signed = −5. Rule of thumb: negate by inverting all bits and adding one; the sign bit tells you the value is negative.