Computing Systems
Domain 4 - Computing Systems covers hardware, operating systems, storage, virtualization, and system security. Questions ask how instructions execute, where data resides, how an OS manages resources, and which security control addresses a given risk.
CPU, memory, and storage
The CPU repeats fetch-decode-execute: the program counter holds the next instruction address, the instruction register holds the current instruction, and the arithmetic logic unit performs operations. Registers are fastest, then cache (L1-L3), RAM, SSD/HDD, and offline storage; costs and capacities generally move in opposite directions. RAM is volatile; SSDs retain data without power and have no moving parts. Virtual memory maps virtual addresses to physical frames through page tables; a page fault loads a missing page. Caches exploit temporal and spatial locality.
OS and security
An OS schedules processes and threads, allocates memory, manages files and permissions, and handles interrupts. A process has its own address space; threads in a process share it. Virtualization runs multiple isolated guests on one host; cloud service models are IaaS (infrastructure), PaaS (platform), and SaaS (software). Security controls include authentication, least privilege, patching, encryption, backups, and firewalls. Malware categories include viruses, worms, trojans, ransomware, and spyware.
Worked example
A cache has a 90% hit rate, 1 ns hit time, and 101 ns miss penalty. Average memory access time = 0.90 x 1 + 0.10 x 101 = 0.9 + 10.1 = 11 ns. Raising the hit rate to 95% gives 0.95 x 1 + 0.05 x 101 = 5.95 ns, showing why hit rate dominates performance. The formula assumes the miss penalty includes the time to fetch from the next level.
Traps that cost marks
Calling RAM nonvolatile; confusing cache with virtual memory; assuming more threads always mean more parallel execution on a single core; treating virtualization as the same thing as emulation; and confusing authentication (who you are) with authorization (what you may do).
Networks and Impacts of Computing
Domain 5 - Networks and Impacts of Computing covers how data moves between devices, network protocols and security, and the social, legal, and ethical consequences of computing. Expect scenario questions: choose a protocol, interpret an address, or evaluate an impact.
Models and protocols
Layering separates concerns. The TCP/IP model has application, transport, internet, and network-access layers; OSI has seven layers from physical to application. IP addresses identify interfaces and route packets between networks; MAC addresses identify hardware on a local link. TCP is connection-oriented, acknowledges data, and retransmits lost segments; UDP is connectionless with lower overhead, used for DNS queries, streaming, and VoIP where latency matters more than perfect delivery. DNS resolves names to addresses; HTTP/HTTPS carries web traffic; DHCP assigns addresses; switches forward within a LAN, routers between networks. Firewalls filter traffic; WPA2/WPA3 protect Wi-Fi.
Impacts
Computing creates benefits and harms: automation changes employment, social media affects attention and misinformation, and data collection raises privacy concerns. Intellectual property covers copyright, patents, trademarks, and trade secrets. The digital divide describes unequal access to devices, connectivity, and skills. Security and privacy differ: security protects data from unauthorized access; privacy governs appropriate collection and use. E-waste and energy use are environmental impacts; bias in algorithms can reproduce discrimination.
Worked example
192.168.10.0/26 has a 26-bit prefix and 6 host bits, so each subnet has 2^6 = 64 addresses, of which 62 are usable (network and broadcast addresses are reserved). The first subnet is 192.168.10.0 through 192.168.10.63, with broadcast .63; the next begins at .64. Four such subnets fit in the final octet: .0, .64, .128, and .192. A /26 does not provide 64 usable hosts.
Traps that cost marks
Confusing MAC and IP addresses; saying UDP is reliable; treating HTTPS as making a site trustworthy; using /26 and counting 64 usable hosts; and confusing privacy with security.