Bhutan Civil Service Main Examination (ICT Category) cheat sheet

CPU instruction cycle: fetch → decode → execute → store; the program counter holds the address of the next instruction, and the instruction register holds the current one.

Memory hierarchy by speed and cost per bit: registers > cache (L1/L2/L3) > main memory (RAM) > SSD/HDD > tape; capacity grows as speed falls.

Process vs thread: a process has its own address space; threads of one process share code, data, and open files but keep separate stacks and registers.

Deadlock needs four simultaneous Coffman conditions: mutual exclusion, hold-and-wait, no preemption, circular wait; breaking any one prevents it.

Page replacement: FIFO can suffer Belady's anomaly (more frames, more faults); LRU and Optimal do not; Optimal is a theoretical lower bound only.

Thrashing = excessive paging because the sum of working sets exceeds physical frames; the fix is reducing multiprogramming degree, not adding swap.

Disk scheduling: SSTF minimizes seek but can starve far requests; SCAN (elevator) sweeps one direction then reverses; C-SCAN returns without servicing.

File allocation: contiguous is fast but fragments; linked has no external fragmentation but poor random access; indexed uses an index block for direct access.

UNIX permissions rwx = 4/2/1 per owner, group, other; chmod 754 gives owner rwx, group r-x, others r--.

Array access is O(1) by index; insertion or deletion in the middle is O(n) because elements shift.

Singly linked list: search O(n), insert/delete at a known node O(1); it cannot traverse backwards without a prev pointer.

Stack is LIFO (push/pop, O(1)); queue is FIFO (enqueue at rear, dequeue at front); a circular queue avoids the false-full condition of a linear array queue.

Binary search requires a sorted array and runs in O(log n); each comparison halves the search interval.

Balanced BST height is O(log n); an unbalanced BST degrades to O(n) on sorted insertion order; AVL and red-black trees rebalance on insert.

Hash table average lookup is O(1); worst case is O(n) when all keys collide; open addressing vs chaining are the two main collision strategies.

Big-O ordering: O(1) < O(log n) < O(n) < O(n log n) < O(n^2) < O(2^n) < O(n!).

Merge sort is O(n log n) worst case and stable but needs O(n) extra space; quicksort averages O(n log n), worst O(n^2), and is in-place.

Graph traversal: BFS uses a queue and finds shortest paths in unweighted graphs; DFS uses a stack/recursion and suits cycle detection and topological sort.

ACID: Atomicity (all-or-nothing), Consistency (constraints hold), Isolation (concurrent transactions do not interfere), Durability (committed data survives failure).

Normalization: 1NF removes repeating groups; 2NF removes partial dependency on part of a composite key; 3NF removes transitive dependency; BCNF every determinant is a candidate key.

SQL clause evaluation order: FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY → LIMIT; WHERE filters rows, HAVING filters groups.

INNER JOIN returns matching rows only; LEFT JOIN keeps all left rows with NULLs for non-matches; FULL OUTER JOIN keeps unmatched rows from both sides.

A candidate key is a minimal superkey; the primary key is the chosen candidate key; a foreign key references a primary or unique key and enforces referential integrity.

Indexes speed SELECT and slow INSERT/UPDATE/DELETE; a B+ tree index supports range scans and keeps all data in leaf nodes.

Concurrency anomalies: dirty read (uncommitted data), non-repeatable read (row changes between reads), phantom read (new rows appear); isolation levels trade these against throughput.

RAID 1 mirrors for redundancy; RAID 5 stripes with distributed parity and survives one disk failure; RAID 0 stripes with no redundancy.

A view is a stored query, not stored data; it can restrict column exposure and simplify complex joins.

OSI layers top-down: Application, Presentation, Session, Transport, Network, Data Link, Physical; TCP/IP collapses these to Application, Transport, Internet, Network Access.

TCP is connection-oriented, reliable, ordered, with flow and congestion control; UDP is connectionless, unreliable, and lower latency, suited to streaming and DNS.

TCP three-way handshake: SYN → SYN-ACK → ACK; teardown uses FIN/ACK in both directions.

IPv4 has 32-bit addresses; IPv6 has 128-bit addresses; private IPv4 ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.

Subnetting: usable hosts = 2^(host bits) − 2; a /24 leaves 8 host bits, giving 254 usable addresses.

DNS resolves names to addresses using A (IPv4), AAAA (IPv6), MX (mail), CNAME (alias), and PTR (reverse) records; default port 53.

Well-known ports: HTTP 80, HTTPS 443, FTP 20/21, SSH 22, SMTP 25, DNS 53, DHCP 67/68.

CSMA/CD detects collisions on shared Ethernet; CSMA/CA avoids them in Wi-Fi; switches forward by MAC at Layer 2, routers by IP at Layer 3.

DHCP uses DORA: Discover, Offer, Request, Acknowledge, assigning address, mask, gateway, and DNS to a host.

SDLC phases: requirements, analysis, design, implementation, testing, deployment, maintenance; maintenance typically consumes the largest share of lifetime cost.

Waterfall suits stable, well-understood requirements; iterative/Agile suits changing requirements and delivers working increments each sprint.

Requirement types: functional (what the system does) vs non-functional (performance, security, usability, availability); a constraint limits design choices.

DFD symbols: process (circle), data store (open rectangle), external entity (rectangle), data flow (arrow); a level-0 context diagram shows one process and all external entities.

ER modeling: entity → table, attribute → column, relationship → foreign key or junction table for many-to-many; cardinality is 1:1, 1:N, or M:N.

Coupling should be low and cohesion high; content coupling is worst, data coupling is best; functional cohesion is strongest.

Black-box testing uses equivalence partitioning and boundary value analysis; white-box testing uses statement, branch, and path coverage.

Verification asks 'are we building the product right?' against the spec; validation asks 'are we building the right product?' against user needs.

UML: use case diagrams capture actors and goals; class diagrams capture structure; sequence diagrams capture time-ordered message exchange.

CIA triad: Confidentiality (encryption, access control), Integrity (hashing, checksums), Availability (redundancy, DDoS protection).

Symmetric ciphers (AES, DES) use one shared key and are fast; asymmetric ciphers (RSA, ECC) use a public/private pair and are slow but solve key distribution.

Hashing (SHA-256) is one-way and collision-resistant for integrity; encryption is reversible for confidentiality; a MAC combines a key with a hash for authenticity.

Digital signature = hash of message encrypted with sender's private key; verification uses the sender's public key, giving authenticity, integrity, and non-repudiation.

TLS handshake authenticates the server via certificate, negotiates a session key, then encrypts traffic; HTTPS is HTTP over TLS on port 443.

Attack classes: phishing (social engineering), SQL injection (unsanitized input), XSS (injected script), MITM (interception), ransomware (encrypt-and-extort).

Defense in depth layers controls: physical, network (firewall, IDS/IPS), host (antivirus, patching), application (input validation), and data (encryption, backup).

Least privilege and separation of duties limit blast radius; multi-factor authentication combines something you know, have, and are.

Incident response phases: preparation, identification, containment, eradication, recovery, lessons learned.

HTML structures content, CSS styles presentation, JavaScript adds behavior; separation of concerns keeps markup, style, and logic in separate files.

HTTP methods: GET retrieves and is idempotent, POST submits data and is not idempotent, PUT replaces, PATCH partially updates, DELETE removes.

Status codes: 2xx success, 3xx redirect, 4xx client error (404 not found, 401 unauthenticated, 403 forbidden), 5xx server error.

REST constraints: statelessness, client-server separation, uniform interface, cacheability; resources are nouns addressed by URL, actions are HTTP verbs.

Responsive design uses fluid grids, flexible images, and media queries to adapt layout to viewport width; mobile-first CSS starts with small screens.

Client-side state: cookies (sent automatically, size-limited), localStorage (persistent, ~5 MB), sessionStorage (per tab); none should hold secrets.

Same-origin policy blocks cross-origin reads; CORS headers let a server explicitly permit selected origins.

Mobile app models: native (platform SDK, best performance), cross-platform (React Native, Flutter), and hybrid/webview (Cordova); each trades performance against code reuse.

MVC separates model (data), view (UI), and controller (input); MVVM binds the view to observable view-model state.

Supervised learning uses labeled data (classification, regression); unsupervised learning finds structure in unlabeled data (clustering, dimensionality reduction); reinforcement learning learns from reward signals.

Overfitting = low training error, high validation error; remedies are more data, regularization, dropout, and cross-validation.

Precision = TP/(TP+FP); recall = TP/(TP+FN); F1 is their harmonic mean, useful when classes are imbalanced.

Cloud service models: IaaS (VMs, storage), PaaS (managed runtime), SaaS (finished application); the provider manages more as you move from IaaS to SaaS.

Cloud deployment models: public, private, hybrid, community; NIST defines five essential characteristics including on-demand self-service and measured service.

IoT architecture layers: perception (sensors/actuators), network (connectivity), and application (services); constrained devices favor MQTT or CoAP over HTTP.

Big Data 'Vs': volume, velocity, variety (plus veracity, value); Hadoop HDFS handles storage and MapReduce batch processing, while Spark keeps data in memory.

CAP theorem: a distributed system can guarantee only two of consistency, availability, and partition tolerance; partition tolerance is unavoidable in practice.

Bhutan's ICT governance is led by the Ministry of Industry, Commerce and Employment (MoICE) through its Department of ICT and Telecom, with the Bhutan Infocomm and Media Authority (BICMA) as regulator.

The Information, Communications and Media Act 2018 provides the legal framework for telecom, broadcasting, and online content regulation in Bhutan.

The Electronic Transactions Act 2003 (amended 2010) gives legal recognition to electronic records and digital signatures.

The Bhutan Penal Code and the Information, Communications and Media Act address cybercrime, including unauthorized access and online harassment.

e-Government initiatives include the Government Data Centre, the national digital identity system, and G2C services delivered through the gov.bt domain and citizen service portals.

Bhutan's National Digital Identity (NDI) system provides verified online identity for citizens and supports digital service delivery.

Digital Drukyul is Bhutan's flagship digital transformation initiative, covering digital identity, e-services, and ICT infrastructure.

Universal service goals are pursued through rural connectivity programs, community centres, and mobile network expansion by Bhutan Telecom and Tashi InfoComm.

Digital inclusion policy emphasizes Dzongkha-language content, accessibility, and bridging the rural-urban connectivity gap.

Percentage change: increase by x% then decrease by x% gives a net decrease of x^2/100 percent, never zero.

Work formula: if A finishes in a days and B in b days, together they take ab/(a+b) days.

Speed, distance, time: distance = speed × time; for equal distances at speeds u and v, average speed = 2uv/(u+v).

Simple interest = P × R × T / 100; compound interest = P[(1 + R/100)^T − 1].

Profit and loss: profit % is calculated on cost price; selling price = cost price × (1 + profit%/100).

Permutations of n distinct items = n!; combinations of r from n = n! / (r!(n−r)!); arrangements of a word with repeats divide by factorials of repeat counts.

Probability of an event = favorable outcomes / total equally likely outcomes; P(A or B) = P(A) + P(B) − P(A and B).

Ratio and mixture: alligation gives the ratio of two quantities as the difference of each from the mean, taken crosswise.

Ratio and proportion: if a:b = c:d then ad = bc; for direct proportion both quantities scale together, for inverse one rises as the other falls.

The Constitution of the Kingdom of Bhutan was adopted in 2008 and establishes a constitutional monarchy with the King as head of state.

The Parliament (Tshogdu) is bicameral: the National Council with 25 members (20 elected, 5 appointed by the King) and the National Assembly with 47 elected members.

The Prime Minister is the leader of the majority party in the National Assembly and is appointed by the King; the cabinet is the Lhengye Zhungtshog.

The Constitution mandates that the state maintain a minimum of 60 percent forest cover and that at least 60 percent of Bhutan remains under forest.

Gross National Happiness (GNH) is the guiding development philosophy, measured through nine domains including living standards, health, education, and ecological diversity.

The Election Commission of Bhutan conducts national elections; the two-party system in the National Assembly is established by the Constitution.

Bhutan became a member of the United Nations in 1971 and joined the World Trade Organization in 2024.

The Royal Government operates through ministries headed by ministers; Dzongkhag administrations are led by Dzongdag and Gewog administrations by Gups.

Bhutan's foreign policy is guided by the Constitution's principles of peace, non-alignment, and friendly relations, with close ties to India.