NEB 12 Computer Science flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Two's complement range

In an n-bit two's complement system the representable range is −2^(n−1) through 2^(n−1)−1, so 8 bits cover −128 to 127 and there is only one representation of zero.

Hexadecimal digit to bits

Each hexadecimal digit corresponds to exactly four binary bits, so hex F is 1111 and hex A is 1010; grouping binary digits in fours from the right converts back to hex.

De Morgan's theorems

The complement of a sum equals the product of the complements, (A+B)' = A'·B', and the complement of a product equals the sum of the complements, (A·B)' = A' + B'.

Universal gates

NAND and NOR are called universal gates because either one alone can be wired to produce NOT, AND and OR functions, so any logic circuit can be built from it.

Half adder outputs

A half adder adds two bits and produces Sum = A XOR B and Carry = A AND B; it has no carry input, so it cannot add multi-bit columns alone.

Von Neumann stored program

Instructions and data are held together in the same main memory and fetched sequentially by the program counter, which is the defining idea of the stored-program computer.

Cache memory purpose

Cache is small, fast memory between the CPU and main RAM that holds recently used instructions and data to reduce average memory access time.

Volatile versus non-volatile memory

Volatile memory such as RAM loses its contents when power is removed, while non-volatile memory such as ROM, flash and magnetic disks retains data without power.

Multiprogramming versus multitasking

Multiprogramming keeps several jobs in memory so the CPU always has work when one job waits for I/O; multitasking time-slices the CPU so several tasks appear to run at once.

Compiler versus interpreter

A compiler translates the whole source program into object code before execution and reports all errors together; an interpreter translates and runs line by line, stopping at the first error.

Call by value

In call by value the function receives a copy of the argument, so changes made inside the function do not affect the caller's original variable.

Stack versus queue

A stack is last-in-first-out with push and pop at one end, while a queue is first-in-first-out with insertion at the rear and deletion at the front.

Circular queue advantage

A circular queue reuses freed front positions by wrapping the rear index around, avoiding the false overflow that a linear array-based queue shows when space is free.

Primary key

A primary key is a column or column set that uniquely identifies each row of a table, cannot contain null values, and is usually indexed.

Foreign key

A foreign key is a column whose values must match a primary key in another table or be null, enforcing referential integrity between related tables.

DDL versus DML

Data definition language covers CREATE, ALTER and DROP for defining structures; data manipulation language covers INSERT, UPDATE, DELETE and SELECT for working with rows.

OSI reference model layers

The seven OSI layers from bottom to top are physical, data link, network, transport, session, presentation and application, each handling a distinct communication concern.

TCP versus UDP

TCP is connection-oriented, acknowledges segments and retransmits lost data, so it is reliable but slower; UDP is connectionless with no guarantees and suits streaming.

Usable hosts in a subnet

For a prefix length p, total addresses are 2^(32−p) and usable host addresses are that total minus two, because the network and broadcast addresses are reserved.

Star topology

In a star topology every device connects to a central hub or switch; a single cable failure affects only one device, but failure of the centre disables the whole network.

GET versus POST

GET appends form data to the URL as visible name-value pairs and is length-limited; POST sends data in the request body, hiding it from the address bar.

Static versus dynamic web page

A static page is a fixed file returned exactly as stored, while a dynamic page is generated per request by server-side code, often from a database.

Computer virus versus worm

A virus inserts itself into a host file and spreads when that file is executed, whereas a worm self-replicates and propagates across networks without a host program.

Phishing

Phishing is a social-engineering attack that sends fraudulent messages imitating a trusted organisation to trick recipients into revealing passwords or financial details.

Firewall

A firewall inspects incoming and outgoing network traffic and permits or blocks packets according to configured rules, forming a barrier between trusted and untrusted networks.

Digital divide

The digital divide is the gap between groups with reliable access to computers, internet and digital skills and those without, often following income, region and gender lines.

Binary search precondition

Binary search requires the array to be sorted; it compares the target with the middle element and discards half the remaining range each step, giving O(log n).

Bubble sort behaviour

Bubble sort repeatedly compares adjacent elements and swaps them when out of order, pushing the largest remaining value to the end each pass; worst case is O(n²).

Recursion base case

A recursive function must include a base case that returns without calling itself, otherwise the calls never terminate and the stack overflows.

Flowchart decision symbol

A diamond in a flowchart represents a decision with one input and two labelled outputs, typically Yes and No, branching the flow of control.

C string terminator

A C string is a character array ending with the null character '\0'; functions such as strlen and printf rely on it to know where the text stops.

fopen modes

fopen opens a file and returns a FILE pointer; mode "r" reads an existing file, "w" creates or truncates for writing, and "a" appends at the end.

Algorithm properties

A valid algorithm must be finite, definite in each step, accept zero or more inputs, produce at least one output, and be effective enough to carry out.