Evil J277 Computer Science Revision

Flashcards covering OCR GCSE Computer Science (J277) topics including CPU architecture, memory and storage, data representation, networks, protocols, and network security.

According to systems architecture, what is the definition of a computer?

A computer is an electronic device that takes an input, processes the input, and then delivers an output.

What are the three stages of the CPU fetch-decode-execute (FDE) cycle and what occurs in each?

1. Fetch: Retrieves the next instruction from main memory (RAM) and brings it into the CPU.
2. Decode: The Control Unit (CU) inspects the instruction to determine what needs to be done.
3. Execute: Carries out the instruction (e.g., fetching data from memory, performing a calculation, or storing data back in memory).

In the provided Von Neumann architecture diagram, what value is stored in the Accumulator, and what address is held in the Program Counter (PC)?

The Accumulator holds $23$, and the Program Counter (PC) holds $2$.

What are the specific functions of the Memory Address Register (MAR) and the Memory Data Register (MDR)?

The MAR holds the memory address where data is to be fetched or stored. The MDR holds the data fetched from memory, or data to be written to memory.

What are the functions of the Arithmetic Logic Unit (ALU) and the Control Unit (CU)?

The ALU makes arithmetic calculations and logical decisions. The CU sends signals to control how data moves around the CPU.

What does a CPU clock speed of $3.2\,\text{GHz}$ mean, and what is overclocking?

A clock speed of $3.2\,\text{GHz}$ means $3.2\text{ billion}$ instructions (cycles) are fetched per second. Overclocking is increasing clock speed to perform more instructions per second; it increases performance but can lead to glitches or bugs.

Why does doubling the number of CPU cores not necessarily double processing performance?

CPU cores must communicate with each other, which consumes time, and many software programs are not designed to utilize multiple cores.

What is an embedded system, and what proportion of processors are manufactured for embedded systems?

An embedded system is a computer system with a dedicated function within a larger mechanical system. $98\%$ of processors are manufactured as components of embedded systems.

What are four key properties of embedded systems compared to general-purpose computers?

Low power consumption, small size, rugged operating systems, and low cost per unit (balanced against limited processing resources).

What are the key functional differences between RAM and ROM?

ROM is non-volatile, read-only, relatively small, and holds the computer startup instructions (bootstrap). RAM is volatile (lost when power is cut), read-and-write, larger, and holds temporary data and instructions currently in use by the processor.

Identify the three categories of storage illustrated in this hierarchy and describe the function of each.

1. Primary storage: Holds RAM, ROM, registers, and cache for fast access during execution.
2. Secondary storage: Holds the operating system, programs, and data when not in use.
3. Tertiary storage: Used for backing up and archiving large amounts of data.

What is virtual memory, and where is it located?

Virtual memory is temporary storage located on the hard disk (secondary storage), used when physical RAM is full. Programs not currently executing are transferred out to virtual memory and brought back to RAM when needed.

How do optical, magnetic, and solid-state storage differ in terms of physical mechanisms and performance?

Optical uses light (lasers) to read pits and lands, has the slowest access speeds, and is prone to scratches. Magnetic storage uses magnetized platters and moving heads, offering cheap high capacity but fragility. Solid-state has no moving parts, silent operation, very fast access, and durability, but is more expensive per byte and has limited read/write cycles.

What is the difference between a bit, a nibble, and a byte, and how do their unit symbols differ from network speeds?

A bit is a binary digit ($0$ or $1$), a nibble is $4\text{ bits}$, and a byte is $8\text{ bits}$. Storage file size uses capital letters (e.g., $\text{MB}$ for megabytes), while data transfer speed uses a lowercase second letter (e.g., $\text{Mb}$ for megabits), meaning $1\,\text{MB} = 8\,\text{Mb}$.

Based on the provided breakdown, calculate the sound file size in bits and bytes for a sample rate of $6\,\text{Hz}$, duration of $3\,\text{s}$, and bit depth of $4\,\text{bits}$.

The calculation is $\text{Sample rate} \times \text{Duration} \times \text{Bit depth} = 6 \times 3 \times 4 = 72\,\text{bits} = 9\,\text{bytes}$.

How do binary left and right shifts affect the denary value of an integer?

A binary left shift multiplies the denary number by $2$. A binary right shift divides the denary number by $2$.

What is a character set, and how many characters can standard 7-bit ASCII represent versus 24-bit Unicode?

A character set is a defined list of characters recognized by computer hardware and software, where each character is represented by a single number. Standard 7-bit ASCII represents $2^7 = 128$ characters, while 24-bit Unicode represents $2^{24} = \text{over } 16\text{ million}$ characters.

What is the difference between a bitmap image and a vector image, and what is image metadata?

A bitmap image is made up of coloured pixel squares where each pixel has a binary value. A vector image stores the mathematical formulas required to draw shapes. Metadata is additional data stored with an image defining its width, height, colour depth, and colour palette.

What are sample rate and bit depth in digital sound recording, and what are standard values for a CD?

Sample rate is the number of sound wave amplitude samples recorded per second (measured in $\text{Hz}$; typical CD is $44{,}100\,\text{Hz}$). Bit depth is the number of bits stored per sample (CD uses $16\text{-bit}$, representing one of $65{,}536$ amplitudes).

What is the difference between lossy and lossless compression, and which file types require lossless?

Lossy compression permanently discards data to greatly reduce file size and lowers quality (suitable for images, sound, and video). Lossless compression reduces file size by encoding data without losing any information, allowing full reconstruction. Lossless must be used for text documents and executable files.

What are the structural differences between a LAN and a WAN?

A Local Area Network (LAN) covers a single site within a small geographical area using hardware owned by the organisation. A Wide Area Network (WAN) connects LANs over a large geographical area using telecommunication infrastructure that is leased and managed by external providers.

What is network bandwidth, and what factors impact network performance?

Bandwidth is the amount of data that can be sent and received successfully in a specified period (measured in bits per second). Performance is affected by bandwidth, number of users, transmission media (fibre vs copper), error rate, and latency (transmission delays).

How do client-server and peer-to-peer (P2P) network models differ in terms of management and points of failure?

In a client-server model, a central server manages security, file storage, and backups, but acts as a single point of failure. In a peer-to-peer model, each computer is an equal peer responsible for its own security, backups, and shared files, eliminating single-point hardware failure.

What are the respective networking functions of a NIC, a switch, and a router?

A Network Interface Controller (NIC) connects a device to a network using protocols. A switch connects computers on a LAN and intelligently directs frames using MAC addresses. A router forwards data packets between separate networks (e.g., LAN to WAN) using IP addresses.

According to the diagram, what are the two components of an IPv4 address, and how are they partitioned in $107.56.94.111$?

The two components are the Network identifier and the Host identifier. In $107.56.94.111$, the Network identifier is 3 bytes ($107.56.94$) and the Host identifier is 1 byte ($111$).

How do IPv4 and IPv6 addresses differ in size, structure, and address capacity?

IPv4 is $32\text{-bit}$ ($4\text{ bytes}$), written as four denary numbers ($0\text{--}255$) separated by dots, providing approximately $4\text{ billion}$ addresses. IPv6 is $128\text{-bit}$ ($16\text{ bytes}$), written as eight groups of four hexadecimal values separated by colons, providing approximately $340\text{ trillion trillion trillion}$ combinations.

What role does a Domain Name Server (DNS) play in internet communication?

A DNS converts human-readable domain names (e.g., www.google.com) into IP addresses (e.g., $8.8.8.8$) so web browsers can locate and request web pages from the appropriate host server.

Explain what the encryption and decryption pipeline shows in this diagram.

A plain text message ('HELLO WORLD') is scrambled into encrypted ciphertext ('CZGGJ RJMGY') using an encryption key to prevent interception, then reverted to plain text at the destination using a decryption key.

What are the four layers of the TCP/IP stack and what does each layer handle?

1. Application: Selects the protocol for user applications (e.g., HTTP, FTP, SMTP).
2. Transport: TCP establishes end-to-end connections, splits data into packets, and attaches sequence numbers and port numbers.
3. Network (Internet): IP adds source and destination IP addresses to route packets across routers.
4. Link: Represents physical connections and attaches MAC addresses of source and destination devices.

How do brute-force, Denial of Service (DoS), and SQL injection attacks function?

A brute-force attack repeatedly guesses passwords through trial-and-error using automated software. A DoS attack floods a server with useless traffic to overwhelm it and render it unavailable. SQL injection enters malicious SQL commands into user text inputs to execute queries on backend databases to expose, alter, or delete records.