IGCSE Computer Science Flashcards

40 flashcards for IGCSE Computer Science (0478)

How is sound recorded digitally?

Amplitude measured at intervals, converted via ADC, samples encoded as binary, readings approximate sound wave.

What is the function of the Control Unit (CU) in a CPU?

Manages data flow within the CPU.

What is the purpose of the Program Counter (PC)?

Stores the address of the next instruction to be executed.

What is the function of the Memory Address Register (MAR)?

Holds the address of the data in RAM that the CPU wants to access.

What is the role of the Data Bus in a computer system?

Transfers data between the CPU and RAM.

What is the function of the Accumulator in the CPU?

Stores the results of calculations performed by the CPU.

What is the purpose of the Current Instruction Register (CIR)?

Holds the current instruction being decoded and executed.

Describe the Fetch stage of the Fetch-Decode-Execute (FDE) cycle.

PC → MAR (address bus), MAR → RAM (request), RAM → MDR (data bus), MDR → CIR, PC increments.

Why is a smartwatch not considered a general-purpose computer?

Limited functions, cannot install new software, no peripheral support, dedicated microprocessor.

How does Run-Length Encoding (RLE) compress the string 'AAAAABBBB'?

» It is a form of lossless/reversible file compression

» It reduces the size of a string of adjacent, identical data (e.g., repeated colours in an image)

» A repeating string is encoded into two values:– the first value represents the number of similar data items (e.g., characters) in the run– the second value represents the code of the data item (such as the ASCII code if it is a keyboard character) 

» RLE is only effective where there is a long run of repeated units/bits

Explain the steps involved in checksum error detection.

Sender calculates checksum, appends to data, receiver recalculates & compares, mismatch → retransmission.

How does the ARQ protocol correct errors in data transmission?

» ARQ uses positive and negative acknowledgements (messages sent to the receiver indicating that data has/has not been received correctly) and timeout (this is the time interval allowed to elapse before an acknowledgement is received)

» the receiving device receives an error detection code as part of the data transmission (this is typically a Cyclic Redundancy Check – refer to Section 2.1.1); this is used to detect whether the received data contains any transmission errors

» if no error is detected, a positive acknowledgement is sent back to the sending device

» however, if an error is detected, the receiving device now sends a negative acknowledgement to the sending device and requests re-transmission of the data

» the sending device uses a time-out by waiting a pre-determined amount of time ….

» ... and if the sending device has received no acknowledgement of any type within this time limit, it automatically resends the data until a positive acknowledgement is received ….

» ... or until a pre-determined number of re-transmissions has taken place.

» mobile phone networks often use ARQ to guarantee data integrity

How can Jane receive encrypted messages using asymmetric encryption?

Jane generates public/private key pair, shares public key, others encrypt with Jane's public key, Jane decrypts with private key.

Describe two methods used by anti-virus software to detect viruses.

Signature-based (compares to known virus DB) and Heuristic (detects suspicious behavior patterns).

Why is disk defragmentation needed for Hard Disk Drives (HDDs)?

Files fragment across sectors, read head moves more → slower access, rearranges blocks contiguously.

What is the 3-version backup rule?

1. Internal HDD/SSD (current), 2. Portable SSD (local), 3. Cloud storage (remote).

Why are USB driver descriptors important?

VID/PID identify hardware type; unique serial numbers prevent conflicts.

List five functions of an Operating System (OS).

• -managing files
  – handling interrupts
  – providing an interface
  – managing peripherals and drivers
  – managing memory
  – managing multitasking
  – providing a platform for running applications
  – providing system security
  – managing user accounts

What is firmware? Give examples.

Software in ROM. Examples: BIOS, bootloader.

Define a) Resolution, b) Color depth, and c) Lossy compression in the context of images.

a) Pixels (width × height), b) Bits per pixel, c) Permanently reduces quality (e.g., JPEG).

Explain the interaction between a sensor and a microprocessor in a train door sensor system.

1. A proximity/pressure/infrared sensor is used
2. Senor collects data and converts it to digital using an ADC

3. The microprocessor compares it to preset values

4. If data is within range/outside range/matches, appropriate measures are taken
5. The actuator closes the door

6. process continues until the door closes

What does managing memory involve in computing?

Allocating RAM to processes, Preventing memory conflicts, Virtual memory handling.

Contrast System software and Application software, with examples.

System: Manages hardware (e.g., OS), Application: User tasks (e.g., Word).

Compare ASCII and Unicode character encoding.

ASCII has fewer characters

ASCII covers a limited languages

ASCII 7 to 8 bits a character

ASCII 128 to 256 characters

Unicode 16 to 32 bits per character

Unicode has approx 60 thousand to 4 billion characters

Give two key differences between a Compiler and an Interpreter.

1. Compiler: Entire code → executable; 2. Interpreter: Line-by-line execution.

What are the key differences between IPv4 and IPv6?

IPv4: 32-bit (e.g., 192.168.0.1), dots(.); IPv6: 128-bit (e.g., 2001:db8::) colons(:).

How do lasers read data from CDs in optical storage?

Laser reflects off pits (0) and lands (1), Sensor detects intensity changes.

Why is compiled software distributed instead of source code?

Hides source code, No interpreter needed, Faster execution.

Calculate the number of 100x100 pixel, 16-bit images that can be stored in 5MB.

Approximately 262/250 images.

How can an error be found in a parity block error detection system?

Check row/column parity; Intersection locates error.

What is the main purpose of the CPU, and what are its stages?

Processes instructions. Stages: Fetch → Decode → Execute.

Explain how sensors operate in an automated water tap system.

The motion sensor detects hands, ADC converts the analog signal, the Microprocessor compares the value, and opens the valve if below the threshold.Process is continuous

List six features commonly found in an Integrated Development Environment (IDE).

1. Code editor, 2. Run-time environment, 3. Error diagnostics, 4. Auto-completion, 5. Auto-correction, 6. Prettyprint.

What happens during an interrupt handling routine?

1. Interrupt Trigger

   • A hardware device (e.g., a keyboard, printer) or a software event (e.g., a division error) sends an interrupt signal to the CPU.

2. Immediate CPU Response

   • The CPU pauses execution of the current task.

   • Critical state information (including the program counter, flags, and register values) is saved to the stack. This ensures the original task can resume exactly where it left off.

3. Identify the Interrupt Source

   • The CPU checks the Interrupt Vector Table (IVT) to locate the address of the correct Interrupt Service Routine (ISR).

4. Execute the ISR

   • The CPU runs the ISR (a small program) to handle the interrupt.

   • The ISR:

     • Acknowledges the interrupt (e.g., tells the keyboard its keypress was received).

     • Performs required actions (e.g., reads input data, handles errors).

   • The stack enables safe temporary storage during this process.

5. Restore and Resume

   • The CPU retrieves the saved state from the stack (reversing the save operation).

   • The original program resumes execution seamlessly.

Compare the storage mechanisms of Hard Disk Drives (HDDs) and Solid State Drives (SSDs).

HDD: Magnetic platters, moving parts; SSD: NAND flash, no moving parts. SSD Benefits: Faster, lighter, less power.

What are the benefits and drawbacks of packet switching networks?

• No single point of failure, + Reroutes around faults, - Packet reordering delay, - Unsuitable for real-time.

What are the advantages of serial transmission in USB connections?

Fewer wires → less interference, Longer distance reliable, Cheaper hardware.

Calculate the check digit for the ISBN-13 number 978034098382.

The check digit is 9.

What are three key roles of a router in a network?

1. Directs packets between networks, 2. Assigns IPs via DHCP, 3. Connects LAN to internet (gateway).

Why is data redundancy used in cloud storage systems?

Multiple server copies, Ensures access during maintenance, Prevents data loss.

List five types of utility software.

1. Anti-virus, 2. Defragmentation, 3. Backup, 4. Security, 5. Device drivers,6. screensavers

How does lossless text compression work?

Identifies repeated words/phrases, Replaces with shorter codes, Indexes positions.

Contrast Parity and Checksum methods for error detection.

Parity: Single-bit errors (per byte), Checksum: Multi-bit errors (per block).

Recommend one type of touchscreen technology and justify your choice.

Capacitive: Better visibility, multitouch; Resistive: Works with any object, cheaper; Infrared: Shatter-resistant, no calibration.

What is the key feature of the Von Neumann architecture?

Shared memory for instructions + data, Sequential FDE cycle.