OCR Computer Science A level paper 1 / Questions & Expertly Verified Answers, 2025/ 2026.
PC - Answer: Program counter, contains the address of the next instruction
CIR - Answer: Current Instruction Register: stores the address of the next instruction currently being executed and decoded MAR - Answer: Memory Address Register, holds relevant memory address (to read from or write to) MDR - Answer: Memory Data Register, stores data being transferred to and from memory, acts as a buffer ALU - Answer: Arithmetic and Logic Unit, does all mathematical calculations and makes all logical decisions Accumulator - Answer: A storage register in the ALU that holds data temporarily while the data is processed and before it is transferred to memory.RISC - Answer: Reduced Instruction Set Computer: Only simple instructions taking one clock cycle are executed - currently more widely used than CISC
Advantages: allows for pipelining, execution will be quicker or as fast as CISC
Disadvantages: Compiler has to do more work, more RAM required 1 / 3
CISC - Answer: Complex Instruction Set Computer: Large instruction set is used to minimise lines of code required - mainly used for microcontrollers and embedded systems
Advantages: compiler has very little work to do, not much RAM required
Disadvantages: Building in specialised instructions can be inefficient when only ~20% get used per program GPU - Answer: Graphics Processing Unit - dedicated graphics processor on a graphics card, used to render images stored in the RAM of the graphics card.Multi-core CPU - Answer: A CPU with more than one core on the same chip - makes use of parallel processing for optimum efficiency What affects processor performance? - Answer: - The number of processor cores: more cores -> better performance
- The processor's clock speed: faster clock speed -> better performance
- Amount/type of cache memory: more/better cache memory -> better performance
- Memory management
- Interrupt service routines
- Processor scheduling
- Backing store management
- Input and output management 2 / 3
RAM - Answer: Random Access Memory - stores programs and data currently in use. Volatile ROM - Answer: Read Only Memory - holds information which must be permanently in memory eg. BIOS What are the functions of an operating system? - Answer: - Providing a user interface
What are the types of memory management? - Answer: Paging: Available memory is divided into fixed size pages, process in memory can be held in non-contiguous pages, mapped to logical addresses by a page table
Segmentation: Divides address space logically into segments of varying length
What is virtual memory? - Answer: A technique that uses a portion of the computer's hard disk as an extension of RAM - swaps files in and out of RAM by a process called disk thrashing What is a distributed operating system? - Answer: Form of parallel processing system which splits tasks over multiple computers - OS coordinates so that the system appears to the user as a single system What is a multitasking operating system? - Answer: An OS which allows each user to run more than one job at a time.What is a multi-user operating system? - Answer: Allows multiple users to use a computer system by allocating each one a slice of processor time - uses a scheduling algorithm What does a mobile operating system comprise of? - Answer: Mobile OSes are made up of two
separate operating systems:
A main system operating the user interface/application software A low-level real-time operating system which is hardware-specific What is an embedded operating system? - Answer: An OS on a chip instead of on a hard drive, designed to do simple things adapted for specific hardware/functions What is a real-time operating system - Answer: Processes data as it comes in - in real time
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