OCR A Level Computer Science – Paper 1 Detailed Revision Notes
Topic 1.1: Structure and Function of the Processor
The CPU (Central Processing Unit) is the core of a computer system, responsible for
processing instructions and managing operations.
Key Components:
• Arithmetic Logic Unit (ALU):Performs all arithmetic calculations
(addition, subtraction, multiplication, division) and logic operations (AND, OR,
NOT, XOR). It processes the actual data.
• Control Unit (CU):Directs the flow of data between the CPU and other
components. It controls the fetch-decode-execute cycle by interpreting instructions
and sending signals to other parts of the system.
• Registers:Very fast, small storage areas within the CPU. Key registers
include:
• Program Counter (PC): Holds the address of the next instruction to
execute.
• Memory Address Register (MAR): Holds the address of the memory location
to read/write.
• Memory Data Register (MDR): Holds data being transferred to/from
memory.
• Current Instruction Register (CIR): Holds the instruction currently
being decoded/executed.
• Accumulator (ACC): Holds intermediate arithmetic and logic results.
Buses:
• Address Bus: Carries memory addresses from the CPU to memory. It is
unidirectional (CPU → Memory).
• Data Bus: Transfers actual data between CPU, memory, and I/O devices.
It is bidirectional.
• Control Bus: Sends control signals (e.g., read/write commands,
interrupt requests).
Fetch-Decode-Execute Cycle:
1. Fetch: The CPU fetches the instruction from memory. The PC provides the
address to the MAR; the instruction is loaded into the MDR and then into the CIR.
The PC increments to point to the next instruction.
2. Decode: The CU decodes the instruction to understand the operation and
operands.
3. Execute: The instruction is executed — this could involve calculations
by the ALU, data transfer, or I/O operations.
Clock Speed:
• Measured in hertz (Hz), indicating the number of cycles per second.
Higher clock speeds mean more instructions can be executed per second.
Performance Factors:
• Number of Cores: More cores allow parallel processing of instructions,
increasing throughput.
• Cache Memory: Small, fast memory close to the CPU to store frequently
accessed data and instructions, reducing access time.
• Pipelining: A technique where multiple instruction stages are
overlapped to increase CPU efficiency.
Architectural Designs:
• Von Neumann Architecture: Uses a single shared memory and bus for both
instructions and data, which can cause a bottleneck called the von Neumann
bottleneck.
• Harvard Architecture: Uses separate memory and buses for instructions
and data, allowing simultaneous access and improving speed.
Topic 1.2: Types of Processor
CISC (Complex Instruction Set Computer):
• Large and complex instruction set with multi-step operations in a
single instruction.
, • Instructions may vary in length and complexity.
• Fewer instructions per program but each instruction takes longer to
execute.
• Examples: Intel x86 processors.
RISC (Reduced Instruction Set Computer):
• Smaller, highly optimized set of simple instructions.
• Each instruction generally takes one clock cycle.
• Requires more instructions for complex operations but executes faster
overall.
• Used in ARM processors common in mobile devices.
Multicore Processors:
• Multiple independent cores within a single CPU chip.
• Can execute multiple threads or processes simultaneously (parallelism).
• Improves performance for multitasking and multithreaded applications.
Parallel Systems:
• Multiple CPUs working together on a single task, often in
supercomputers or large-scale data centers.
• Can divide tasks into smaller chunks processed simultaneously.
Co-processors:
• Specialized processors designed to handle specific types of tasks, such
as floating-point arithmetic or graphics processing.
• The GPU (Graphics Processing Unit) is the most common co-processor,
optimized for highly parallel processing.
GPU (Graphics Processing Unit):
• Contains thousands of cores designed for parallel processing.
• Excellent at rendering images and video, and increasingly used for AI
and scientific computation due to parallelism.
Topic 1.3: Input, Output and Storage Devices
Input Devices:
• Devices used to enter data into a computer system. Examples:
• Keyboard and mouse (user input)
• Microphone (audio input)
• Scanner (image input)
• Sensors (temperature, light, motion) for embedded systems and IoT
devices.
Output Devices:
• Devices that present data from the computer to the user or environment.
Examples:
• Monitor (visual output)
• Printer (hard copy output)
• Speakers (audio output)
• Actuators in robotics (mechanical movement).
Storage Devices:
• Primary Storage:
• RAM (Random Access Memory): Volatile memory used for temporary storage
of programs and data being executed. Data is lost when power is off.
• ROM (Read-Only Memory): Non-volatile memory storing firmware or
bootstrap programs essential for system start-up.
• Secondary Storage:
• Hard Disk Drive (HDD): Uses spinning magnetic disks, large capacity but
slower access speed.
• Solid State Drive (SSD): Uses flash memory, faster than HDDs, no moving
parts, more durable.
• Optical Storage: CDs, DVDs, Blu-ray discs use laser technology for
reading and writing data. Mainly used for media distribution.
• Flash Storage: USB flash drives and memory cards using solid-state
technology. Portable, fast, and non-volatile.
• Tertiary Storage: Used for backups and archival, often magnetic tapes
Topic 1.1: Structure and Function of the Processor
The CPU (Central Processing Unit) is the core of a computer system, responsible for
processing instructions and managing operations.
Key Components:
• Arithmetic Logic Unit (ALU):Performs all arithmetic calculations
(addition, subtraction, multiplication, division) and logic operations (AND, OR,
NOT, XOR). It processes the actual data.
• Control Unit (CU):Directs the flow of data between the CPU and other
components. It controls the fetch-decode-execute cycle by interpreting instructions
and sending signals to other parts of the system.
• Registers:Very fast, small storage areas within the CPU. Key registers
include:
• Program Counter (PC): Holds the address of the next instruction to
execute.
• Memory Address Register (MAR): Holds the address of the memory location
to read/write.
• Memory Data Register (MDR): Holds data being transferred to/from
memory.
• Current Instruction Register (CIR): Holds the instruction currently
being decoded/executed.
• Accumulator (ACC): Holds intermediate arithmetic and logic results.
Buses:
• Address Bus: Carries memory addresses from the CPU to memory. It is
unidirectional (CPU → Memory).
• Data Bus: Transfers actual data between CPU, memory, and I/O devices.
It is bidirectional.
• Control Bus: Sends control signals (e.g., read/write commands,
interrupt requests).
Fetch-Decode-Execute Cycle:
1. Fetch: The CPU fetches the instruction from memory. The PC provides the
address to the MAR; the instruction is loaded into the MDR and then into the CIR.
The PC increments to point to the next instruction.
2. Decode: The CU decodes the instruction to understand the operation and
operands.
3. Execute: The instruction is executed — this could involve calculations
by the ALU, data transfer, or I/O operations.
Clock Speed:
• Measured in hertz (Hz), indicating the number of cycles per second.
Higher clock speeds mean more instructions can be executed per second.
Performance Factors:
• Number of Cores: More cores allow parallel processing of instructions,
increasing throughput.
• Cache Memory: Small, fast memory close to the CPU to store frequently
accessed data and instructions, reducing access time.
• Pipelining: A technique where multiple instruction stages are
overlapped to increase CPU efficiency.
Architectural Designs:
• Von Neumann Architecture: Uses a single shared memory and bus for both
instructions and data, which can cause a bottleneck called the von Neumann
bottleneck.
• Harvard Architecture: Uses separate memory and buses for instructions
and data, allowing simultaneous access and improving speed.
Topic 1.2: Types of Processor
CISC (Complex Instruction Set Computer):
• Large and complex instruction set with multi-step operations in a
single instruction.
, • Instructions may vary in length and complexity.
• Fewer instructions per program but each instruction takes longer to
execute.
• Examples: Intel x86 processors.
RISC (Reduced Instruction Set Computer):
• Smaller, highly optimized set of simple instructions.
• Each instruction generally takes one clock cycle.
• Requires more instructions for complex operations but executes faster
overall.
• Used in ARM processors common in mobile devices.
Multicore Processors:
• Multiple independent cores within a single CPU chip.
• Can execute multiple threads or processes simultaneously (parallelism).
• Improves performance for multitasking and multithreaded applications.
Parallel Systems:
• Multiple CPUs working together on a single task, often in
supercomputers or large-scale data centers.
• Can divide tasks into smaller chunks processed simultaneously.
Co-processors:
• Specialized processors designed to handle specific types of tasks, such
as floating-point arithmetic or graphics processing.
• The GPU (Graphics Processing Unit) is the most common co-processor,
optimized for highly parallel processing.
GPU (Graphics Processing Unit):
• Contains thousands of cores designed for parallel processing.
• Excellent at rendering images and video, and increasingly used for AI
and scientific computation due to parallelism.
Topic 1.3: Input, Output and Storage Devices
Input Devices:
• Devices used to enter data into a computer system. Examples:
• Keyboard and mouse (user input)
• Microphone (audio input)
• Scanner (image input)
• Sensors (temperature, light, motion) for embedded systems and IoT
devices.
Output Devices:
• Devices that present data from the computer to the user or environment.
Examples:
• Monitor (visual output)
• Printer (hard copy output)
• Speakers (audio output)
• Actuators in robotics (mechanical movement).
Storage Devices:
• Primary Storage:
• RAM (Random Access Memory): Volatile memory used for temporary storage
of programs and data being executed. Data is lost when power is off.
• ROM (Read-Only Memory): Non-volatile memory storing firmware or
bootstrap programs essential for system start-up.
• Secondary Storage:
• Hard Disk Drive (HDD): Uses spinning magnetic disks, large capacity but
slower access speed.
• Solid State Drive (SSD): Uses flash memory, faster than HDDs, no moving
parts, more durable.
• Optical Storage: CDs, DVDs, Blu-ray discs use laser technology for
reading and writing data. Mainly used for media distribution.
• Flash Storage: USB flash drives and memory cards using solid-state
technology. Portable, fast, and non-volatile.
• Tertiary Storage: Used for backups and archival, often magnetic tapes
