System performance (Cambridge International AS Computer Science)

Cores

What is a core?

• A core acts like a mini CPU, it can fetch, decode, and execute instructions on its own

• Each core has its own:

  • Control Unit (CU)
  • Arithmetic Logic Unit (ALU)
  • Registers

• A CPU with more than one core is a multicore CPU

• Allows for parallel processing, multiple instructions are processed at the same time

• Example:

  • A dual-core processor has 2 cores
  • A quad-core processor has 4 cores

• More cores = better performance, especially for powerful programs

• Example:

  • A quad-core CPU can run up to four sequences of instructions at the same time, one on each core

• A dual-core processor isn't always twice as fast as a single-core

  • Some time is used for organising tasks between cores

• Not all tasks can be split across cores

  • Some tasks are sequential and must be done step-by-step

Clock speed

What is clock speed?

• The clock controls the timing of operations inside the CPU

• It constantly switches between 0 and 1, each switch is called a state change

• A state change can represent one step in the fetch-execute (F-E) cycle

  • Some instructions may take more than one cycle

• Clock speed measures how many state changes happen per second

• 1 cycle per second = 1 Hz

• A typical clock speed is around 2.3 GHz

  • That’s 2.3 billion cycles per second

• A higher clock speed means the CPU can execute more instructions per second

• This helps the computer run tasks more quickly and efficiently

Cache memory

What is cache memory?

• Cache is part of primary storage

• Stores frequently used data and instructions

• Located closer to the CPU than RAM, so it’s faster to access

• Some cache is built directly into each processor core

• Speeds up the performance of the CPU

• More cache = more data stored nearby = less time waiting for data from RAM

• Example:

  • When a program runs a loop, the same instructions are needed again and again
  • After they are first fetched from RAM, they are kept in cache
  • On later passes they are fetched from cache, which is faster than fetching from RAM, so the CPU spends less time waiting

Levels of cache

Diagram of the levels within the cache

Bus width

What is bus width?

• Bus width is the number of bits a bus can carry at once

• A wider bus can transfer more bits in a single operation

• Example:

  • A 32-bit bus transfers 32 bits at a time
  • A 64-bit bus transfers 64 bits at a time

• Increases performance, more data can be moved in the same amount of time

• A wider address bus means:

  • More memory locations can be directly addressed, e.g. a 32-bit address bus can address 2³² memory locations

• Important for high-performance tasks like gaming, video editing, or large data processing