Central Processing Unit (CPU)

Syllabus Detail

  • Functions of processing;CPU, ALU, CU, registers. 11 GEN
  • Functions of ALU, CU, registers, program counter, system clock. 12 GEN
  • Functions of buses (data/address/control) for 11 ATAR(p11), 12 ATAR
  • Purpose of processor architectures for different types of systems. 12 ATAR

 

What is the CPU?

  • CPU stands for Central Processing Unit
  • The CPU can be seen as the "brains" of a computer system
  • When talking about processors, we're generally referring to the CPU
  • The CPU is responsible for gathering and executing instructions (programs) in order, as well as controlling peripherals

 

CPU Components and their purpose

  • The ALU stands for arithmetic logic unit
    • The ALU performs all arithmetic (mathematic) and logic operations
    • Arithmetic operations are things like 1 + 1 = 2
    • Logical operations are things like comparing to see if 1 is greater than 2 (1 > 2 = FALSE)

  • The CU stands for control unit
    • The CU is responsible for extracting the instructions from memory and decoding them
    • The ALU is called upon when necessary for arithmetic and logical processing
    • The CU also monitors attached hardware (peripherals) and ensures the program's instructions for the hardware are run
      • For example, the control unit will ensure that a program's "Scan" feature is directing its instructions towards the scanner peripheral
    • Essentially, the control unit tells the computer's memory, ALU and input/output devices what to do

  • Registers are small amounts of memory available to the CPU
    • Registers are at the top of the memory hierarchy, as they can be accessed the fastest
    • This is due to the positioning of the registers (as they're placed directly inside a CPU)
    • Registers are typically measured by the amount of bits they can hold (e.g. 8-bit registers, 32-bit registers)
    • CPU's are designed to only act upon data once stored in a register (Read The Computing Teacher's article on the Fetch-Execute Cycle here)

  • The program counter is a special type of register that contains the address of the instruction being executed
    • Memory is split up in to memory locations
    • The program counter stores the address (location) of the instruction being processed at any point in time
    • As each instruction gets fetched, the program counter increments (increases) by 1
    • This allows the computer system to indicate where the program is in its instruction sequence

  • The system clock is a microprocessor that regulates the timing and speed of a computer system's functions
    • The system clock chip contains a special crystal that vibrates at a specific frequency when power is applied to the computer
    • The shortest time a computer is able to perform is known as one clock, meaning one vibration of the clock chip
    • The speed of a computer's CPU is measured in Hertz
    • Typical processors have a speed in GigaHertz (GHz)
      • For example, a processor with a speed of 3.8GHz means 3.8 billion clock cycles (vibrations) per second
    • The system clock also helps the computer keep the correct time (the number of second from an arbitrary value, e.g. the 1st of January 1970 on Linux systems)

 

Buses (Address, Control and Data)

  • Buses are communication systems inside a computer that transfers data between components
  • A computer utilizes buses to send and received binary data as encoded electrical signals

  • The address bus specifies the the location of a physical memory address
    • The location refers to the memory address that the processor will read data from or write data to
    • This includes primary memory, secondary memory and other connected peripherals
    • The width of an address bus is responsible for the amount of memory that can be addressed
    • For example, a 32-bit bus can address 4,294,967,296 (2 ^ 32) bytes of memory
    • This is why there is a limitation of 4GB of RAM available on a 32-bit computer

  • The control bus is used by CPUs for communicating with other components and peripherals in a computer system
    • This bus is responsible for managing information flow between components
    • It indicates whether the operation is read or write
    • The control bus also ensures that the operation is happening at the correct time

  • The data bus contains the actual data being sent around the computer system
    • The data bus is responsible for connecting the CPU to other devices on the system
    • Unlike the address bus, the data bus is capable of two way operations
    • This means it supports both read and write operations
    • The data bus is also responsible for transferring program instructions to the CPU

 

Processor Architectures

  • A processor architecture refers to the complex art of designing and constructing CPUs
  • RISC or reduced instruction set computers is one type of CPU structure.
    • used in high end CPUs for applications that need video processing, supercomputers etc
    • has no memory unit and uses separate hardware to process instructions
    • Advanced RISC Machine (ARM) is most common in many smartphones
    • very fast
  • CISC or complex instruction set computers is another type of CPU structure
    • used in low end CPUs for applications like modern cars, home security, home automation such as vacuum cleaners, smart fridges
    • has a memory unit to conduct complex instructions
    • not so fast

 

Further Research

  1. To be added.
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