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Computer Architecture | O Level Computer Science 2210 & IGCSE Computer Science 0478 | Detailed Free Notes To Score An A Star (A*)

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Computer Architecture

  • Focuses on how a computer actually works.
  • Important components
    • Central Processing Unit (CPU)
      • Also called processor or microprocessor
      • A computer cannot work without this part
      • Usually a single microchip contains the CPU as an integrated circuit.
      • All processing and execution occurs here.
      • 3 Main Parts
        • Control Unit (CU)
        • Arithmetic and Logic Unit (ALU)
        • Registers and Buses
  • Von Neumann Architecture
    • Early computers
      • Required data to be fed side by side as it was working
      • No stored programs
      • Needed a lot of human labor
    • Jon von Neumann
      • Solves this situation back n 1940s
      • Stored program computer
        • The basics of all computer systems today
      • Main features that were new to the system back in 1940s
        • CPU
        • Accessing the memory directly by CPU
        • Memories can stores data and programs
        • Stored programs have instructions that can be executed when the program is run.
    • Main Parts of the CPU
      • Arithmetic and Logic Unit (ALU)
        • Arithmetic operations
          • + – and shifting
        • Logic operations
          • AND and OR
        • Runs these operations when the program is being run
        • More than one ALU can be present in a computer for specific instructions
        • Multiplication and Division are not functions that ALU performs directly
          • Instead, it performs it using sequences of additions, subtraction or logical shift operations
      • Control Unit (CU)
        • Reads instructions from memory
        • The address where the instruction is stored is found in Program Counter (PCC)
        • The Fetch-Decode-Execute cycle is used to interpret the instruction
        • Signals are generated along the control bus
          • It tells other parts in the computer about what to do based on the instructions
        • Control Unit
          • Synchronizes the data flow and program instructions throughout the computer system
        • System Clock
          • Produces timing signals on the control bus to ensure synchronization
          • A computer can’t work without the system clock
        • Backing Store
          • CPU takes data and programs that are stored here.
          • Backing store is basically a secondary memory
          • This memory is permanent and non-volatile.
          • Example is hard disk drive
          • The data taken from here is paced in the RAM temporarily
    • RAM
      • All data and programs are contained on the RAM that are to be accessed by the CP
      • Remember, RAM is not the permanent storage
      • It is a volatile/ temporary memory – also the primary memory.
      • It is also called Immediate Access Store (IAS)
      • Read/ Write operations can be completed faster in RAM compared to backing storage
      • Any important data required by an application will be stored in RAM
      • Speeds the operations
    • Registers
      • Extremely important part of the architecture
      • Two types
        • General
        • Specific Purpose
          • Current Instruction Register (CIR)
            • Stores the current instruction that is being decoded and executed.
          • Accumulator (ACC)
            • During ALU calculations, data is stored here temporarily
          • Memory Address Register (MAR)
            • Stores address of the memory location currently being read or written
          • Memory Data/ Buffer Register (MDR)
            • Stores data that has just been read from memory
            • Or data that is about to be written in the memory
          • Program Counter (PC)
            • Address of the next instruction to be read
    • Memory
      • Partitions in computer memory
        • Each partition has contents and address
        • Address uniquely identifies every location in the memory
        • Contents stored in the location is binary
        • The steps are as follows
          • Read operation
            • The address of the location of the memory specified is written onto the MAR
            • A read signal is sent to computer memory
            • Content of the memory location is put into the MDR
          • Write operation
            • Data to be stored is first written onto the MDR
            • Data has to be written to a specified address – this address is written into the MAR
            • Write signal is sent to computer memory and the content will be written into the memory address
    • Input and Output Devices
      • Input devices convert external data to computerized form
      • Output devices convert computer data to human understandable form.
    • System Buses
      • Used in computers as parallel transmission components
      • Each write transmits one bit of data
      • 3 common buses
        • Address Bus
          • Carrier addresses throughout the computer system
          • Unidirectional between CPU and memory
            • Bit travel in one direction
            • Prevents addresses being carried back to the CPU
          • Bus width is extremely important
            • Wider bus can address more memory locations at the same time.
            • For example, a bus width of 32 can address 2^32 memory locations concurrently (4294967296 memory locations)
        • Data Bus
          • Data bus is bidirectional
          • Data ca be carried to and from CPU to memory/ or to and from input and output devices
          • Data can be an address, numerical value or instruction
          • Width of the bus is imporant
            • A wider bus can transport larger word length
            • Word in this case means a group of bits that can be regarded as one single unit. For example 64-bit.
            • Larger word length provides better overall performance
        • Control Bus
          • Bidirectional
          • Signals are carried from control unit (CU) to other system components
          • Usually 8-bits wide.
          • No wider bus needed because only control signals are sent.
    • Fetch-Decode-Execute Cycle
      • CPU first fetches some data/ instructions from memory and stores them in suitable registers
        • Address bus and data bus are used.
      • Each instruction has to be decoded
      • Then executed.
      • The cycle
        • Fetch
          • Data and instructions are stored in MDR
          • The next instruction is fetched from memory address stored in MAR
          • Instruction stored in MDR
          • Contents of MDR are copied on Current Instruction Register (CIR)
          • PC is incremented by 1 (increased by 1)
            • This provides the next instruction address
        • Decode
          • Interpret the instruction
        • Execute
          • The decoded instructed is passed by the CPU as a set of control signals to the appropriate components within the computer system
          • Each instruction is carried out in logical sequence
  • Cores, Cache and Internal Clock
    • What determines the performance of CPU
      • System clock
        • Defines the clock cycle
        • Synchronizes the computer operations
        • Control bus transmits the timing signals for synchronization
        • With increased clock speed, computer processing speed also increases.
        • For example, when we say a 4.2 GHz processor, it means it has 4.2 billion clock cycles a second.
        • However, speed increase does not mean that overall performance has been increased
      • Other Factors
        • Width of the address and data bus.
        • Over clocking
          • BIOS (Basic Input/ Output System) can be used to change the clock speed.
          • However, a clock speed higher than the computer’s supported design can result in problems
            • Unsynchronized operations when instructions are overclocked
            • One instruction not properly completed before the next one occurs.
            • Frequent crashes and instability
            • It can result in overheating of CPU, causing damage and unreliable performance
        • Use of Cache memories
          • Temporary memory inside the CPU
          • Faster access time than RAM
          • Frequently performed instructions are contained here.
          • First cache checked and then RAM accessed
          • Larger cache ensures better performance
        • Different number of Cores
          • (Quadcore, Octa-core etc.)
          • One core has one ALU, CU and registers
          • Multiple cores allow more sets of ALU, CU and registers
          • Clock speed does not need to be increased continually
          • However, increasing cores does not necessarily improve performance
            • CPU has to communicate with each core.
            • Reduce overall performance

Instruction Set

  • A set of operations that are decoded in a sequence is called instruction
  • Each operation
    • Instructions ALU and CU
  • Operation
    • Made of opcode and operand
    • Opcode informs CPU about what operations must be done
    • Operand is the data that is acted on or the register in the memory
  • Only a limited number of opcodes can be used
    • Called instruction set
  • All software on the computer will have a set of instructions
    • They need to be converted to binary form
  • The Fetch-Decode-Execute cycle is the step sequence that CPU uses to process each instruction.
  • X86 CPU instruction set
    • Common one
    • Share identical instruction set if based on it.

Embedded Systems

  • Combination of hardware and software that can carry out specific instructions
  • Such systems can be electrical, electronic or electro-mechanical
  • Can be based on
    • Microcontrollers
      • A CPU plus some RAM and ROM, along with peripherals all embedded together in a single chip
      • Can carry specific tasks together
    • Microprocessor
      • Integrated circuit which only bas a CPU
    • SoC
      • System on chips
      • A microcontroller present as one of the component
        • It has a CPU, memory, input/output (I/O) ports and secondary storage on a single microchip.
  • General work methods
    • Specific set of tasks
    • User interface
    • Some form of output
    • Very latest embedded systems have powerful CPUs and multiple I/O connections so multiple applications can occur
    • Can contain mechanical components, software, actuators and sensor inputs
    • Analogue or digital input
    • Feedback oriented systems
    • Either automatic data input or an operator can enter data.
    • Functions are performed on the data
    • Signals are sent to the controlled components to perform certain actions
    • Embedded system may be programmable or non-programmable.
      • Non-programmable devices are replaced to update software
      • Programmable devices have two methods of upgrade
        • Download the updates by connecting the device to the system.
        • Automatic updates via an internet source, for example Wi-Fi or cellular.
  • Benefits
    • Small size so fit easily
    • Lower costs to make
    • Dedicated to single task so simple interface
    • No operating system required
    • Minimal power consumption
    • Controlled remotely
    • Fast reaction to the input, so even real time work possible
    • Reliable due to mass production
  • Drawbacks
    • Difficult to upgrade
    • Specialists required to troubleshoot them
    • Confusing interface
    • Accessible to the hackers and viruses
    • Thrown away instead of repairing due to the difficulties in the repair
  • An computer is not an embedded system
    • It is multi-functional
  • Examples
    • Motor vehicles
      • Many parts are based on embedded functions
      • For example the GPS system or ABS system
    • Set-top box
      • Uses an embedded system to record and play television programs
      • Can be operated remotely
      • Has RAM, SSD, keypad interface, sensor input and output
    • Lighting systems
      • Architectural lighting uses these systems to sense light and adjust artificial lighting accordingly
    • Vending systems
      • Use micro-controller
        • To make selection
        • To show position of the gates
        • Coin counters
        • Temperature sensors
        • Actuators
        • Display systems
        • Wireless modems
    • Washing machines
      • White goods such as microwaves, refrigerators and washing machines etc. have embedded systems
      • Determine things like temperature, cycle time, water quantity, display results, have input system etc.
Hunain Zia

Hunain Zia has previously achieved considerable success in the high-school educational stream, gaining a massive 154 Total A Grades and 7 Major Distinctions, a process in which he broke/ set 11 different world records, including the most A grades ever achieved, most subjects ever appeared in, most distinctions, gaining distinctions across two separate boards (Pearson Edexcel and CAIE) within the same year/ ever, and even 19 hours of constant examination. All records stand intact today. Hunain pursued his Honors Accounting Degree from the prestigious Bentley University, and then completed an LLB (in Honors) from University of London. Currently, he manages multiple social and commercial projects, has founded digital streams and works tirelessly in the education sector.

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