Computer Science; Component 2 Computer systems

Computer Science; Component 2 Computer systems

1. Processor

   • Control Unit: Part of the processor which runs the bulk of the machine code. Has the responsibility of coordinating the rest of the processor
   • Arithmetic Logic Unit (ALU): Responsible for performing calculations and logic. Also acts as a gateway to the processor – all inputs and outputs to and from the processor have to pass through the ALU
   • Memory Unit (MU): Responsible for accessing the memory. CPU must pull instructions and data from main memory to the registers. An instruction can only run on data which is inside a register. The MU will store currently running programs currents files and the OS

2. Buses

   • Control Bus- this sends commands from the control unit to different parts of the processor
   • Buses are like pathways (like mini cables) used to send data around the processor
   • Data Bus- this carries data from one register to the other
   • Address Bus- this carries the location address to which the data is going

3. Von Neumann Harvard architecture has separate data and instruction busses, allowing transfers to be performed simultaneously on both busses. A von Neumann architecture has only one bus which is used for both data transfers and instruction fetches, and therefore data transfers and instruction fetches must be scheduled - they cannot be performed at the same time.

4. Hardware: Physical components that make up a computer system.

5. Software: Programs, data and code used to make the hardware work/produce a useful result

   • Types of software: Driver software: controls hardware such as hard disks and printers. Application software: allows user to perform tasks with the computer. Utility software: performs specific tasks

6. A CPU will contain many registers some of which will be reserved for specific calculations whilst others are used of general purpose calculations. A register is a small block of memory, normally about 4/8 bytes which is used for instructions to be processed. Machine instructions can only work if they are loaded into registers. Programs tend to load and save data into general purpose registers. These can be used for any purpose the programmer chooses

   • PC Program Counter. A special purpose register which stores the address of the next instruction to be executes. is incremented once read and controls the sequence of instructions
   • CIR Current Instruction Reader.A special purpose register which stores the instruction currently being executed by the CPU.Once an instruction is in the CIR it can be split into different parts and sent to the relevant locations
   • MDR Memory Data Register.A special purpose register which stores the data fetched using the address found in the MAR
   • ACC Accumulator.Will store the result of any calculation performed by the CPU
   • MAR Memory Address Register.A special purpose register which stores the address of the next instruction or data to be fetched into the CPU.This is where the address that was read by the PC is sent

7. Fetch-Decode-Execute Cycle. Process by which programs are run. The Fetch part retrieves an instruction from the memory unit and places it in the CIR. By looking at the mnemonic (instruction part of the code), the processor can work out what needs to be done. Decode. The Execute part is what happens next. So normally some data will be retrieved from the MU into the MDR, then the instruction is carried out

8. Memory. Two main types; ROM and RAM

   • ROM (read Only) means the data is stored and cannot be altered or deleted. Data is always in the ROM even if the computer is off . Used to store the computers BIOS and instructions needed to boot
   • RAM data is whipped when computer is turned off- volatile. Enables computer to do more than one thing

9. Different types if CPU. There are 3 types. To increase efficiency, an additional co-processor may be installed to carry out specific operations

   • Parallel Processor: these use multiple CPU’s to complete more tasks per unit time
   • Array Processors: (or vector processors) all work together on the same task
   • Mulit-core Processors: these can support multiple paths of execution (multireading) e.g. modern quad or dual core processors

10. Instruction Sets

    • Each CPU had an instruction set that it supports
      • Most modern CPU’s have a : Complex Instruction Set Computer or CISC architecture. Each instruction may take more than one cycle to complete
      • Some CPU’s have simple instructions. These are called: Reduced Instruction Set Computers or RISC architecture. Each instruction is simple and only takes one clock cycle to complete
    • Key difference between CISC and RICS
      • Instructions: RISC had fewer instructions than CISC. CISC generally hundreds of instructions whereas RISC has a lot less. RISC combines simpler instructions that are more efficient at performing simply tasks
      • Physical Size: in order to handle all the instructions, CISC CPU’s are larger
      • Speed: RISC CPU’s run at a much lower clock speed that CISC. They perform simpler tasks and are used in devices like tablets and smart phones where for examples advanced games don’t run
      • Energy Consumption: As CISC CPU’s are larger that use more electricity. RISC’s are designed to use less power and go into ‘sleep mode’ when no actively running
      • Design: Smartphones and tablets are designed to be a system on a chip (SoC). RISC’s are smaller, more functions (like memory and hardware) can be combined with the CPU on one chip. CISC’s are usually build into a system which has a fan to cool them down. Smartphones and tablets using RISC’s don’t need this hardware (fans).
      • Cost: RISC’s use less power and are cheaper to make

11. Operating Systems

    • An operating system is what allows the applications software and the hardware to communicate
    • It provides a platform on which the application software to run
    • It allows the computer to communicate with the outside world and the user
    • Single user/ multi-tasking
      • Only one person can use the system at any one time – not only one user can ever use it
      • It will allow you to protect your files with passwords and enable you to carry out actions using hardware and software
      • Will allow you to do more the one thing at a time. Multi-tasking
    • Batch Processing
      • Batch processing performs many similar tasks or jobs serially without the need for user intervention
      • For software to be considered for batch processing it must:
        • Have a large number of similar tasks or jobs
        • Be run with no user intervention (other than to start it)
        • Be able to run during out of office hours (or when there is light load on the computers)
    • Real-time System
      • For a system to be real-time tasks must always have a bounded time to complete.
      • Respond immediately to input otherwise there could be negative consequences
      • When an input in entered a response comes back immediately
      • This happens without human interaction
      • Transactional based systems are also considered to be real-time (think of more them one travel agent booking from the same database
    • Networking and Distributed Systems
      • Most modern day OS are network OS (NOS’s)
      • A NOS is different from a multi-user OS because even though the data is stored centrally, all of the processing is done locally (on your machine)
      • Once you log in you are enabled access to particular shared drives, every time you open a file a local copy will be created for editing
    • Multi- User
      • A good example of this is a supermarket checkout system- even though it may seem like each terminal is run independently, they are all in fact run from one central OS
      • If a database of information needs to be stored centrally and accessed by many, a mulit-user operating systems is used.