Computers

Computers

1. Machines and computational models

   • Input-process-output model
     • The Input stage represents the flow of data into the process from outside the system
     • The processing stage includes all tasks required to effect a transformation of the inputs
     • The output stage is where the data and information flow out of the transformation process
     • The storage stage keeps data when the computer is switched off
   • Parallel algorithms
     • A parallel algorithm is an algorithm that can be executed a piece at a time on different processing devices
     • Parallel algorithms are executed concurrently
     • Non-parallel, non-concurrent algorithms are often referred to as sequential algorithms
   • Sequential algorithms
     • A sequential algorithm is an algorithm that is executed sequentially
   • Multi-agents
     • Multi-agents don't have to be on different machines; they could be multiple processes on a single chip or machine
     • They act without centralised control
     • Multi-agent systems are often used in artificial intelligence
   • Virtual machines
     • Any virtual machine is a combination of a real machine and virtualising software
     • Types of virtual machines - Whole system virtual machines are more dynamic and more complex than any other type of virtual machines
       • Whole system virtual machines provide a complete syatem platform in order to support the running and the execution of a complete operating system
       • An advantage of system virtaul machines is that the user can have multiple virtual machine operating on the same computer
       • The main disadvantage is that a virtual machine is less efficient than a real machine as it accesses the hardware indirectly

2. Hardware

   • Hardware components of a computer
     • Motherboard
       • The motherboard links the CPU to the memory and other hardware
       • The CPU is connected to the other hardware components via a bus
       • The CPU computes data and uses the motherboard to receive and send signals
       • The motherboard have speeds too, called the bus speed. The speed of the bus is measured in megahertz (MHz).
       • Motherboards have many buses; each one transfers data from one computer to another
   • CPU
     • Control unit: fetches, decodes, executes instructions
     • Arithmetic logic unit: performs arithmetic and logical operators
     • Registers: on-chip memory inside in the CPU
     • Internal buses to connect the comonents
     • Logic gates to control the flow of information
   • Fetch-decode-execute cycle
     • Firstly, it fetches the instruction from the counter/control unit in the processor, which has the instructions to be undertaken.
     • Next, the processor copies this information to the main memory
     • Then, the instructions are copied to the current instructions register for decoding and execution
     • Finally, the decoder interprets the instructions
     • The decode cycle is used for interpreting the instruction that was fetched in the fetch cycle
   • Memory
     • The CPU can't fetch data direct from the hard disk
     • Cache
       • The cache is very high speed memory and it draws the data from memory called RAM
       • RAM is much faster thatn the hard disk, however, RAM is not as fast a the CPU
       • Cache technology uses very fast but small amounts of memory
     • Cache organisation
       • Temporal Locality: the CPU accesses a data source, the cache knows it is likely to need to access the same source again
       • Spatial Locality: the CPU accesses a particular location in memory, the cache knows that it is portable that it will also need to access data that is close to the original data
       • Sequentiality Locality: the CPU accesses location (a), it is likely that a reference to the location of (a) + 1 will also be needed
   • Categories of memory
     • There are 2 main categories of chip-based memory: Volatile and Non-volatile
     • Volatile memory
       • Volatile memory is computer memory that requires a power supply
       • RAM is a type of volatile memory
       • Volatile memory can't store data when the computing device is turned off
       • Virtual memory is a section of the hard disk used as if it were RAM to supplement the amount of main memory available to the computer
     • Non-volatile memory
       • The purpose of non-volatile memory is secondary or long-term persistent storage
       • Read-only memory is store for data in a computer that cannot be overwritten
       • Data in ROM is always available and is not lost when the computer is turned off