Components of a Computer System - Part 1

Components of a Computer System - Part 1

1. Computer systems

   • A computer is a machine that processes data
     • The purpose of a computer is to take data, process it, then output it. Computers were created to help process data and complete tasks more efficiently than humans
     • A computer system consists of hardware and software that work together to process data/complete tasks
       • Hardware - Hardware is the physical stuff that makes up your computer system, like the CPU, motherboard, monitor and printer
       • Software - Software is the programs or applications that a computer system runs. e.g. an operating system, a word processor or video game
     • There are many types of computer system. These range from small devices like calculators and watches, up to large supercomputers used by banks or for scientific applications. Computers may be general-purpose (designed to perform many tasks) or dedicated systems (designed for one particular function)
   • Embedded systems are Computers inside a larger system
     • Embedded systems are computers built into other devices, like dishwashers, microwaves and TVs. They are usually dedicated systems.
     • Embedded systems are often used as control systems - they monitor and control machinery in order to achieve the desired result, for instance, a dishwasher has an embedded system that controls the water pumps and water release mechanisms, manage the various dishwasher cycles and control the thermostat to keep the water at an appropriate temperature
     • As they're dedicated to a single task, embedded systems are usually easier to design, cheaper to produce, and more efficient at doing their task than a general-purpose computer
   • Computers contain components which work together
     • Power supply - supplies power to the motherboard, optical and hard drives and other hardware
     • Case cooling fan - extracts hot air from the computer case
     • CPU heat sink cooling fan - keeps the CPU at a steady temperature (CPUs generate a lot of heat)
     • CPU - the most important component. Does all the processing
     • Optical drive - for read/writing of optical disks
     • RAM sticks - Random Access Memory
     • Motherboard - the main circuit board in the computer, where the hardware is connected
     • Hard disk drive - internal secondary storage

2. The CPU

   • The CPU is the central processing unit
     • The CPU is the brain of the computer system
     • It processes all of the data nad instructions that make the system work
     • The processing power of the CPU depends on different characteristics, like its clock speed, number of cores and cache size
     • The CPU architecture describes the main components of the CPU, how they interact with each and with other pats of the computer system. Von Neumann and Harvard are the two main types of architecture.
   • The CPU has three main parts
     • The Control Unit (CU)
       • The control unit is in overall control of the CPU. Its main job is to execute program instructions by following the fetch-decode-execute cycle
       • It controls the flow of data inside the CPU (to registers, ALU, cache) and outside the CPU (to main memory and input/output devices)
     • The Arithmetic Logic Unit
       • The ALU basically does all the calcuations
       • It completes simple addition and subtraction, compares the size of numbers and can do multiplications and divisions using repeated addition and subtraction
       • It performs logic operations such as AND, OR and NOT and binary shifts - remember computers process binary
       • It contains the accumlator register
     • The Cache
       • The cache is very fast memory in the CPU. It's slower than the registers but faster than RAM
       • It stores regularly used data so that the CPU can access it quickly the next time it's needed. When the CPU requests data, it checks the cache first to see if the data is there. If not it will fetch it from the RAM.
       • Caches have a very low capacity and are expensive compared to RAM and secondary storage
       • There are different levels of cache memory - L1, L2 and L3. L1 is quickest but has the lowest capacity. L2 is slower than L1 but can hold more. L3 is slower than L2 but can hold more
     • The CPU also contains various registers which temporarily hold tiny bits of data needed by the CPU. They are super quick to read/write to, much quicker than any other form of memory. You need to know about the program counter, memory address register (MAR), a Memory data register (MDR) and the accumulator
   • Von Neumann's design revolutionised computing
     • The Von Neumann architecture describes a system where the CPU runs programs stored in memory. Programs consist of instructions and data which are stored in memory addresses
     • The program counter (PC) holds the memory address of the instruction for each cycle
     • The accumulator stores intermediate results of calculations in the ALU.
     • The memory address register (MAR) holds any memory address about to be used by the CPU. The address might point to data or a CPU instruction
     • The memory data register (MDR) holds the actual data or instruction. This may have been fetched from memory, or be waiting to be written to memory
     • The memory holds the program instructions and the program data
   • CPUs follow the fetch, decode and execute cycle
     • Fetch instruction - Copy memory address from the program counter to the MDR. Copy the instruction stored in the MAR address to the MDR. Increment (increase) the program counter to point to the address of the next instruction, ready for the next cycle
     • Decode instruction - The instruction in the MDR is decoded by the CU. The CU may then prepare for the next step, e.g. by loading values into the MAR or MDR
     • Execute Instruction - The instruction is performed. This could be: load data from memory, write data to memory, do a calculation or logic operation (using the ALU), change the address in the PC, or halt the program