Computers

Computers

1. Hardware

   • storage
     • secondary storage
       • Magnetic storage
         • Inside hard disk drive are platter (stacks of disks). Each surface has a thin magnetic coating which can be individually magnetised. Magnetised=1 demagnetised=0. A platter has 2 read-write heads. To write data the heads alter the magnetic state of the disk
       • Optical storage
         • CD, DVD etc - made of several layers. A player uses 2 lasers; one to read data on the disk, one to write
       • Solid-state storage/drive
         • Represent bits by little pools of trapped electrons on a microchip.
     • main memory
       • RAM has a collection of storage locations with its own unique memory address. These can be accessed directly by the CPU, and the CPU can also read and write data to the memory
   • CPU(Central Processing Unit)
     • Buses
       • Address Bus - carries address of the memory location. Unidirectional
       • Data bus - holds value being read or written to memory. Bidirectional
       • Control bus - carries command from CU to others components so they know whether to read or write data back to CPU
     • Control Unit (CU) - decodes instructions from main memory and organises actions of other parts of CPU to execute them
     • Arithmetic logic unit(ALU) - performs arithmetic and logic operation on data
     • Registers
       • Provide fast and temporary storage for instructions and data. Some are general purpose some have a designated function
         • Instruction register holds instruction currently being executed
         • Accumulator holds resulats of calculations peformed by ALU
     • Clock - send out electrical signals at regular intervals to synchronise actions. Faster the clock the more instructions the CPU can execute per second
     • Fetch-decode-execute cycle
       • Fetch:
         • 1) CU places the memory address of the next instruction on the address bus
           • 2) It sends a read signal to the RAM along the control bus
             • 3) The RAM receives the signal and copies the content of the memory location from the address bus onto the data bus
       • Decode:
         • 4) The CU decodes the instruction
       • Execute
         • 5) The CU sends signals to the other components of the CPU instructing them what to do

2. software