MAIN MEMORY

1. The main memory stores programming instructions, data and results.
   
   
2. It consists of a long list (millions and billions) of numbered locations called memory locations.
   
   
3. Each memory location contains a string of digits of fixed length.
   
   
4. Each digit can take on the values of zero or one only; it is called a binary digit or a bit.
   
   
5. A bit is the smallest unit of memory.
   
   
6. Typically each memory location contains 8 bits or a byte.
   
   
7. Each memory location is identified by a number called its memory address.
   
   
8. Data item (such as a large integer) too big to fit in a single byte is stored using several adjacent bytes; this chunk of memory is still called a memory location; the address of the first byte is used as the memory address for this chunk of memory location.
   
   
9. Coding schemes are used to interpret the strings of zeros and ones as numbers, letters, instructions, or other types of information.
   
   
10. There are 2 types of main memory:
    
    
    • ROM - Data or instructions stored in Read-Only-Memory can be read by the computer but cannot be re-written. ROM is used to store vital information when a computer boots up, for example.
      
      
    • RAM - Data stored in the RAM can be immediately access in any order.
      Both reading and writing are allowed (writing will over-writes previously stored data).
      RAM comes in chips of different memory capacities and speeds.
      Information stored in the main memory can be retrieved or re-written at incredible speeds.
      Information stored is only temporary, and disappears when the power is turned off.
      Compared with secondary memory, they are more expensive.