The smallest unit of Computer Memory: bit

  • Computers use memory devices built with electronics

  • The smallest memory device used by the computer works like a switch:

  • The smallest memory device can be in one of 2 states:

      • off state (which we call state 0)
      • on state (which we call state 1)   -- (0 and 1 are called binary digits)  

  • The smallest memory device is called a bit (= binary digit)

Building computer memory that can store larger numbers

  • A bit can be in one of 2 states:   0 or 1

      off = 0   on = 1

    Therefore, a bit can store (= remember) one of 2 values:   0 or 1

  • A row of n bits can be in one of 2n states

    Each switch can be in 2 states.
    The total # combinations = 2 × 2 × ... × 2 = 2n

Byte memory

Byte memory (or "byte" for short):

  • Byte = 8 bits

    = a memory device that can retain (= remember) 8 binary digits

Each byte memory can store one of the following 256 possible patterns of binary numbers: 

 00000000  00000001  00000010  00000011  00000100  00000101  00000110  00000111
 00001000  00001001  00001010  00001011  00001100  00001101  00001110  00001111
 00010000  00010001  00010010  00010011  00010100  00010101  00010110  00010111
 00011000  00011001  00011010  00011011  00011100  00011101  00011110  00011111
 00100000  00100001  00100010  00100011  00100100  00100101  00100110  00100111
 00101000  00101001  00101010  00101011  00101100  00101101  00101110  00101111
 00110000  00110001  00110010  00110011  00110100  00110101  00110110  00110111
 00111000  00111001  00111010  00111011  00111100  00111101  00111110  00111111
 ....
(And so on... too many to list)

(1 bit can store 21 = 2 patterns. So 1 byte (= 8 bits) can store 28 = 256 patterns.)

Contrasting how humans and computers store data/information

Humans are very flexible and can use many (different) methods to store data/information.
Example: a commonly used human method to save data/information:

 

Computers can only store data/information as binary numbers (using electrical switches)
This is how a computer (must) stores (= "writes") the letter A (which is 01000001):

BTW: an international committee decided that 01000001 represents the letter A long time ago...

Memory address and memory content

  • Each memory cell is identified by a unique address (= a number)

  • Each memory cell stores a number

What does the binary number system has to do with computer memory ?

  • The computer uses the binary number system to represent the numbers:

  • Each memory cell is identified by a unique address as a binary number

  • Each memory cell stores a number as a binary number