Dracula's Definitions of Computer Memory Sizes
=== by Bob Sutherland ===
Traditional Measuring Units for Computer Memory
On this web page I will list the traditional definitions of computer memory sizes that I first learned. These are the definitions that I taught my students throughout my teaching career in the classroom.
I have also created a web page Modern Measuring Units for Computer Memory but I suggest you wait until after reading this page before viewing it.
- A bit is the smallest unit of computer memory size.
- The word bit was created from the words binary digit after removing most of the letters.
- A computer is made up of wires. A single wire can store a bit of memory.
- By turning an electric switch on (such as a light switch on a wall) you allow an electric current to flow through a wire. This ON state with electric current flowing through the wire is represented by the binary digit 1 (one).
- If you turn the electric switch off there will be no electric current flowing through the wire. This OFF state in which there is no electric current flowing through the wire is represented by the binary digit 0 (zero).
- We can therefore represent a single bit of computer memory using the numbers one or zero to indicate whether or not there is an electric current flowing through a computer wire.
- Modern electrical appliances that you can buy in a store now often label the power on/off switch with the symbols 0 and 1.
- In computer logic the on state or 1 is refered to as True and the off state or 0 is refered to as False. This allows us to work with simple true or false type questions.
A bit of computer memory can be used to store the answer to any question for which there is only two possible answers. For example, is it raining outside? Are you a boy or a girl? Is your bathing suit wet or dry? Are your shoes black or white? If you want to allow more than two answers, such as maybe your shoes are brown, then you will need more than one bit of computer memory to store the answers.
In real life if you were to hold a piece of bare wire stretched out a meter or more between your two outstretched hands then there would always be at least a minimal amount of electric current flowing through the wire. Your body, the magnetic field of the earth as it spins on its axis, nearby machinery, overhead electical power lines, and naturally occurring electromagnetic fields in the environment around you will all cause an electric current to flow through a metal wire. You may have difficulty finding a machine sensitive enough to measure the electric current flowing along the wire but it will be there.
Computer scientists when they were building the first computers recognized this problem of the background environment causing an electric current to flow along a wire. They therefore defined the OFF state as being an electric current of less than 3 volts. An electric current of 3 to 6 volts - with 5 volts being the desired target voltage - was defined as the ON state for the early computers.
1 Nibble = 4 Bits
- A nibble is sometimes spelt as nybble.
- A nibble is a unit of computer memory size that is four times as large as a bit.
- It requires four wires to store one nibble of computer memory.
- We represent a nibble using four binary digits such as 1101 or 0110.
In Canada, the old telephone wires in many homes contained four thin insulated wires within the outer protective insulation. I have therefore used recycled old telephone wires for electical experiments in school to demonstrate the storage or transport of one nibble of computer memory.
Here is a list of the sixteen different combinations of binary digits that can be stored in a nibble of computer memory:
Dracula is getting thirsty! At first he was making small insect sized pecking bits along the back of your hand and arm. Then he started to nibble your ears. Now he wants to BYTE YOUR NECK!
1 Byte = 2 Nibbles = 8 Bits
- A byte is a very commonly used unit of computer memory size.
- A byte can be represented using eight binary digits such as 00101101 or 10111000.
- There are 256 different combinations of binary digits that can be stored in a single byte of computer memory.
- Each time you press a key on an English keyboard your computer will require one byte of computer memory to store the letter, number or symbol you typed.
1 Kilobyte = 1024 Bytes
1 Megabyte = 1024 Kilobytes
= 1024 x 1024 = 1048576 Bytes
1 Gigabyte = 1024 Megabytes
= 1024 x 1024 = 1,048,576 Kilobytes
= 1024 x 1024 x 1024 = 1,073,741,824 Bytes
1 Terabyte = 1024 Gigabytes
= 1024 x 1024 = 1,048,576 Megabytes
= 1024 x 1024 x 1024 = 1,073,741,824 Kilobytes
= 1024 x 1024 x 1024 x 1024 = 1,099,511,627,776 Bytes
I think Dracula got a bit carried away. He went way beyond his own ability to Byte and gave us definitions that might be useful for measuring the bites of dinosaurs and whales.
Summary of Traditional Measuring Units
Here is a summary chart of Dracula's definitions for computer memory size using the traditional measuring units. Remember these units are based on computer wires and binary numbers.
|Bit||is the smallest unit of computer memory size.|
|Nibble||= 4 bits|
|Byte||= 2 nibbles = 8 bits|
|Kilobyte||210 bytes||10241 bytes||= 1024 bytes|
|Megabyte||220 bytes||10242 bytes||= 1024 kilobytes|
|Gigabyte||230 bytes||10243 bytes||= 1024 megabytes|
|Terabyte||240 bytes||10244 bytes||= 1024 gigabytes|