MORE NEAT STUFF ABOUT BITS AND
BYTES
How many 8-bit panels or bytes were there in a standard IBM or IBM-compatible
computer in 1980? Take a guess. Ten thousand panels, twenty thousand panels, one
hundred thousand panels? The answer is 640,000 8-bit panels or 640,000 bytes.
This is usually written as 640 KB meaning 640 kilobytes (kilo means thousand).
The total number of bytes is commonly referred to as the computer's
memory capacity. And remember, each of these 640,000 bytes can hold any
of 256 possible conditions representing numbers from 0 to 255. Awesome!
So far we have discussed how the computer holds numbers but we have not
mentioned letters of the alphabet. Obviously if a computer can do word
processing it must understand the alphabet, right? Wrong! Computers have no idea
what the alphabet is. Remember, the only thing a computer
understands is whether switches are off or on. Every letter of the
alphabet and every punctuation mark is represented by a numerical value which is
held in a single byte. The uppercase letter "A" has a numerical value of 65 and
the lowercase letter "a" has a numerical value of 97. A comma (,) is number 44,
a colon (:) is number 58. Even a blank space ( ) has a number, 32. So how does
the computer know that a byte containing the value 65 is the letter "A" and not
the number 65? Once again it is the CPU or "brain" that keeps track of every
byte and knows which is which.
A old computer with a memory capacity of 640 kilobytes is surprizingly
versatile. It can be used for word processing, accounting, filing, games, etc.
In order to perform any of these functions, you must provide the computer's
memory with an appropriate set of instructions to follow. These instructions,
commonly called software, are usually loaded into the
computer's memory from a disk. If you type a business letter with word
processing software, everything you type also goes into the computer's memory.
If you turn the power to the computer off, everything you have typed as well as
the word processing software will disappear from the computer's memory. The only
way to get the instructions back into the computer is to turn the computer on
and load the instructions from disk again. The name for this computer memory is
RAM, which means Random Access
Memory. Since the CPU can access any portion of this memory as
it is needed, the term Random Access Memory is appropriate.
Earlier we mentioned that the CPU, or brain, of the computer is pre-programmed
at the factory to know everything that is going on in the computer's memory. In
addition to the CPU, there is a whole series of instructions pre-programmed at
the factory called the Operating System or
OS. This portion of the Operating System contains essential
instructions which do not disappear when the computer
is turned off. You can think of the Operating System as a bunch of switches
which have been permanently set with crazy glue! These instructions are
responsible for making the computer operate properly from the moment you turn it
on. Since these instructions never disappear and never change they are called
ROM or Read Only Memory.
To summarize, we have learned that computers operate with thousands of tiny
switches that can be either off or on. The ROM contains switches which have been
permanently set at the factory to control how the computer operates. The RAM
contains switches which are all turned off when the power to the computer is
turned off. The RAM switches are activated by whatever we choose to use the
computer for. If we load a word processing program from disk the RAM switches
will be set according to how they were stored on the disk. If we type a letter,
additional RAM switches will be set according to whatever we decide to type. If
we load a computer game, the RAM switches will be set according to how they were
stored on the game disk and give us another unique use for the computer.