Have you ever thought about what a computer actually is? I like to think
about computers as a set of very complex circuits with a bunch of switches
up front. We control which circuits are run and what output is important
by turning some switches off and on. Imagine our computer has a row of
16 switches. Maybe setting the first 3 switches to on-on-off tells the
computer we're going to add two numbers. A different arrangement like
off-on-off might tell the computer we want to load a number into memory,
or jump to a new location in the program.
+---------------------------------------------------------------+
+ 1| 2| 3| 4| 5| 6| 7| 8| 9| 10| 11| 12| 13| 14| 15| 16|
+---------------------------------------------------------------+
| X | X | - | - | X | - | - | X | - | - | - | - | - | - | - | - |
+---------------------------------------------------------------+
A bank of 16 switches Where X is "on" and - is "off"
The computer also has a set of lights that can be on or off. When toggle
the switches some lights go on or off depending on a specific arrangement of
the switches. For example, our computer has 4 banks of lights, each with 16
bulbs. Let's say I set the switches to load a pattern of lights that
represent the number 3 into the 2nd bank of lights. Then, I set the
switches to load a 4 in the 3rd bank of lights. If I look at the lights, I
see the pattern for 3 and the pattern for 4 in banks 2 and 3.
Now, when I flip the switches I can enable the add circuit, setting its
inputs to the contents of the lights in banks 2 and 3 and to set the result
into the bank number 4. Now, I expect the fourth bank of lights to hold the
pattern for a 7. If I use a different arrangement of switches I might
subtract the numbers or jump to a location in memory.
We call an arrangement of the switches an instruction. Instead of physical
switches we have digital 'bits' which are either on or off. Instead of
lights we have 'registers' which store the values on which the computer
instructions operate. A specific set of instructions for a particular model
of processor are the instruction set for that processor.
A sixteen switch computer is a sixteen bit computer which were popular in
the 70's for mini-computers and in the mid to late 80's for micro computers.
There are still 16 bit processors but they mostly are used for things like
the computer in your car's brakes. These ubiquitous but invisible processor
are called embedded processors. Most computers that you buy at the store
and use at work are 64 bit computers.
But where do we hold these instructions? I used to have a computer that I
had to program every time I used it because I couldn't afford the cassette
recorder that was a popular way of storing data for home computers. Now we
store instructions on the large disks that come with our computers. Or
maybe we just download the instructions from the internet when we need them.
But loading one insruction at a time from disks or the interenet much
slower than the processor can execute instructions. We store the working
data and the instructions we are currently executing in the computer's
memory. The processor has some special high-speed memory that it uses to
temporarily hold the instructions it will immediately process, while the
main memory holds the instructions that are part of the program we're
executing.
So we have the processor that is executing instructions and moving them in
and out of memory and on or off the disk or the internet. There are other
devices like video cards, network cards, sound cards, etc. The processor
has special instructions to move instructions and data to those devices to
enable sound, graphis, and other services. But at its heart, with all that
complexity, is basically a box that executes whatever circuit we tell it to
execute. Nothing more and nothing less.
