How Computers Work by Roger Young

• Computers are the most complex machines that have ever been created. Very few people really know how they work.
• The ground wire is not shown because it is assumed that one connection of every light is always connected to the bottom of the battery by a ground wire in diagrams. Diagrams are simpler to draw than pictures that mean the same thing.
• A key is a flat piece of springy steel that is bent up so that the key only touches the wire to the key's right when the key is pressed down by someone's finger.
• An electromagnet is a coil of (plastic coated) wire. An electromagnet becomes magnetic when electricity goes through it.
• A key and the electromagnet that controls it are, together, called a relay.
• A computer is almost entirely made up of a lot of relays (today, transistors) connected by wires.
• Today, transistors are used instead of relays for lower cost and greater speed.
• Touching wires are connected wires.
• Most relays in a computer are used to make loops, or connect the loops together.
• A light bulb that is controlled by a loop is called a "pixel".
• When the key is down and the light bulb is glowing, one says that the loop has value '1' and the pixel is 'on'.
• When the key is up and the light bulb is not glowing, one says that the loop has value '0' and the pixel is 'off'.
• A key is called 'closed' when electricity can flow through it from a wire on its left to a wire on its right. A key is called 'open' when electricity can't flow through it from a wire on the left to a wire on the right.
• Any piece of wire that is powered is said to have value '1'. Any piece of wire that is not powered is said to have value '0'.
• Writing to a latch will not clear any bits that were previously 1, so always clear a latch before copying (writing) data to it.
• The next most important part of a computer, after memory, is the processor. A processor changes the values in memory as instructed by instructions stored in memory. An instruction is a group of bits (loop values) in memory that tell the processor to do something. A group of instructions that instruct the processor to do some task is called a program.
• A transistor-based computer works in the same way as a relay-based computer.
• A 16-bit computer has 16 bits in each 'word' and 65536 words of memory. This is because there are 65536 possible 16-bit addresses.
• The data is the program. An instruction written with instructions that are just 1's and 0's is a machine language instruction. Machine language instructions are called 'machine code'.
• A lookup table can be made to do any function with a limited number of possible inputs.
• Modern computers use two types of transistors, which correspond to the two types of relays. An N-channel transistor corresponds to a normally open relay. A P-channel transistor corresponds to a normally closed relay.
• It takes about twice a s many transistors as relays to do something. Of course, the high speed and low cost of transistors make transistors vastly superior in spite of the extra design effort required.
• In modern designs, it is common for the ensuing instruction to be started before the prior instruction finishes, so a few instructions can be executed at once. This is called pieplining.