But How Do It Know? - the Basic Principles of Computers for Everyone by J Clark Scott Read Free Book Online Page A

“What code is in that byte?” It will be pretty obvious from the context which way it is being used.

 
    Back to the Byte
    Do you remember the memory byte we drew a few chapters ago? It was eight memory bits with their ‘s’ wires all connected together. Almost every time that we need to remember a byte inside a computer, we also need an additional part that gets connected to the byte’s output. This extra part consists of eight AND gates.
    These eight AND gates, together, are called an “Enabler.” The drawing on the left shows all of the parts, the drawing on the right is a simpler way to draw it.

    The second input of all eight AND gates are connected together and given the name ‘enable,’ or ‘e’ for short. When ‘e’ is off, whatever comes into the Enabler goes no further, because the other side of each AND gate is off, thus the outputs of those gates are all going to be off. When ‘e’ is on, the inputs go through the Enabler unchanged to the outputs, ‘o.’
    By the way, when gates are used for something like this, the name “gate” starts to make some sense. An Enabler allows a byte through when the bit ‘e’ is 1 and stops the byte when it is 0. So ‘e’ being on is like opening a gate, and ‘e’ being off is like closing a gate.
    We will take our byte, and connect it to an enabler, as shown in the left hand drawing. To simplify once again, we can draw it as shown on the right.

    Now we have a combination that can store eight bits. It captures them all at the same time, and it can either keep them to itself, or let them out for use somewhere else. This combination of a Byte and an Enabler, has a name, it is called a Register, thus the ‘R’ in the drawing.
    There will be a few places in this book where there are registers that never need to have their outputs turned off. In those cases, we will draw a register that only has a ‘set’ bit, and no ‘enable’ bit. We should probably refer to these devices as ‘bytes,’ but we will call them registers nonetheless.
    Register simply means a place to record some kind of information, like a hotel register where all the guests sign in, or a check register where you write down each check that is written. In the case of this computer part, you record the state of the eight input bits. This register is very limited though, in that it can only hold one set of values; in a hotel register there is a new line for each guest. Every time you store a new state in a computer register, the previous state of the eight memory bits is lost. The only thing that is in there is the most recently saved value.

 
    The Magic Bus
    There are many places in a computer where eight wires are needed to connect registers together. Our register, for example, has eight memory bits, each of which have an input and an output. To simplify our diagrams, we will replace our eight wires with a double line.
    So our register can look like one of these:

    Or, we can simplify, and replace it with one of these:

    It’s exactly the same thing, we will just save a lot of ink in our drawings, and they will be easier to understand.
    When there is a connection between two of these bundles of wires, one wire of each bundle is connected to one wire of the other bundle as shown in the diagram on the left. But we will simplify it, and just draw it like the diagram on the right.

    Now, this grouping of eight wires is so common inside computers that it has a name. It is called a bus. Why is it called a bus? Well, it probably has to do with the old electrical term ‘buss,’ that means a bar of metal used as a very large wire in places like power generating plants. But there is also an interesting similarity to the kind of bus that people use for transportation.
    A bus is a vehicle that commonly travels along a predetermined route, and makes many stops where people get on or off. They start somewhere, and the bus takes them to some other place they need to be. In the world of computers, a