Code: The Hidden Language of Computer Hardware and Software

by Petzold Charles

The time required for one cycle is called the period of the oscillator.

The frequency of the oscillator is 1 divided by the period.

But cycles per second isn’t used much any more. In commemoration of Heinrich Rudolph Hertz (1857–1894), who was the first person to transmit and receive radio waves, the word hertz is now used instead.

Thus, we can say that our oscillator has a frequency of 20 hertz, or (to abbreviate) 20 Hz.

The strangeness of this circuit is that sometimes when both switches are open the light is on, and sometimes when both switches are open, the light is off.

We can say that this circuit has two stable states when both switches are open. Such a circuit is called a flip-flop

A flip-flop circuit retains information. It “remembers.”

In particular, the flip-flop shown previously remembers which switch was most recently closed. If

Although it might not be apparent yet, flip-flops are essential tools. They add memory to a circuit to give it a history of what’s gone on before.

What I’ve just shown is the simplest and is called an R-S (or Reset-Set) flip-flop.

The two inputs are called S for set and R for reset. You can think of these verbs as meaning “set Q to 1” and “reset Q to 0.”

This is called a function table or a logic table or a truth table. It shows the outputs that result from particular combinations of inputs.

The final row of the table indicates that a situation in which the S and R inputs are both 1 is disallowed or illegal

So when you’re designing circuitry that uses the R-S flip-flop, avoid situations in which the S and R inputs are both 1.

The R-S flip-flop is certainly interesting as a first example of a circuit that seems to “remember” which of two inputs was last a voltage.

This circuit is called a level-triggered D-type flip-flop. The D stands for Data. Level-triggered

But for now, the Clock input simply indicates when the Data input is to be saved:

This circuit is also known as a level-triggered D-type latch, and that term simply means that the circuit latches onto one bit of data and keeps it around for further use.

This latch is capable of saving 8 bits at once.

When the Clock signal is 1, the 8-bit value on the D inputs is transferred to the Q outputs. When the Clock signal goes back to 0, that 8-bit value stays there until the next time the Clock signal is 1.

For this reason, such a circuit—in which the output is routed back to the Data input of a flip-flop—is also known as a frequency divider.

We have now persuaded telegraph relays to add, subtract, and count in binary numbers.

The two improved adding machines of the last chapter illustrate clearly the concept of data paths. Throughout the circuitry, 8-bit values move from one component to another.

The word byte originated at IBM, probably around 1956. The word had its origins in the word bite but was spelled with a y so that nobody would mistake the word for bit.

Because bytes show up a lot in the internals of computers, it’s convenient to be able to refer to their values in as succinct a manner as possible.

We write and we later read. We save and we later retrieve. We store and we later access.

Telegraph relays too—when assembled into logic gates and then flip-flops—can store information.

a flip-flop is capable of storing 1 bit.

we encountered the level-triggered D-type flip-flop, which is made out of an inverter, two AND gates, and two NOR gates:

But whenever we want to store the Data In signal in the flip-flop, we make the Write input 1 and then 0 again. As I mentioned in Chapter 14, this type of circuit is also called a latch because it latches onto data.

Three switches can represent eight different values: 000, 001, 010, 011, 100, 101, 110, and 111.