Unit 1: Signals

A computer only knows two things

Inside a computer there are no words, no pictures and no numbers. There is only electricity in a wire — and it is either flowing or it is not.

We write 1 for on and 0 for off. That is it. Everything else you have ever seen a computer do is built out of those two.

Where does the thinking happen?

A wire on its own is just a wire. The interesting part is a gate: a tiny thing that looks at the wires coming in and decides what goes out.

A computer is millions of gates. And here is the surprise you are going to prove for yourself: they can all be the same kind of gate.

Meet Nand

Nand has two wires going in, called a and b, and one coming out, called out.

The rule is short: out is 0 only when a and b are both 1. Every other time, out is 1.

TABLE shows you every possible input and what comes out — that is called a truth table.

HDL

TABLE Nand;

It prints

a b | out
0 0 | 1
0 1 | 1
1 0 | 1
1 1 | 0

Why this one?

Nand is not special because it is clever. It is special because everything else can be made out of it.

Real factories like it for the same reason: one gate to design, one gate to test, printed millions of times over. In this track you are going to start with exactly one Nand and end with a working calculator and a memory.

How to describe wiring

You do not write instructions here — there is nothing to run. You describe what is connected to what, and the wires do the rest, all at once.

IN lists the wires coming in, OUT the wires going out, and under PARTS: you name a gate and say where each of its pins is wired.

HDL

CHIP Not {
    IN in;
    OUT out;
    PARTS:
    Nand(a=in, b=in, out=out);
}

One gate, one trick

Feed the same wire into both inputs of a Nand. Now the only way to get 0 out is for that wire to be 1.

So the answer always comes out the opposite of what went in. That chip is called Not, and it is your first one.

HDL

CHIP Not {
    IN in;
    OUT out;
    PARTS:
    Nand(a=in, b=in, out=out);
}

TABLE Not;

It prints

in | out
0  | 1
1  | 0

Testing one row at a time

A truth table shows every row at once. Sometimes you want just one.

USE picks the chip, SET holds a wire at a value, and SHOW prints what is coming out right now.

HDL

CHIP Not {
    IN in;
    OUT out;
    PARTS:
    Nand(a=in, b=in, out=out);
}

USE Not;
SET in=1;
SHOW out;

It prints

out=0

Chips are parts too

Once you have built a chip you can use it inside the next one, exactly like a Nand. That is the whole game: small pieces become parts of bigger pieces.

Here Same uses Not twice. The wire between them is called flipped — a name we made up, because we needed somewhere for the middle answer to sit.

HDL

CHIP Not {
    IN in;
    OUT out;
    PARTS:
    Nand(a=in, b=in, out=out);
}

CHIP Same {
    IN in;
    OUT out;
    PARTS:
    Not(in=in, out=flipped);
    Not(in=flipped, out=out);
}

TABLE Same;

It prints

in | out
0  | 0
1  | 1