Introduction

This is the second kit of the Hardware Basics series. In it we build what every processor counts with — the adder — and go all the way: from a single bit to a calculator that adds and subtracts signed numbers.

The kit is standalone: it starts from the transistor and takes nothing from other kits. Inside:

• gates NOT, NAND, NOR, AND, OR, XOR — built in the very first lesson;
• adders — from a one-bit half adder to the 8-bit ADDSUB8, each one made of the ones before;
• negative numbers and subtraction — with the same adder, no separate "subtractor";
• flags C, Z, N, V — what the processor makes its decisions by;
• comparison — how the CMP instruction finds out which number is bigger.

The finished calculator is 1,208 transistors.

What you get

Five lessons: half adder → 4 bits → subtraction and flags → 8 bits and comparison → the ± calculator. Bonus lesson 6 is decimal math, as in pocket calculators. If you are tired after the fifth lesson — the kit is already complete.

How to read binary numbers

Switches and lamps show numbers in binary: the top digit is on the left. In the check tables, next to every number it says which switches to turn on: 1 — on, 0 — off.