Reading But How Do It Know? Build each part of the computer live
But How Do It Know? The Basic Principles of Computers for Everyone, by J. Clark Scott, explains a computer to readers with no technical background. In the book's words, it will "show the one simple part that computers are made of, and then connect a bunch of them together until we have built a complete computer": bits and bytes, RAM, a bus and registers, an ALU, a clock and a stepper, a control section and finally a program. It is sold in print, as an e-book and on Kindle. This page points each part of that computer to a live circuit or lesson on digiwleea, so you can build what you just read about.
What the book does best
It assumes nothing. Every new word is defined when it is first used, there is nothing to memorize, and each chapter adds one idea to the ones before, so a reader with no background ends up understanding a whole computer. Readers have built on it, too: the book's site (as archived in December 2022; it did not load when we checked on 2026-10-25) shares a browser model of the computer by Siegbert Filbinger, a Logisim version with a 24-video series by Ross McGowan, an FPGA build by Patrick LeBoutillier and his 32-video breadboard companion series. (digiwleea is an independent project, not affiliated with the author.)
Building the parts yourself
The book explains each part in words and diagrams and, by its author's choice, has no exercises: it is meant to demystify, not to drill. digiwleea is where you build the parts and find out what happens when one is wired wrong: a NAND gate from transistors, then a memory bit, a register, a bus, an adder and ALU, a clock-driven counter, RAM, and finally a small 8-bit CPU that you program in assembly, with every part checked as you go. digiwleea's computer is its own design, not the one in the book, so details such as the instruction set differ, but the principles are the same.
| But How Do It Know? | digiwleea | |
|---|---|---|
| Format | A book for general readers | A browser lab, theory lessons and a guided course |
| Starts from | One simple part: the NAND gate | CMOS transistors, then gates |
| Exercises | None, by design | Every part is a level, checked against its truth table |
| The computer | The book's own simple design | digiwleea's own small 8-bit CPU |
| Price | The book is paid | Free lab and Act I; Premium for the full course |
The parts, one by one
In the order the book builds them up, with the digiwleea circuit or lesson to try for each:
- The NAND gate. Build it from four transistors as the CMOS NAND, then make NOT, AND and OR from it: NAND as a universal gate.
- A memory bit. NAND gates fed back into each other hold a bit: the SR latch from NAND gates, then the D latch. Lessons: the SR latch, a one-bit register.
- Bytes and registers. Eight memory bits side by side: the 8-bit register, and live, the 4-bit register with load.
- The bus. One set of wires shared by every register, where only one may drive at a time: buses and the tri-state buffer.
- RAM. Many bytes, one picked by its address: RAM, the 3-to-8 decoder that picks a row, and the memory simulator.
- The ALU. Adding, comparing and logic on two bytes: the full adder, the comparator and the 4-bit ALU.
- The clock and the stepper. The clock; to see one active step move along with every tick, try the ring counter.
- The control section. Turning an instruction into the right enables and sets: control signals and the fetch, decode and execute cycle.
- Programs. Machine code, assembly language, and a CPU to run them in the CPU simulator.
No sign-up is needed to start: open the lab and place your first transistor, or read the free theory course. The account (and the AI tutor) are optional.
See for yourself: build a logic gate from transistors in the simulator, then follow the path from sand to an 8-bit CPU.
Open the digiwleea lab →Frequently asked
Is there a simulator for the computer in But How Do It Know?
Readers have made several, listed on the book's site as archived in 2022: a browser model by Siegbert Filbinger, a Logisim circuit by Ross McGowan and an FPGA build by Patrick LeBoutillier. digiwleea is different: instead of running the book's finished design, you build each part yourself in a free browser lab.
What is the one simple part a computer is made of?
In the book, the NAND gate: its output is 0 only when both inputs are 1. Every other gate, memory and in the end a whole computer can be built from NAND gates, which you can try with NOT, AND and OR made from NAND.
Do I need to read But How Do It Know? before using digiwleea?
No. digiwleea's free lessons start from a single switch, so either works on its own. They pair well: read a chapter, then build the part.
Is digiwleea's computer the same as the one in the book?
No. digiwleea's course builds its own small 8-bit CPU from the parts you make in earlier levels, with its own instruction set. The principles match: gates make memory, registers and an ALU share a bus, and a clock and control section step through each instruction.