From One Transistor to a Full CPU — Circuit Copilot Glossary

Every computer you use is built on one primitive: the transistor. Understanding the layers that turn it into a CPU is the whole story of modern computing.

Layer 0: One transistor

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a switch controlled by voltage. Apply voltage to the gate, and current flows between source and drain. Remove it, current stops. That's it. That's the fundamental unit.

Layer 1: Gates

Combine 2-4 transistors and you get a logic gate: AND, OR, NOT, NAND, NOR, XOR. A NAND gate is 4 transistors. Everything a computer does can be built from NAND gates alone.

Layer 2: Combinational and sequential circuits

Wire gates together to build:

Adders — for arithmetic
Multiplexers — for choosing between inputs
Latches and flip-flops — for storing 1 bit of state (a "memory cell")
Registers — arrays of flip-flops for storing numbers

Layer 3: Functional blocks

Wire circuits together to build:

ALU (Arithmetic Logic Unit) — the calculator part of the CPU
Register file — the CPU's scratchpad
Program counter — tracks which instruction to run next
Instruction decoder — reads bits and figures out what operation they represent

Layer 4: The datapath

Connect the functional blocks with wires (buses) so data flows through them in the right sequence. Add a control unit that generates the signals to coordinate everything. Now you have a CPU that can execute one instruction: fetch, decode, execute, write back.

Layer 5: Optimizations that make it fast

A "textbook" CPU processes one instruction at a time. Real CPUs use:

Pipelining: multiple instructions in progress at once, like a factory assembly line
Superscalar execution: multiple instructions started per cycle
Out-of-order execution: reorder instructions to keep the pipeline full
Branch prediction: guess where the code will go next
Cache: keep frequently used data close

Layer 6: Multiple cores + a system

Put several full CPUs on one die, share caches between them, add a memory controller, PCIe interface, and now you have a modern chip. An Apple M3 Max has ~92 billion transistors, all built from this exact stack.

Layer 7: Software

The OS treats the CPU as a machine that executes instructions and manages memory. Applications treat the OS as a machine that provides services. Every layer of abstraction hides the one below — but under all of it, transistors switching.