Skip to content
Computer Internals
NewAll 84 chapters are online, in English and French

How a computer really works, layer by layer.

Start from the apps you use and the algorithms behind them, then follow a single line of C down through assembly, machine code, micro-operations and buses to the logic gates and transistors that run it, and step through every one of them live.

Start with the levelsHow the levels fit together
levels of abstraction
10
levels of abstraction
chapters planned
84
chapters planned
runnable examples
24
runnable examples
installs or sign-ups
0
installs or sign-ups

The simulator

Stop imagining what the CPU does. Watch it.

A compiler, an emulator and a debugger that explain themselves, running entirely in your browser.

int c = a + b;→mov eax, DWORD PTR [rbp-4]mov ecx, DWORD PTR [rbp-8]add eax, ecxmov DWORD PTR [rbp-12], eax
if (c > 7)→cmp DWORD PTR [rbp-12], 7jle .L1

C → assembly, in your browser

A real compiler with types, pointers, structs and strings. Every assembly line maps back to the C that produced it.

OF0
SF1
ZF0
AF1
PF0
CF1

cmp eax, 5 · 3 − 5 = −2 → jl taken

Every flag, every bit

The full RFLAGS register, what each instruction wrote or changed, and which conditional jump will be taken, and why.

▸ factorial(n=3) · ← factorial(n=4) ← main()
[rbp-4]argn0x00000003
[rbp]fpsaved rbp (factorial)0x7fffff98
[rbp+8]retreturn → factorial+140x00401011

The stack, labeled

Frames, locals, arguments, saved frame pointers and return addresses, named from your C source. Watch a buffer overflow smash one.

CPUMAR · MDR
Address0x7fffffc8
Data0x7fffffe0 →
ControlMEMW
RAMstack

Inside the CPU

Fetch, decode, address calculation, bus reads and writes, ALU, write-back: each micro-operation animated on the system bus.

Ten levels

From a click to an electron, ten levels deep

Every level is implemented by the one below it. Pick any level to zoom in.

Explore the ten levels
  1. L0Applications & the userWhat you see and do: clicking, typing, loading a page, and what each of those really sets in motion.
  2. L1Problems & algorithmsBefore any language: what a computation is, how to solve a problem step by step, and how to measure the cost.
  3. L2Programming languagesWhat you write: variables, functions, pointers, and what a compiler turns them into.
  4. L3Assembly languageOne line per instruction: registers, flags, jumps: the view debuggers and disassemblers give you.
  5. L4Operating systemThe layer that turns one CPU into many processes: virtual memory, system calls, files.
  6. L5Instruction set architectureThe contract between software and silicon: instruction formats, encodings, addressing modes.
  7. L6MicroarchitectureHow one instruction becomes fetch, decode, execute: datapath, buses, caches and pipelines.
  8. L7Digital logicGates, adders, the ALU, flip-flops and memory cells, where 0 and 1 become arithmetic.
  9. L8Devices & chipsTransistors as switches, CMOS gates, how chips are made, and the storage and I/O devices built from them.
  10. L9Physics of computingThe real bottom: electrons, semiconductors and the field effect: how a bit is physically stored and moved, and the limits physics sets.

Foundations

The number systems every level relies on.

Levels of abstraction and a short history of computersBinary and hexadecimalTwo’s complement and signed numbersFloating point (IEEE 754)Characters, ASCII and UnicodeEndiannessParity, Hamming codes and error correctionUnits: kilo, kibi and friends

Start at the top. Go as deep as you like.

No install, no account. Every chapter has live demos to play with.

Start with the levels