Johnny simulator alternative: from von Neumann model to a CPU you build

Johnny is an educational simulator of a simple von Neumann computer, originally written by Peter Dauscher, with manuals in English, German, Italian and French. A web version by David Laubersheimer (AGPL-3.0) is hosted by the German digital schoolbook inf-schule.de; its README says it is no longer actively maintained and points to a newer version by TobisMa at tobisma.github.io/johnny. Johnny shows memory as rows of address, data and assembly, an accumulator with a zero test, and the data bus, and lets you click the individual micro-operations yourself: RAM to data bus, data bus to RAM, data bus to accumulator, accumulator to data bus, clear, increment, decrement, plus and minus. An experimental version adds a pixel framebuffer and keyboard addresses for simple games. Johnny lets you click through the micro-steps of a model computer; digiwleea has you build the computer's hardware itself, gate by gate.

What Johnny is good at

Showing students that an instruction is really a sequence of tiny transfers between memory, bus and accumulator, without the complexity of a real processor or assembly language. Clicking ram-->db, db-->acc and plus by hand, then watching a program do the same steps, makes the fetch-execute idea concrete, and it is free and in the browser. (digiwleea is an independent project, not affiliated with Johnny, its authors or inf-schule.de.)

How digiwleea is different

digiwleea goes one level further down: the bus, the accumulator and the control signals are circuits you build. In the browser you start from CMOS transistors, make NOT and NAND, then adders, an ALU, registers, RAM, a bus with tri-state buffers, a program counter and a control unit, through a 40+ level course where every part is checked against its truth table, with an AI tutor (Wleea). The CPU you finish with is a small 8-bit accumulator machine, close in spirit to Johnny, whose capstone runs a stored LOAD, ADD, STORE, HALT program. The microprogrammed control unit page shows how micro-operations like Johnny's become control words in a ROM.

	Johnny	digiwleea
Focus	Micro-operations of a model von Neumann computer	Digital logic from transistors to a simple CPU
The machine	Given: memory, data bus, accumulator	Built by you, part by part
Micro-steps	Clicked by hand or run by a program	Control signals you wire, then a control unit
Runs on	Browser (web versions); original desktop version	Any modern browser
Price	Free, open source	Free lab and Act I; Premium for the full course

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.

More comparisons: CPU and assembly simulators

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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

What is the Johnny simulator?

An educational simulator of a simple von Neumann computer, originally by Peter Dauscher, used in German schools through inf-schule.de. It shows memory, the data bus and an accumulator, and lets you run each micro-operation by hand.

Is there an online version of Johnny?

Yes. A web version by David Laubersheimer is hosted by inf-schule.de, and a newer maintained version by TobisMa runs at tobisma.github.io/johnny.

Can I build a von Neumann computer myself?

In digiwleea you build a small one from transistors up: registers, an ALU, RAM, a bus, a program counter and a control unit, then watch a stored program run on it.