Like the MARS MIPS simulator, but you build the processor
MARS, the MIPS Assembler and Runtime Simulator, is a lightweight interactive development environment for MIPS assembly language by Pete Sanderson and Ken Vollmar, aimed at teaching with Patterson and Hennessy's Computer Organization and Design. It is written in Java and needs a Java runtime, and it offers an editor, register and memory views you can edit in hexadecimal or decimal, variable-speed single-stepping both forwards and backwards, a command-line mode, and tools such as a cache performance analyzer. Version 4.5 dates from August 2014, and its authors describe it as a legacy application that remains very useful but will not be developed further. If that is what brought you here, digiwleea is a free, in-browser alternative with a different angle: you build a computer from the transistor up.
What MARS is good at
It does its job very well: it is simple, quick to learn, matches the textbook's MIPS, and stepping backwards through a program is a wonderful way to find a bug. (digiwleea is an independent project, not affiliated with MARS or its authors.)
How digiwleea is different
MARS runs MIPS programs on a simulated processor that you never see inside. digiwleea is about the inside: in your browser you start from CMOS transistors and wire gates, adders, an ALU, registers, RAM and control logic into an 8-bit CPU, following a guided 40+ level course with an AI tutor (Wleea), then program it in a small assembly language (LOAD, ADD, SUB, STORE, JMP, JZ, OUT, HALT) and watch each instruction travel through the datapath and the control unit. It does not run MIPS code. MARS is the better pick for a MIPS assembly course; digiwleea is the better pick for seeing how a processor executes instructions, and the pipelining lesson covers the idea behind the textbook's five-stage MIPS pipeline.
| MARS | digiwleea | |
|---|---|---|
| Instruction set | MIPS32, with floating point | One small 8-bit accumulator ISA |
| Runs on | A Java desktop application | The browser, nothing to install |
| You see | Registers and memory, stepping both ways | Every gate and wire of the CPU |
| The CPU | Simulated, not shown inside | Built by you, from transistors up |
| Learning path | Pairs with Patterson and Hennessy | 40+ level course + theory lessons |
| AI tutor | No | Yes (Wleea) |
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 digiwleea a good MARS alternative?
Yes, if you want to understand digital logic from the ground up. digiwleea is a free, in-browser simulator with a guided 40+ level course where you build every gate from CMOS transistors and work up to a real 8-bit CPU, with an AI tutor to help.
Is digiwleea free?
The sandbox lab and the first act of the course are free and need no sign-up. An optional account adds the AI tutor and cross-device sync; the full course and expanded AI tutoring are included in the monthly or annual Premium subscription.
Do I need to install anything?
No. digiwleea runs entirely in your browser, on desktop or mobile, and works offline once loaded.