An implementation of the Simple-As-Possible computer (SAP-1) built in Logisim-evolution 3.8.0. This project reproduces the 8-bit architecture popularized by Malvino's Digital Computer Electronics, using Logisim's digital logic simulator so you can inspect, modify, and extend the design visually.
- Logisim-evolution 3.8.0 or newer. Download the JAR file directly here.
- The official release page is available on GitHub.
- Launch Logisim-evolution.
- Open
circuits/sap1.circ. - Load or edit the ROM contents to experiment with instruction sequences.
- Use the Control and Execution panels to single-step or run programs while monitoring the registers and bus.
If macOS Gatekeeper flags the Logisim-evolution app, go to System Preferences → Privacy & Security and click Open Anyway to allow the application to run.
circuits/sap1.circ– main Logisim-evolution project containing the SAP-1 schematic, wiring, and control logic.docs/images/– high-resolution references that document default ROM contents, program examples, and macOS Gatekeeper instructions.docs/logisim-evolution-download.txt– direct URL for the Logisim-evolution 3.8.0 JAR used to develop and test this circuit.
The SAP-1 supports a minimal instruction set. The address routine table below shows the mapping between opcodes and their corresponding routines:
The control unit uses a ROM to generate micro-instructions for each machine cycle. The table below details the contents of the control ROM:
The following example demonstrates a program that computes 2 + 3 + 4 - 1 = 8:
Feel free to fork the project, adapt the ROM contents, or upgrade the design (e.g., additional instructions, memory, or I/O). Pull requests are welcome—just keep the documentation in sync with any new features.
This project is licensed under the terms of the MIT License. See LICENSE for details.