This project is a software emulator for Zeal 8-bit Computer: a homebrew 8-bit computer based on a Z80 CPU.

Click here for a live demo

The goal of this project is to reproduce the exact same behavior of the real machine, in order to be able to execute and mainly debug the programs written for it directly from a host computer, without the need to flash any EEPROM or NOR Flash in order to test it. This makes development and test cycles much faster and more convenient.

The main goal is to integrate this emulator as part of zeal8bit.com website. Thus, choosing HTML/CSS and Javascript was a bit obvious on this side.

On the other side, another goal of this project is also to let anybody test or write programs for Zeal 8-bit Computer on any platform (Linux/Windows/Mac OS X) without the need of a toolchain installed.

The emulator has been tested on Opera and Google Chrome.

The emulator is slower than the real hardware, even though the real hardware is using a Z80 running at 10MHz. This is due to the overhead of Javascript itself and the web browser itself. But as I said above, the main feature this was made for is debugging programs and for this, there is no doubt it is efficient.

Moreover, the emulator is not really accurate it terms of timings, more details below, in the features section.

Writing a native emulator in C (w/ SDL) would cover the requirement of a full-speed emulator, which would be more accurate in terms of timings. However, this needs some time investment. Feel free to contact me or contribute if such project interest you.

Currently, the following features from Zeal 8-bit Computer are emulated:

• Z80 CPU (thanks to Molly Howell)
• Z80 PIO: all modes supported, both port A and B. Implementation is independent from connected devices.
• 22-bit MMU
• 256KB ROM
• 512KB RAM
• Video chip: 640x480 text mode w/ colors, 640x480 graphic mode w/ colors
• PS/2 Keyboard, with interrupts
• UART: it is possible to send and receive bytes. It is also possible to send files. The baudrate can be changed from the GUI
• I2C: bus emulated, supporting write/read/write-read operations
• I2C RTC: always returns the current date

Features of the emulator itself implemented:

• Debugger: breakpoints, step, step over instruction, continue until next breakpoint
• Load a binary file, loaded into ROM directly
• Load a dump file generated from z88dk-dis to view assembly code while doing step by step debugging
• Set breakpoints with either a hexadecimal PC address or a symbol (from the dump file)
• View memory content when doing step by step debugging
• Dedicated tab for the UART view

Of course a lot of things are still remaining to do. On the emulation side:

• PIO: complete the implementation for the user port (A) to act as a regular GPIO, and system port (port B) to activate/deactivate the interrupts, the H-blank, V-blank, I2C, UART, etc... (Done, PIO is now a regular GPIO component, separated from connected components)
• UART emulation (needs PIO port B) (Done, it is possible to read and write characters, baudrates are hardcoded at the moment. Would be interesting to be able to send a file through UART Done)
• I2C RTC (needs PIO port B) (Partially implemented: it always returns the current browser date, so writing to it will have no effect)
• I2C 32KB EEPROM emulation (needs PIO port B)
• Sound support¹
• SD Card emulation²
• Video chip: 320x240 text mode, 320x240 graphic mode², sprites², 4-bit palettes², etc...

¹: feature that may be modified on the real hardware.

²: features not in high priority as they have not been implemented on real hardware yet.

On the project/debugger side itself:

• A better interface for the debugger's buttons, with some shortcuts
• A better way to parse breakpoint input. A know bug is providing a label starting hex letters would be interpreted as a PC value instead of a label. For example, inputting date_routine as a label to break in would result in the addition of a breakpoint at address 0xda instead of the address of label date_routine (because date starts with hex letters da)
• Refactoring. Some part of the code, mainly in zeal.js are a bit dirty in the sense that several different things are managed by this file: breakpoints, disassembly view, memory viewer, Zeal emulation, etc... It should be cleaned and split up between multiple files.
• A disassembler! Today, the disassembler view only takes the disassembly from a file generated by z88dk-dis toolchain. A much better (or complementary) way is to disassemble the instructions on the fly and show them in the disassembly view at the bottom of the screen.
• An integrated code editor with an assembler! In the (far?) future, integrating a code editor, ideally CodeMirror, would be a very nice addition. It would let anyone write code, assemble it, inject it in the emulator and test it directly. No need to have a toolchain, or an assembler installed. Everything would be available directly from the browser. It would also be possible to connect the debugger to the code editor in order to be able to debug the code written more easily.

Anyone can contribute to this project, contributions are welcome!

Feel free to fix any bug that you may see or encounter, implement any feature that is present in the TODO list or a new one that you find useful or important.

To contribute:

• Fork the Project
• Create your feature Branch (optional)
• Commit your changes. Please make a clear and concise commit message (*)
• Push to the branch
• Open a Pull Request

(*) A good commit message is as follow:

Module: add/fix/remove a from b

Explanation on what/how/why

For example:

Video chip: implement 320x240 text-mode

It is now possible to switch to 320x240 text-mode and display text.

Z80.js file is distributed under the MIT licence (originated from Molly Howell repository: https://github.com/DrGoldfire/Z80.js)

All the other files are distributed under the Apache 2.0 License. See LICENSE file for more information.

You are free to use it for personal and commercial use, the boilerplate present in each file must not be removed.

For any suggestion or request, you can contact me at contact [at] zeal8bit [dot] com

For features requests, you can also open an issue or a pull request.