Don Williamson shared his real-time web-based code editor & interpreter for the DCPU-16 Virtual Machine. It makes nice use of the canvas tag for a beautiful blue console, features a real-time recompile as you type, and thanks to some custom DCPU-16 support for Marijn Haverbeke's wonderful CodeMirror syntax highlighting textarea library, it'll give you real-time error feedback as you type.
It also supports the dat instruction, to make it easy to store strings — something I haven't seen supported in any other web-based compiler so far.
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