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UndertaleModTool and game compilation 

  • VM (Virtual Machine): In this mode, the game's code is stored in the data file as bytecode, which UndertaleModTool can decompile and edit.
  • YYC (YoYo Compiler): This method compiles the game's code into native C++ machine code and embeds it directly into the executable file.
  • Conflict: UndertaleModTool cannot decompile or modify the machine code in a YYC-compiled game. When the tool detects a YYC game, it will warn you that the code is inaccessible. 

What can you do with a YYC game?

  • You can still use UndertaleModTool to edit other assets, such as images, sounds, and rooms.
  • For code-level modifications, you will need to use different tools that can work with machine code, such as YYToolkit, which allows for custom C++ code to be run on top of the game. 

This video explains how to use YYC in GameMaker Studio: