Get it via the latest MAME distribution toolset.
Unlike the older .BIN / .CUE format, which separates audio tracks and data into multiple files, a CHD file wraps the entire game (data, audio tracks, and audio cues) into a single, highly compressed file. Why Switch from BIN/CUE to CHD?
When users search for "free ROMs," they are entering a legally complex area. It is important to understand the distinction between abandonware and copyright.
