What are .chd files? What is a CHD?
CHD stands for “(C)ompressed (H)unks of (D)ata” – the odd acronym owes it self to the fact that the format was originally “(C)ompressed (H)ard (D)isk” but has subsequently be expanded to fit a variety of media storage (CD-ROM, etc.). These files are not technically ROM files, since they contain a drive image, rather than a ROM chip image. Some games had disk drives in them with extra data, large hard disks, or CDs are substantially cheaper than large ROM chips. Because of this, CHD files will typically be much larger than any ROM files you have. Please see the “Technical Information” section for more detailed information.
