The CPCEMU disc image format (conventional extension .DSK) is conventionally used for images of Amstrad CPC, Amstrad PCW, and Spectrum +3 floppy disks.
It is thus the de facto disc image format for most 3" floppy disks. (There's probably no technical reason it couldn't image other disk types, but typically it doesn't.)
Such disk images will often (but not always) contain a CP/M file system.
- Original format specification
- Extended DSK (EDSK) specification (extensions to handle representing copy protection, etc -- Marco Vieth, Ulrich Doewich, Kevin Thacker)
- cpcwiki.eu -- (FIXME just the same as above, or more information?)
- simonowen.com -- extensions to handle further copy protection schemes