The docs for the Ultimate-II+ expansion cartridge for the Commodore 64 state:
It is able to read D64 files, as well as D71 and D81 files (no partitions), T64 files.
So what are D64, T64, D71 and D81 files? Where did they come from?
These are disk- and tape images files for the Commodore 64 and computers using the same floppy drives and disk formats, like the VIC-20, or the Commodore 128.
- D64 are single-sided 5.25" disk images ("1541 images" for the Commodore 64), sometimes also called D41.
- D71 are double-sided 5.25" disk images ("1571 images")
- D81 are double-sided 3.5" disk images ("1581 images")
- T64 are tape images for the Commodore 64
The Dxx formats are straightforward sequential dumps of all sectors on a Commodore disk in ascending order, i.e. starting with track 1, sector 0, followed by track 1, sector 1, until the disk "ends" (in the 1541 case) at track 35, sector 16. More detailed documentation on the internal structure, including the structure created by Commodore DOS, can be found here.
According to the German C-64 Wiki, the D64 name was introduced with the C64s emulator by Miha Peternel in 1994, even though disk images with the same structure had been around since the 1980s. Miha Peternel also created the T64 format (see also here).
All of these are common in CBM emulation, and CBM drive emulators. There are more formats, and they are supported by a number of tools.
Since I've been writing code to interact with various systems' disk image formats over the past few weeks I'd like to add a little more information that's not in the current answers.
Disk images and tape images are usually of a few different types and subtypes. I'm using my own terminology here so forgive me if there is standard terminology I'm not yet aware of.
Disk images
- "High level": These typically represent the physical tracks and sectors of a disk. Sometimes they might represent logical tracks, sectors, and/or blocks though. The two main subtypes I might call "raw" and "headered". "Raw" images expect the tools using them to know things such as the disk geometry, or be able to make correct assumptions based on external metadata such as the file extension or file length. "Headered" images provide extra information such as a magic word identifiable signature, version number, information about the geometry, etc.
- "Low level": These typically represent the disk as the drive controller sees it. These typically allow disks with various kinds of copy protection to be represented. There will often be several low-level image formats or versions as emulator writers find games or tricks that can't be represented in earlier image formats. These also come in "raw" and "headered" subtypes.
- "Very high level": These represent the filesystem rather than the disk geometry. Sometimes just a collection of files with their metadata.
Tape images
- "Audio". These represent the sound of the tape to a high enough fidelity that it can be played as a sound for a real computer to interpret as though it's connected to a tape player, or to be converted to an accurate byte representation. They may just be a plain existing audio file format such as WAV, or have a wrapper or extra header. Just like a real tape it could include multiple programs or even analogue audio.
- "High level". These represent the data rather than the sound. They can be loaded into an emulator without having to go to the trouble of emulating the audio of the tape player, but there are usually features of the emulator or other tools to convert them to audio. They are usually "headered", but that extra information can vary from very minimal to more extensive to cover custom speedloaders, multiple files on a tape, etc.
In the case of .d64, .d71, and .d81, they are all "high level" "raw" disk image formats, and are all closely related in the same family. A program implmeneting one can fairly trivially be extended to work with the others.
As for .t64, it's a "high level" "headered" tape image format which includes quite a bit of structured metadata about the data on the tape.
They are image files that contain the data from cassette tapes (T) and floppy disks (D).
