A D81 file is a disk image format used primarily for Commodore 1581 floppy disks. The Commodore 1581 was a 3.5-inch floppy disk drive for the Commodore 128 personal computer. D81 files are essentially exact copies of the data stored on these disks, saved as a single file on a modern computer. They are used for archiving, preserving, and emulating software and data originally stored on Commodore 1581 disks. Emulators like VICE (Versatile Commodore Emulator) can read and use these disk images, allowing users to run old Commodore software on modern systems. The format contains the raw sector data from the disk, along with any necessary metadata for the emulator to properly interpret the disk's structure and contents. These files are crucial for preserving Commodore software and data for future generations, as the original floppy disks are prone to degradation and eventual data loss.