A D81 file is a specialized disk image format used primarily by Commodore 64 and Commodore 128 computer emulators. It represents the exact digital contents of a 3.5-inch double-sided, double-density floppy disk originally used with the Commodore 1581 disk drive. Unlike the more common D64 format, which mimics the lower-capacity 5.25-inch disks, the D81 format supports a significantly larger capacity of 800 KB. This is achieved through a structure of 80 tracks, each containing 40 sectors of 256 bytes. The file serves as a raw byte-for-byte copy of the physical media, preserving the original directory structure, file allocation headers, and the actual program or data files stored on the disk. In the modern era, D81 files are essential for the retro-computing community, enabling the preservation and execution of vintage software, games, and utilities on modern hardware. These images can be loaded into emulators or written back to physical media using specialized hardware interfaces, allowing users to experience legacy software exactly as it functioned on original Commodore hardware.