D64

Overview
The D64, D71 and D81 file formats (and some variants, like X64 and F64) are Commodore 64 sector-based disk images. They contain a fixed amount of 256 byte sectors, and, optionally, a table with one byte for each sector to contain error information. This was an early attempt at supporting some of the older copy protection schemes which relied on CBM DOS returning some error code when trying to read a certain sector on disk

Resources

 * A library to read/write D64 (and variant) image files here
 * Lots of C64 disk image documentation here (e.g. D64, D71, D81, G64)
 * Documentation with some code snippets here