A PCX format file stores one page image in the form of a rectangular bitmap. PCX files have a 128-byte header followed by compressed image data. The PCX file format allows for multiple colors each with multiple shades, but Fax Mode transfers require black and white, no shades. The PCX file format also allows for different resolutions, but a PCX file for Fax Mode transfer is interpreted as having 200 pixels per inch both horizontally and vertically, no matter what the file header says. The PCX file format can handle very large page sizes, but for Fax Transfer mode, the bitmap width must not exceed 1728 pels(about 8.5 inches wide). Lengths are unlimited, since fax paper comes in rolls, but 2200 pels is 11 inches, and that is the recommended maximum length for easy handling by the receiver. Pel locations can be given in X,Y coordinates where the origin is at the top left corner of the page, X increases toward the right, and Y increases toward the bottom.
The DCX file format allows multiple pages to be stored in one file. A DCX file has a header followed by pages. Each page is exactly the same as a PCX file, including the PCX header for each bitmap.
The DCX file header starts with a 4-byte integer ID used to verify the file type. This ID is always 987654321 (decimal). After the ID is a list of offsets into the file for the pages of the document. The list is a simple array of 4-byte integers, the last of which has a value of 0 to mark the end of the array. The first page is 0, and the offset from the beginning of the file of the n'th page is stored at array location n. The array has a maximum of 1024 entries, including the 0 terminator. This means that up to 1023 pages can be stored in one DCX file, and that the maximum size of the index is 4096 bytes.