DUMP, DDT and DEBUG
In the CP/M 8 bit operating system used on early personal computers, the standard DUMP program would list a file 16 bytes per line with the hex offset at the start of the line and the ASCII equivalent of each byte at the end. Bytes outside the standard range of printable ASCII characters (20 to 7E) would be displayed as a single period for visual alignment. This same format was used to display memory when invoking the D command in the standard CP/M debugger DDT. Later incarnations of the format (e.g. in the DOS debugger DEBUG) changed the space between the 8th and 9th byte to a dash without changing the overall width.
This notation has been retained in operating systems that were directly or indirectly derived from CP/M, including DR-DOS, MS-DOS, OS/2 and MS-Windows. On Linux systems, the command hexcat produces this classic output format too. The main reason for the design of this format is that it fits the maximum amount of data on a standard 80 character wide screen or printer, while still being very easy to read and skim visually.
1234:0100 FF D8 FF E0 00 10 4A 46-49 46 00 01 02 01 00 48 ......JFIF.....H 1234:0110 00 48 00 00 FF ED 0A 96-50 68 6F 74 6F 73 68 6F .H......Photosho 1234:0120 70 20 33 2E 30 00 38 42-49 4D 04 04 07 43 61 70 p 3.0.8BIM...CapRead more about this topic: Hex Dump