In what situations is octal base used?

Question

Octal is used when the number of bits in one word is a multiple of 3. Examples are ancient systems with 18bit word sizes, systems with 9bit bytes, or unix file permissions with 9bits or 12bits.

If the number of bits in your word is a multiple of 4, however, please do use hex, by all means.

Octal is used as a shorthand for representing file permissions on UNIX systems. For example, file mode rwxr-xr-x would be 0755.

Octal is used when the syntax is a relic from the ages when it perhaps made sense on some platform (system words haven't always been a multiple of 8 bits). Nowadays hex is the thing to use.

Didn't think of this but Digital displays!

Several other uses from: http://en.wikipedia.org/wiki/Octal

One of the main reasons octal used to be more frequently used was that it is easier to convert between octal and binary in your head than hex to binary: you only have to remember the binary representation of the 8 octal digits (0-7).

Back in the days when debugging meant reading register contents from a row of LEDs, or entering data with an array of toggle switches, this was a big concern. The panels on many of these early computers grouped the LEDs and switches in groups of threes to facilitate this.

However, hex began to win out as word sizes that are multiples of 8-bit bytes began to win out, and the need to read and enter data in binary became unecessary (with console text UI and later GUI debuggers).