In what situations is octal base used?

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Octal is used when the number of bits in one word is a multiple of 3, or if the grouping of the bits makes sense to notate in groups of 3. Examples are

  • ancient systems with 18bit word sizes (mostly historical)
  • systems with 9bit bytes (mostly historical)
  • unix file permissions with 9bits (3*3bits, “rwxr-x—” 0750)
  • unix file permissions with 12bits (the same as the 9bit version but adding three bits in front for setuid, setgid, and sticky, 01777 but the letters are more complicated here)

I have not encountered any uses of octal other than unix file permission bits during my about 25 years in IT.

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

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