Source : Free On-Line Dictionary of Computing

moby
     
         /moh'bee/ (From {MIT}, seems to have been in use
        among model railroad fans years ago.  Derived from Melville's
        "Moby Dick", some say from "Moby Pickle") 1. Large, immense,
        complex, impressive.  "A Saturn V rocket is a truly moby
        frob."  "Some MIT undergrads pulled off a moby hack at the
        Harvard-Yale game."
     
        2. (Obsolete) The maximum {address space} of a computer (see
        below).  For a 680[234]0 or {VAX} or most modern 32-bit
        architectures, it is 4,294,967,296 8-bit bytes (four
        {gigabytes}).
     
        3. A title of address (never of third-person reference),
        usually used to show admiration, respect, and/or friendliness
        to a competent hacker.  "Greetings, moby Dave.  How's that
        address-book thing for the Mac going?"
     
        4. In backgammon, doubles on the dice, as in "moby sixes",
        "moby ones", etc.  Compare this with {bignum}: double sixes
        are both bignums and moby sixes, but moby ones are not bignums
        (the use of "moby" to describe double ones is sarcastic).
     
        5. The largest available unit of something which is available
        in discrete increments.  Thus a "moby Coke" is not just large,
        it's the largest size on sale.
     
        This term entered hackerdom with the Fabritek 256K memory
        added to the MIT AI PDP-6 machine, which was considered
        unimaginably huge when it was installed in the 1960s (at a
        time when a more typical memory size for a {time-sharing}
        system was 72 kilobytes).  Thus, a moby is classically 256K
        36-bit words, the size of a PDP-6 or PDP-10 moby.  Back when
        {address registers} were narrow the term was more generally
        useful, because when a computer had {virtual memory} mapping,
        it might actually have more physical memory attached to it
        than any one program could access directly.  One could then
        say "This computer has six mobies" meaning that the ratio of
        physical memory to address space is six, without having to say
        specifically how much memory there actually is.  That in turn
        implied that the computer could timeshare six "full-sized"
        programs without having to swap programs between memory and
        disk.
     
        Nowadays the low cost of processor logic means that address
        spaces are usually larger than the most physical memory you
        can cram onto a machine, so most systems have much *less* than
        one theoretical "native" moby of {core}.  Also, more modern
        memory-management techniques (especially paging) make the
        "moby count" less significant.  However, there is one series
        of widely-used chips for which the term could stand to be
        revived --- the Intel 8088 and 80286 with their incredibly
        {brain-damaged} segmented-memory designs.  On these, a "moby"
        would be the 1-megabyte address span of a segment/offset pair
        (by coincidence, a PDP-10 moby was exactly one megabyte of
        nine-bit bytes).
     
        [{Jargon File}]
     
        (1997-10-01)