moby

jargon /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)