From: utzoo!decvax!genradbo!mitccc!barmar
Newsgroups: net.unix-wizards
Title: Re: Information on Unix/Vax peculiarities
Article-I.D.: mitccc.222
Posted: Tue Dec 14 03:33:00 1982
Received: Wed Dec 15 05:37:10 1982
References: sri-unix.4758

The Honeywell 68/DPS and DPS/8 computers, which run the Multics
time-sharing system (no, the name is NOT a hack on UNIX, but
in fact the reverse is true!), uses double-words for its pointers
when it is running segmented (most of the time), so its
pointers are 72 bits long.  The natural length for int would
be 18 bits because the accumulator is 36 bits wide (it is possible
to do double-word arithmetic using the combination of the
Accumulator and Quotient registers called the AQ, but it is
not as "natural").  These pointers are also more precise than
*charyou can address arbitrary bit boundaries.  Note that all
the actual addressing information fits in less than 36 bits;
these 72-bit pointers contain additional information, such as 
ring-number and fault tags (there are nine bits eventually left over
that Multics Maclisp uses to implement typed-pointers).
An 18-bit int would be a fine subscript, though, as 18 bits is
enough to address any word in a segment (20 bits are necessary
if you want to address any character, though).
By the way, there are places in Multics that pass around pointers
(the 36-bit "packed pointer" type I alluded to earlier) as PL/I
type "fixed bin (35)", i.e. 35-bits plus sign.  This is generally
done so that the program is callable from languages that
do not support pointers, such as Fortran and COBOL.