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From: kaufman@Shasta.UUCP
Newsgroups: comp.sources.d,misc.misc
Subject: Re: Lisp operators
Message-ID: <1344@Shasta.STANFORD.EDU>
Date: Thu, 26-Feb-87 11:52:34 EST
Article-I.D.: Shasta.1344
Posted: Thu Feb 26 11:52:34 1987
Date-Received: Sat, 28-Feb-87 05:39:39 EST
References: <1277@beta.UUCP> <517@cod.UUCP> <1297@zeus.TEK.COM>
Reply-To: kaufman@Shasta.UUCP (Marc Kaufman)
Organization: Stanford University
Lines: 25
Xref: utgpu comp.sources.d:364 misc.misc:610

In article <1297@zeus.TEK.COM> dant@tekla.tek.com (Dan Tilque) writes:
>learned that there are essentially two operators in the language.  These
>are "CAR" and "CDR".  ("CDR" *is* pronounced "could-er".)  The interesting
>thing about them is their origin.  It seems that on the machine where
>the language was developed (at Stanford, I think; don't know what machine)
>there were two registers: a Current Address Register (CAR) and a Current
>Decrement Register (CDR).

>I always thought that this was perfect example of a machine independent
>language.

The IBM 709 and 7090 had 36 bit words, which were formatted as follows
for instructions:
   3 bits - Flag
  15 bits - Decrement field (used for index modification)
   3 bits - Tag
  15 bits - Address field (obvious use)

Memory size was 32K words (15 bit address).  What more natural thing to
do in a list structure than store one address in the Address field,
and one address in the Decrement field of a list token.  The Flag bits
were used by the garbage collector marking algorithm, and the Tag bits
for typing (Atoms, etc), or perhaps the other way around.

John McCarthy was at MIT when LISP was invented..