Etymology
Lisp was originally implemented on the IBM 704 computer, in the late 1950s. The 704 hardware had special support for splitting a 36-bit machine word into four parts, an "address part" and "decrement part" of 15 bits each and a "prefix part" and "tag part" of three bits each.
Precursors to Lisp included functions:
- car (short for "Contents of the Address part of Register number"),
- cdr ("Contents of the Decrement part of Register number"),
- cpr ("Contents of the Prefix part of Register number"), and
- ctr ("Contents of the Tag part of Register number"),
each of which took a machine address as an argument, loaded the corresponding word from memory, and extracted the appropriate bits.
The 704 assembler macro for cdr was
A machine word could be reassembled by cons, which took four arguments (a,d,p,t).
The prefix and tag parts were dropped in the early stages of Lisp's design, leaving CAR, CDR, and a two-argument CONS.
Read more about this topic: CAR And CDR
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