Editing CAR and CDR (section)

===704 macros===
The 704 [[assembly language|assembler]] [[macro (programming)|macro]] for <code>car</code> was:<ref name="assm" /><ref name="acm" /><ref name="MIT704"/>
<syntaxhighlight lang="asm">
LXD JLOC 4  # C( Decrement of JLOC ) → C( C )  # Loads the Decrement of location JLOC into Index Register C
CLA 0,4     # C( 0 - C( C ) ) → C( AC )        # The AC register receives the start address of the list
PAX 0,4     # C( Address of AC ) → C( C )      # Loads the Address of AC into Index Register C
PXD 0,4     # C( C ) → C( Decrement of AC )    # Clears AC and loads Index Register C into the Decrement of AC </syntaxhighlight>

The 704 assembler macro for <code>cdr</code> was:<ref name="assm">Portions from NILS' LISP PAGES- [https://archive.today/20041021220505/http://t3x.dyndns.org/LISP/QA/carcdr.html  http://t3x.dyndns.org/LISP/QA/carcdr.html]</ref><ref name="acm">MIT AI Lab Memo 6 [https://web.archive.org/web/20170706114352/ftp://publications.ai.mit.edu/ai-publications/pdf/AIM-006.pdf https://web.archive.org/web/20170706114352/ftp://publications.ai.mit.edu/ai-publications/pdf/AIM-006.pdf]</ref><ref name="MIT704">CODING for the MIT-IBM 704 COMPUTER [http://bitsavers.informatik.uni-stuttgart.de/pdf/mit/computer_center/Coding_for_the_MIT-IBM_704_Computer_Oct57.pdf http://bitsavers.informatik.uni-stuttgart.de/pdf/mit/computer_center/Coding_for_the_MIT-IBM_704_Computer_Oct57.pdf]</ref>
<syntaxhighlight lang="asm">
LXD JLOC 4  # C( Decrement of JLOC ) → C( C )  # Loads the Decrement of location JLOC into Index Register C
CLA 0,4     # C( 0 - C( C ) ) → C( AC )        # The AC register receives the start address of the list
PDX 0,4     # C( Decrement of AC ) → C( C )    # Loads the Decrement of AC into Index Register C
PXD 0,4     # C( C ) → C( Decrement of AC )    # Clears AC and loads Index Register C into the Decrement of AC </syntaxhighlight>

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.<ref name=mccarthy />