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ACL2 Version 3.5 Copyright (C) 2009 University of Texas at Austin. ACL2 is licensed under the terms of the GNU General Public License. See below for details.

Obtaining Common Lisp

ACL2 works on Unix/Linux, Macintosh, and some Windows operating systems (at least including Windows 98, Windows 2000, and Windows XP). It can be built on top of any of the following Common Lisps:

Often we put timing comparisons between different lisps in the ACL2 News.

Obtaining GCL

Gnu Common Lisp (GCL) has probably been the most commonly-used platform for ACL2. IMPORTANT: Here we are referring to the non-ANSI version (sometimes called the "CLtL1 version") of GCL. It is probably impossible to build ACL2 with ANSI GCL just now, but that may be coming. Note: On rare occasions you may see a hard error, sometimes (not always) labeled as "bad plist", that appears to be specific to GCL. We have worked with the main GCL implementor but have not found a way to eliminate these rare errors.

Debian package. You do not need to fetch GCL if you download the binary Debian package for ACL2. Thanks to Camm Maguire for maintaining this package. It may take some time after each ACL2 release for this binary Debian package to be updated for that release.

GCL may be fetched from If that site goes down, you may be able to find useful information from the GCL Temporary Distribution Site. GCL maintainer Camm Maguire suggests the following, in order of preference (most to least):

  1. apt-get -q install gcl gcl-doc if running Debian
  2. Download and install the prebuilt binaries otherwise (if available for your platform)
  3. Download the latest (stable) source tarball and build yourself otherwise
  4. Download the latest (stable) cvs branch and build yourself
As of December 2005, the latest recommended version is 2.6.7, and the latest development version is 2.7.0. But see below for suggestions for building GCL on a Macintosh running OS X. And see the next paragraph:

You may obtain recent CVS versions by executing the following commands if you have CVS installed on your system, which will retrieve the latest development/unstable cvs sources by default.

export CVS_RSH=ssh [or, if using csh: setenv CVS_RSH ssh]
cvs co gcl
If you happen to know a particular version of GCL that you wish to obtain, perhaps by following GCL mailing lists, there are alternatives. The following was recommended in Feb. 2009 by the main GCL developer, and may be the best option.
cvs co -r Version_2_6_8pre -d gcl-2.6.8pre gcl

Macintosh. Robert Krug has provided instructions for building GCL on Mac OS X, which we include here (very slightly modified, in part with help from Camm Maguire) in case others find them helpful.

The normal build process for GCL on Mac OS X assumes that
you have installed fink on your Mac.  (If you do not know
what this is, don't worry; you probably don't have it or
want it.)  Here we give instructions that have worked for
building GCL on OS X without fink.

   A. Obtain recent sources (there is a problem, e.g., with
      gcl-2.6.7).  For example, you can do the following:

      cvs -z9 -q co -d gcl-2.6.8pre -r Version_2_6_8pre gcl

      At some point you may be able to obtain GCL from
      ftp://ftp.gnu/org/, cd gnu, cd gcl, get
      gcl-2.6.8.tar.gz, tar xfz gcl-2.6.8.tar.gz)

   B. Make sure that /usr/local/bin is in your PATH; if not, run:

   C. cd <gcl directory>

   D. You now need to patch file h/powerpc-macosx.defs (this might not be
      necessary starting with GCL 2.6.8):

      Replace the line:
      LIBS := `echo $(LIBS) | sed -e 's/-lncurses/ /'` /sw/lib/libintl.dylib
      LIBS := `echo $(LIBS) | sed -e 's/-lncurses/ /'` /usr/local/lib/libintl.dylib

   E. Configure and start to build gcl:

   F. Install gcl:
      sudo make install

Obtaining Allegro Common Lisp

Allegro Common Lisp is probably the most commonly-used commercial platform for ACL2. You may be able to obtain a trial version from its web site,

Obtaining CMU Common Lisp

CMU Common Lisp (sometimes called CMUCL) is a non-commercial Common Lisp implementation, available from

Obtaining SBCL

SBCL (Steel Bank Common Lisp) is a non-commercial Common Lisp implementation, available from

Obtaining CLISP

CLISP is a non-commercial Common Lisp implementation, available from

Obtaining CCL (OpenMCL)

Clozure CL (CCL) was formerly known as OpenMCL. Quoting from the Clozure CL web page: ``Clozure CL is a fast, mature, open source Common Lisp implementation that runs on Linux, Mac OS X and BSD on either Intel x86-64 or PPC.''

Here is an easy way to obtain and build the latest version (generally recommended) for Linux on running on x86 or x86-64. First execute the following shell command to create a ccl directory.

svn co
Note: if you prefer the latest release, you can obtain that instead, for example as follows.
svn co
Then create a suitable script, say as follows, where DIR is the directory above the new ccl directory.

tmp=`uname -a | fgrep x86_64`
# Start up 64-bit or 32-bit lisp, respectively:
if [ "$tmp" != "" ] ; then \
    DIR/ccl/scripts/ccl64 $* ; \
else \
    DIR/ccl/scripts/ccl $* ; \

Now rebuild the CCL lisp image by invoking the above script and then executing the following commands.
(ccl:rebuild-ccl :full t)
Your script will now start the updated CCL lisp image.

More details if you want or need them:
Step 3 in has more details on building from source. Alternatively, you can download a gzipped tar file; see the main CCL page, or visit the page of stable Clozure CL snapshots for ACL2 users. (Subversion and gzipped tar files are great, but not so much a CCL disk image (.dmg file), as we have had a report of the extracted CCL opening its own window when you start it up.) If you don't want to write your own script (as suggested above) then after obtaining CCL, you may wish to edit file ccl/scripts/ccl or file ccl/scripts/ccl64, depending on whether you want to use a 32-bit or 64-bit version (respectively).

Obtaining Lispworks

Lispworks is a commercial Common Lisp available from

Lispworks note. We initially encountered a problem in getting ACL2 to run under LIspworks 4.2.0. The Lispworks folks provided a patch and suggested that we make the following announcement.

Users with LispWorks4.2.7 should ask us at for the transform-if-node patch. It will be helpful if they quote (Lisp Support Call #11372) when doing so. Also, they must send a bug form generated from their LispWorks image: instructions at

Performance comparisons

You can see recent performance numbers by following this link, or by going to the ACL2 home page and following the link "Recent changes to this page".

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