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ACL2 Version 8.1
Copyright (C) 2018, Regents of the University of Texas

ACL2 is licensed under the terms of the LICENSE file distributed with ACL2. See also the documentation topic, COPYRIGHT.

Table of Contents

Performance comparisons

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

Obtaining Common Lisp

ACL2 works on Unix, GNU-Linux, and Mac OS X, which we call "Unix-like systems", as well as many 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, listed here alphabetically.

Obtaining Allegro Common Lisp

The website for Allegro Common Lisp, a commercial implementation, is You may be able to obtain a trial version there.

Obtaining CCL (OpenMCL)

Clozure Common Lisp (Clozure CL, or CCL) was formerly known as OpenMCL. Quoting from the Clozure Common Lisp web page (July, 2014): ``Some distinguishing features of the implementation include fast compilation speed, native threads, a precise, generational, compacting garbage collector, and a convenient foreign-function interface.''

NOTE: Certain ACL2 features are optimized for 64-bit CCL. Some large developments may even fail with 32-bit CCL; so for CCL, the 64-bit version is preferred. To check if your CCL is a 64-bit CCL, evaluate the following expression in your CCL; the result should be YES.

#+x86_64 'yes #-x86_64 'no

Here are instructions for fetching and installing CCL.

Obtaining CLISP

CLISP is a non-commercial Common Lisp implementation, available from We do not recommend CLISP as a platform for ACL2, for the following reasons.

Obtaining CMU Common Lisp

NOTE: ACL2 Version 8.1 cannot be reliably run on CMUCL, so we have disabled building ACL2 on CMUCL. The CMUCL implementor is aware of the problem, and we are hoping for a fix before the next ACL2 release so that we can remove this note.

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

Obtaining GCL

You might be able to download a binary Debian package for ACL2. Thanks to Camm Maguire for maintaining this package. Note however that it may take some time after each ACL2 release for this binary Debian package to be updated for that release.

Otherwise, it should be easy to obtain and build GCL yourself. Note that ACL2 requires ANSI GCL version 2.6.12 or later. Perhaps simplest is to fetch it via git and then build the executable gcl/gcl/bin/gcl as follows.

git clone git://
cd gcl/gcl
git checkout Version_2_6_13pre
./configure --enable-ansi && make

It may also be possible to fetch it from the main GNU website for GCL or perhaps from, in which case ANSI GCL can be built as shown above:

cd gcl && ./configure --enable-ansi && make

Obtaining LispWorks

LispWorks is a commercial Common Lisp implementation. You can download a free, restricted, version from You may ask the vendor for an evaluation license for the full product if you are considering purchasing a license.

Obtaining SBCL

SBCL (Steel Bank Common Lisp) is a non-commercial Common Lisp implementation, available from You may be able to download a suitable binary distribution from that website, but for some of the older versions (such as 1.2.11, which as of December 2018 has been the latest binary available for MacOS from that website for some years), you will not be able to certify some of the community books. If you download a binary, you can check that the form (member :sb-thread *features*) evaluates to a value other than NIL. To avoid this issue you can build from source, following instructions from the INSTALL file of the SBCL distribution using this command:

sh --with-sb-thread

Depending on the platform, building ACL2 may initially fail with SBCL because of insufficient heap memory. Harsh Raju Chamarthi points out that a fix is to run SBCL with an increased heap size limit. As of 2014 we find that the the option --dynamic-space-size 2000 following works well on 64-bit linux, for example using a script like the following for SBCL.

<sbcl-dir-path>/src/runtime/sbcl --core <sbcl-dir-path>/output/sbcl.core --dynamic-space-size 2000 "$@"

NOTE: The remaining remarks on SBCL installation can probably be ignored for SBCL versions around 1.2.11 or earlier.

If you try to run large jobs using ACL2 build on SBCL, such as building the ACL2+books combined manual, you may fail with an error, "Immobile space exhausted". We have avoided this error by building SBCL from source rather than obtaining a binary, using options as follows:

sh --without-immobile-space --without-immobile-code --without-compact-instance-header

Rather comparable test runs produced a significant speed-up when building with the options above, as shown by the following results when using the "time" command for such runs.

;;; before using the options above in the build
85597.939u 1357.892s 3:27:15.73 699.2%  0+0k 1677680+7635408io 607pf+0w

;;; after using the options above in the build
72693.740u 1565.148s 2:58:36.10 692.9%  0+0k 3778176+7630768io 1250pf+0w

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