This is the source of a public domain FOCAL-81 interpreter I found on the net once, but forgot the original URL of (sorry to the original authors).

Two different versions:

A DOS / *n*x version, in gzipped tar format (source), written by Dave Conroy, and modified and debugged by Akira Kida.
Tested by Akira on the following platforms:

• Sun SPARCstation running SunOS 4.1.3 + sun cc (K&R)
• Sun SPARCstation running SunOS 4.1.3 + gcc 2.6.3 (ANSI)
• Sun SPARCstation running Solaris 2.3 + SunPro cc 2.0.1 (ANSI)
• Sun SPARCstation running Solaris 2.3 + gcc 2.6.3 (ANSI)
• HP9000 model 720 (HPPA) running HPUX 8.05 + hp cc (ANSI)
• HP9000 model 720 (HPPA) running HPUX 8.05 + gcc 2.5.8 (ANSI)
• MS-DOS + MS-C 6.0 (ANSI)
• MS-DOS + LSI C-86 3.30 (very close to ANSI)

The archive hasn't been modified in any way.

A win32 version (Visual Studio 6 source + .exe), ported & tested on Windows 2000 Professional RC2 by me.
The sources have hardly been changed from the other version, with the only intention to make it compile without errors. Changing anything in the include file options will probably make it start generating those again.
The only thing positive about it: it runs without crashing ;-)

And of course, you need something to run on it: try this.