“FreeBSD has several options for using software from other
platforms, such as Wine for Win32 and dosemu for DOS. BSDI, NetBSD,
and OpenBSD binaries will run unmodified, and source code from many
UNIX or Linux programs can be compiled without modification on
FreeBSD. Additionally, FreeBSD includes kernel modules for
Linux, SCO, and SVR4. These modules allow you to run unmodified
binaries for these platforms on your FreeBSD machine. FreeBSD’s
Linux module works well enough that several commercial software
packages for Linux have been included in the FreeBSD ports
system. Similarly, this article was written on Sun StarOffice
5.1 for Linux, on my FreeBSD-current box. Because the last version
of RealVideo for FreeBSD is version 3.0, I run RealVideo 5.0 for
Linux.”
“In FreeBSD 3.3-stable or higher, Linux mode has two components:
the kernel system call module, and the runtime environment.”