A monolithic system has not a well defined structure. It includes virtually all of the operating-system functionality in one large block of code that runs as a single process with a single address space. All the functional components of the kernel have access to all of its internal data structures and routines.
There is not a free BSD system at the end of the eighties, and so many members of MINIX newsgroup ask to Tanenbaum to introduce many modifications for improving the performances of MINIX. Some of these modifications could change the original educational project of Tanenbaum, and so often he said “ NO ” to these requests.
So, Linus Torvalds using a pc 386 with MINIX develops a small but self-contained kernel in 1991 ( Linux 0.01 ), with the major design goal of UNIX compatibility (i.e., satisfying POSIX).
The Linux kernel is distributed under the GNU General Public License (GPL) , the terms of which are set out by the Free Software Foundation .
The main consequence of GPL is that anyone using Linux, or creating their own derivative of Linux, may not make the derived product proprietary; software released under the GPL may not be redistributed as a binary-only product.
For a deeper examination of this subject see www.gnu.org/home.it.html