This is a practical manual on operating systems, which describes a small UNIX-like operating system, demonstrating how it works and illustrating the principles underlying it. The relevant sections of the MINIX source code are described in detail, and the book has been revised to include updates in MINIX, which initially started as a v7 unix clone for a floppy-disk only 8088. It is now aimed at 386, 486 and pentium machines, and is based on the international posix standard instead of on v7. Versions of MINIX are now also available for the Macintosh and SPARC.