Pat Villani explains the construction and operation of DOS-C, the fully functional, portable, single-threaded operating system written in C and distributed by the FreeDOS community on the Internet. DOS-C is a single large kernel with a layered architecture, is non-multitasking with a large number of system calls similar to the MS-DOS APIs, and employs a minimum of assembly language. FreeDOS uses the de facto DOS hardware standards and provides binary compatibility for MS-DOS applications. It compiles with Borland C, Microsoft C, and other C cross-compilers without using their run-time libraries. Villani provides C-to-assembly interface routines where assembly is used. A DOS clone has value as an educational tool, in embedded systems applications, on non-Intel platforms, in establishing independence from Microsoft, and as an extension to a commercial DOS. Villani offers suggestions on creating a development environment using common, low-cost tools and provides clues on where portability issues are most problematic and how to make code portable. The book and companion disk include the full source code and support files for an 80x86 kernel.