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...