  In this paper, I present the design and implementation of Clown--a simulator
of a microprocessor-based computer system specifically optimized for teaching
operating system courses at undergraduate or graduate levels. The package
includes the simulator itself, as well as a collection of basic I/O devices, an
assembler, a linker, and a disk formatter. The simulator architecturally
resembles mainstream microprocessors from the Intel 80386 family, but is much
easier to learn and program. The simulator is fast enough to be used as an
emulator--in the direct user interaction mode.
