Approximately a year and a half ago we began work here at Carnegie Mellon on a project to implement DOS as a user level application based on Mach 3.0 kernel. To date we support DOS as a multi-threaded Mach Task running in conjunction with either the 4.3 BSD Unix Server or with the the OSF/1 Single Server. The Mach DOS software support consists of machine dependent kernel support, and a multi-threaded emulator task which provides virtual devices, BIOS emulation, support for common DOS extensions, emulation of many DOS system calls, and software for integration with the Unix server. This work was outlines in an article presented at the Second USENIX Mach Symposium. That article appears in the proceedings of the Symposium.