MICA was the codename of the operating system developed for the DEC PRISM architecture. MICA was designed by a team at Digital Equipment Corporation led by Dave Cutler. MICA's design was driven by Digital's need to provide a migration path to PRISM for Digital's VAX/VMS customers, as well as allowing PRISM systems to compete in the increasingly important Unix market. MICA attempted to address these requirements by implementing VMS and ULTRIX user interfaces on top of a common kernel that could support the system calls (or "system services" in VMS parlance), libraries and utilities needed for both environments.
MICA was cancelled in 1988 along with the PRISM architecture, before either project was complete. MICA is most notable for inspiring the design of Windows NT. When the PRISM architecture evolved into the DEC Alpha architecture, Digital opted to port OSF/1 and VMS to Alpha instead of reusing MICA.
The original goal for MICA was that all applications would have full and interchangeable access to both the VMS and ULTRIX interfaces, and that a user could choose to log in to an ULTRIX or VMS environment, and run any MICA application from either environment. However, it proved to be impossible to provide both full ULTRIX and full VMS compatibility to the same application at the same time, and Digital scrapped this plan in favour of having a separate Unix operating system based on OSF/1 (this was variously referred to as PRISM ULTRIX or OZIX). As a result, MICA would have served as a portable implementation of a VMS-like operating system, with compatible implementations of DCL, RMS, Files-11, VAXclusters, and the VAX/VMS RTLs and system services. Proposals were made for reinstating Unix compatibility in MICA on a per-application basis so that a MICA application could be compiled and linked against the VMS interfaces, or the ULTRIX interfaces, but not both simultaneously.