P3 was designed in Arden Hills, Minnesota. The P1 was a novel air-cooled, 60-board cabinet designed by a group in Mississauga; the P1 ran on 60 Hz current (no motor-generator sets needed). A fourth high-end 180 model 990 (codenamed THETA) was also under development in Arden Hills. The 180s were initially marketed as 170/8xx machines with no mention of the new 8/64-bit system inside. However, the primary control program is a 180-mode program known as Environmental Interface (EI). The 170 operating system (NOS) used a single, large, fixed page within the main memory. There were a few clues that an alert user could pick up on, such as the "building page tables" message that flashed on the operator's console at startup and deadstart panels with 16 (instead of 12) toggle switches per PP word on the P2 and P3. The peripheral processors in the true 180s are always 16-bit machines with the sign bit determining whether a 16/64 bit or 12/60 bit PP instruction is being executed. The single word I/O instructions in the PPs are always 16-bit instructions, so at deadstart the PPs can set up the proper environment to run both EI plus NOS and the customer's existing 170-mode software. To hide this process from the customer, earlier in the 1980s CDC had ceased distribution of the source code for its Deadstart Diagnostic Sequence (DDS) package and turned it into the proprietary Common Tests & Initialization (CTI) package. The initial 170/800 lineup was: 170/825 (P1), 170/835 (P2), 170/855 (P3), 170/865 and 170/875. The 825 was released initially after some delay loops had been added to its microcode; it seemed the design folks in Toronto had done a little too well and it was too close to the P2 in performance. The 865 and 875 models were revamped 170/760 heads (one or two processors with 6600/7600-style parallel functional units) with larger memories. The 865 used normal 170 memory; the 875 took its faster main processor memory from the Cyber 205 line. A year or two after the initial release, CDC announced the 800-series' true capabilities to its customers, and the true 180s were relabeled as the 180/825 (P1), 180/835 (P2), and 180/855 (P3). At some point, the model 815 was introduced with the delayed microcode and the faster microcode was restored to the model 825. Eventually the THETA was released as the Cyber 990. Cyber 200 series In 1974,