The PDP-15 was the fifth and last of the 18-bit minicomputers produced by Digital Equipment Corporation. The PDP-1 was first delivered in December 1959 and the first PDP-15 was delivered in February 1970. More than 400 of these successors to the PDP-9 (and 9/L) were ordered within the first eight months. In addition to operating systems, the PDP-15 had compilers for Fortran and ALGOL. History The 18-bit PDP systems preceding the PDP-15 were named PDP-1, PDP-4, PDP-7 & PDP-9. The last PDP-15 was produced in 1979. Hardware The PDP-15 was DEC's only 18-bit machine constructed from TTL integrated circuits rather than discrete transistors, and, like every DEC 18-bit system could be equipped with: an optional X-Y (point-plot or vector graphics) display. a hardware floating point option, with a 10x speedup, was offered. up to 128Kwords of core main memory Models The PDP-15 models offered by DEC were: PDP-15/10: a 4K-word paper-tape based system PDP-15/20: 8K, added DECtape PDP-15/30: 16K word, added memory protection and a foreground/background Monitor PDP-15/35: Added a 524K-word fixed-head disk drive PDP-15/40: 24K memory PDP-15/50: PDP-15/76 PDP-15/76: 15/40 plus PDP-11 frontend. The PDP-15/76 was a dual-processor system that shared memory with an attached PDP-11/05.. The PDP-11 served as a peripheral processor and enabled use of Unibus peripherals. Software DECsys, RSX-15, and XVM/RSX were the operating systems supplied by DEC for the PDP-15. A batch processing monitor (BOSS-15: Batch Operating Software System) was also available. DECsys The first DEC-supplied mass-storage operating system available for the PDP-15 was DECsys, an interactive single-user system. This software was provided on a DECtape reel, of which copies were made for each user. This copied DECtape was then added to by the user, and thus was storage for personal programs and data. A second DECtape was used as a scratch tape by the assembler and the Fortran compiler. RSX-15 RSX-15 was released by DEC in 1971. The main architect for RSX-15 (later renamed XVM/RSX) was Dennis "Dan" Brevik. Once XVM/RSX was released, DEC facilitated that "a PDP-15 can be field-upgraded to XVM" but it required "the addition of the XM15 memory processor." The RSX-11 operating system began as a port of RSX-15 to the PDP-11, although it later diverged significantly in terms of design and functionality. Origin of the RSX-15 name Commenting on the RSX acronym, Brevik says: XVM/RSX Later versions of the PDP-15 could run a real-time multi-user OS called XVM/RSX, an