PDP-15

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:
Models.
The PDP-15 models offered by DEC were:
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 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 outgrowth of RSX-15. The "XVM" upgrade to RSX was multi-user, and enabled up to six concurrent teletype-based users. XVM Support for the PDP-15/76 included using an RK05 disk drive.
non-DEC.
One other operating system, developed on the PDP-7, was also available for the PDP-15:
Application software.
DEC provided mathematical, scientific and commercial software application tools.