PDP-15
| Industry | Computer hardware |
|---|---|
| Founded | 1957 |
| Fate | Defunct |
| Headquarters | Maynard , United States |
| Products | PDP-15 minicomputer, among others |
The PDP-15 was an 18-bit minicomputer introduced by Digital Equipment Corporation in February 1970. It represented the fifth and last model in DEC's 18-bit lineup, which began with the PDP-1 back in 1959. Within eight months of its debut, over 400 units had been ordered. A subsequent version, the PDP-15/76, incorporated a complete PDP-11, allowing compatibility with the PDP-11's widely used Unibus peripherals. Production ceased in 1979 after roughly 790 units had been sold.
Essentially an enhanced version of the PDP-9, the PDP-15 was redesigned utilizing small-scale integration integrated circuits instead of the PDP-9’s discrete transistor flip chips. This change resulted in a more compact and cost-efficient machine. For comparison, a basic PDP-9 with 8 kW cost approximately $35,000 in 1968, whereas a PDP-15 equipped with 4 kW was priced at $15,600 in 1970. A fully outfitted PDP-15 system featuring 8 kW memory, punch tape, a KSR-35 terminal, a math coprocessor, and dual DECtape drives was available for $36,000—significantly less expensive than earlier comparable systems.
The PDP-15 supported several operating systems such as DECsys, RSX-15, and DOS-15. It also offered compilers for both Fortran and ALGOL, making it attractive for scientific research and commercial uses.
History
Prior to the PDP-15, DEC produced several 18-bit models including the PDP-1, PDP-4, PDP-7, and PDP-9. The final unit of the PDP-15 line was manufactured in 1979.
Hardware
Unlike previous models, the PDP-15 was DEC’s only 18-bit computer constructed with TTL integrated circuits instead of individual transistors. Consistent with all DEC 18-bit systems, it could be configured with:
- an optional X-Y (vector or point plotting) display
- a hardware floating-point arithmetic unit providing a tenfold increase in speed
- main memory expandable up to 128K words of core storage
Models
The PDP-15 series included multiple configurations:
- PDP-15/10: a 4K-word system relying on paper tape
- PDP-15/20: expanded to 8K words and added support for DECtape
- PDP-15/30: featured 16K words of memory along with memory protection and a foreground/background monitor
- PDP-15/35: included a fixed-head disk drive with 524K-word capacity
- PDP-15/40: equipped with 24K words of memory
- PDP-15/50: known for being "costly to maintain," yet still operational as late as 1982
PDP-15/76
The PDP-15/76 combined the features of the 15/40 model with an attached PDP-11 frontend processor, resulting in a dual CPU system sharing memory with a connected PDP-11/05. This arrangement allowed the use of Unibus peripherals through the PDP-11 acting as a peripheral controller.
Software
DEC provided several operating systems for the PDP-15 including DECsys, RSX-15, and XVM/RSX. A batch processing system named BOSS-15 (Batch Operating Software System) was also available.
DECsys
DECsys was the initial mass-storage operating system offered by DEC for the PDP-15. It functioned as an interactive single-user environment distributed on DECtape reels. Each user received their own copy which could be customized with personal programs and data. A second DECtape was typically used as scratch space for assembling code and compiling Fortran programs.
RSX-15
Introduced by DEC in 1971, RSX-15 was mainly developed by Dennis "Dan" Brevik. Following the release of XVM/RSX, users of RSX-15 were given the option to upgrade to XVM by incorporating the XM15 memory processor.
The RSX operating system family includes RSX-11, which originated as a port of RSX-15 to the PDP-11, eventually evolving into its own distinct architecture and feature set.
RSX-15 Name Origin
The system was originally designated DEX-15 ("Digital's Executive - for the PDP-15") by Brevik, but trademark issues necessitated a change. After considering various acronyms highlighting the impactful "X" sound, Brevik and marketing specialist Bob Decker agreed on "RSX," initially meaning "Real-Time System Executive" before being revised to "Resource Sharing Executive." This final naming decision took place during a meeting held on the third floor of building 5 in DEC's former mill facility.
XVM/RSX
Subsequent PDP-15 models incorporated XVM/RSX, a real-time, multi-user operating system that evolved from RSX-15. This enhancement allowed up to six simultaneous teletype users and added compatibility with the RK05 disk drive on the PDP-15/76 variant.
Non-DEC Systems
The MUMPS operating system, which originated on the PDP-7 outside of DEC in 1966, was also ported to run on the PDP-15.
Software Applications
DEC provided a range of software solutions for mathematical, scientific, and commercial tasks. Among these was a Commercial Subroutine Package (CSP) designed to be compatible with the IBM 1130 commercial subroutine package.