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The PDP-9, the 4th of the five 18-bit minicomputers produced by Digital Equipment Corporation, was introduced in 1966.[1]:P.10 A total of 445 PDP-9 systems were produced,[2] of which 40 were the compact, low-cost PDP-9/L units.[3]


The 18-bit PDP systems preceding the PDP-9 were named PDP-1, PDP-4 and PDP-7.


The PDP-9, which was "two metres wide and about 75cm deep,"[4] was approximately twice the speed of the PDP-7. It was built using discrete transistors, and had an optional integrated vector graphics terminal.

It was DEC's first microprogrammed machine.[5]

A typical configuraton included:[2]

Model 33 Teleprinter,
usable as a computer terminal.
The ASR-33 model above includes a paper tape reader and punch, whereas the PDP-9 console used a KSR-33, which lacks these. The separate reader and punch for the PDP-9 was much faster.

Among the improvements of the PDP-9 over its PDP-7 predecessor were:

  • the addition of Status flags for reader and punch errors, thus providing added flexibility and for error detection[6]
  • an entirely new design for multi-level interrupts, called the Automatic Priority Interrupt (API) option
  • a more advanced form of memory management[7]

User/university-based research projects for extending the PDP-9 included:

  • to design a hardware capability for floating point arithmetic, at a time when machines in this price range used software.[8]
  • to design a PDP-9 controlled parallel computer[9]


The system came with a single-user Keyboard monitor. DECsys, provided an interactive, single user, program development environment for Fortran and assembly language programs.[10]

Both FORTRAN II[2] and FORTRAN IV[11] were implemented for the PDP-9.


The PDP-7, of which 120 were sold,[1]:p.8 was described as "highly successful.".[2] The PDP-9 sold 445 units. Both had submodels, the PDP-7A[NB 1] and the PDP-9/L,[NB 2] neither of which accounted for a substantial percentage of sales.[1][NB 3]


See also[edit]


  1. ^ sometimes identified by customers and media as PDP-7/A
  2. ^ yes, DEC had a "slash" for the 9/L but not for the 7A
  3. ^ directly mentioned for one, calculated in the other case


  1. ^ a b c DIGITAL EQUIPMENT CORPORATION - Nineteen Fifty-Seven To The Present (PDF). Digital Equipment Corporation. 1975. 
  2. ^ a b c d "RI Computer Museum, DEC PDP-9, System Number 319". 
  3. ^ ."The Early Architectures of DEC". 
  4. ^ "David Millson - 50 years". 
  5. ^ "DEC PDP-9". 
  6. ^ Bob Supnik. "Architectural Evolution in DEC's 18b Computers" (PDF). 
  7. ^ "PDP 9 Simulator Configuration". GitHub.com. 
  8. ^ Ben Milton Huey (1969). "Design of a floating point processor for the PDP-9 computer". University of Arizona. 
  9. ^ Wokhlu, Roop Krishen (1969). "The logic design of a PDP-9 controlled parallel computer". 
  10. ^ Bob Supnik (19 June 2006). "Technical Notes on DECsys" (PDF). 
  11. ^ "User Manual - Linkage - PDP-9 Facilities" (PDF). Carnegie-Mellon University Hybrid Computation Laboratory. September 1968. Retrieved 19 September 2017.