6 general registers (16 bit) (Reg 0-5), a Stack Pointer (Reg 6), Instruction Pointer (Reg 7)
(DEC liked to claim 8 general registers, and they were addressed similarily, but you better not add arbitary values to registers 6 & 7)

Instruction List, Page 1, 2, 3, 4, 5, 6, 7, 8, 9

From "Digital at Work" , Digital Press, copyright 1992, page 58

Specification - PDP-11/20 First shipped Spring 1970 Word length 16 bits Speed 800 nanoseconds Primary memory Magnetic core (56 K bytes maximum) Instruction set PDP-11 Software Initially, symbolic editor, debugger, utilities, PAL Architecture UNIBUS Price $20,000 Achievements Became industry standard for 16-bit minicomputers A New Architecture The more people used minicomputers, the more uses they found for them. By the mid-1960s, many customers began to outgrow their machines. As the cost of hardware dropped, the costs of developing software and training rose. Five years after the success of the PDP-8, Digital engineered anew machine with more power at a lower price. The PDP-11 introduced the idea of compatibility as a safeguard against obsolescence and sold close to a million machines. By 1971, all the power of a CPU could be packed onto a sliver of silicon.In another year, floppy disks offered a cheap, portable alternative to built-in hard disks, parallel processing presented an alternative to von Neumann's original step-bystep scheme, and relational databases showed the potential of electronic libraries - asthe video game craze emptied pockets of loose change. By the mid-1970s, computers in medicine performed CAT scans and were used to confirm diagnoses, Wang sold word processing, and the CRAY-1 was the first successful vector processor. A Lasting Success From 1970 to 1990, Digital built four generations of PDP-11 systems, ranging from a small 4-user system to a large 64-user machine. In 1975, a new generation of hardware technology, Large Scale Integration (LSI),furtherstreamlined the PDP-11 design. The "computer on a board" provided greater performance than the PDP-11/20 and maintained compatibility with the PDP-11 systems that preceded it. The PDP-11 was an instant success. In its first week of release Digital received 150 orders. Today more than half a million PDP- 11 systems are still operating around the world. The phenomenal popularity and growth of the PDP-11 product line led to a change in Digital's organizational structure. As larger and more complex PDP-11 systems were engineered, Digital reorganized product lines to correspond more closely to specific applications and markets. Much fanfare attended the design and manufacture of the PDP-11. To escape the fire of PDP-9 wire-wrapping guns, the design team worked below the loading dock of the Mill, coming up regularly for design reviews. PDP-11 Family Timeline 1970 PDP-11/20, first of the PDP-11 series of compatible systems, first UNIBUS product 1972 PDP-11/05, better price/performance for OEM low-end requirements PDP-11/10, end-user version of 11105, for data acquisition and industrial control applications PDP-11/45, fastest in its price range, uses three types of primary memory 1973 PDP-11/40,PDP-11/35 1974 PDP-11/04 1975 PDP-11/03, LSI-11,"computer on a board" incorporates Large Scale Integration (LSI) technology PDP-11/70, internal cache-memory design 1976 PDP-11/34, PDP-11/55 PDT-11/150, Programmable Data Terminal, first terminal-based PDP-11 system, based on the LSI-11 board 1977 LSI-11/2: LST-11 in half the size PDP-11/60,PDP-11/74 1978 PDT-11/110, /130, packaged inside the new VT100 terminal 1979 F-11 chip set MicroPDP-11/23, minicomputer performance and software in micro-sized package, runs RSX-11M operating system PDP-11/44 1981 PDP-11/24, entire computer central processor on single 8 x 10 circuit board GIGI,low-cost graphics generator uses LSI-11 board T-11 chip, first chip-level PDP-11 1982 Professional 300,325,350 personal computers, the "Personal PDP-11" J-ll,a PDP-11/70 in two microprocessor chips 1983 Micro PDP-11/73 1985 PDP-11/84, Professional 380 1985 MicroPDP-11/83 1986 MicroPDP-11/53 1887 MicroPDP-11/53 + 1990 MicroPDP-11/93, MicroPDP-11/94

• The LSI-11 was introduced later. It was a 3 chip set processor that plugged into a Q-Bus, it also required
  • a memory
  • a serial interface card
  to be plugged into the Q-bus
  Oh the LSI-11 card was
  • a big microcode ROM chip
  • a processor chip
  • and an optional Floating Point chip
  • jumper options for starting and power fail
  It did NOT have a front panel with lights and switches. (I felt cheated.) All commands were through a serial interface to a terminal, like a KSR-33, or VT-100.
• -

Keith wrote (February 2004)

this was the first system to run an AT&T UNIX which was written in C. (C was not available for the PDP-7, but was written by AT&T researchers for the 11.) It was also the PDP-11 on which BSD UNIX was first developed.

• Digital Equipment Corporation PDP-11 Manual, 1969, with ISP description of the architecture @5.9MB
• How the PDP-11 Was Born (according to Larry McGowan)