PDP-5
The PDP-5 was Digital Equipment Corporation's first 12-bit computer, introduced in 1963.

History 
An earlier 12-bit computer, named LINC has been described as the first minicomputer and also "the first modern personal computer."  It had 2,048 12-bit words, and the first LINC was built in 1962.

DEC's founder, Ken Olsen, had worked with both it and a still earlier computer, the 18-bit 64,000-word TX-0, at MIT's Lincoln Laboratory.

Neither of these machines was mass-produced.

Applicability 
Although the LINC computer was intended primarily for laboratory use, the PDP-5's 12-bit system had a far wider range of use. An example of DEC's "The success of the PDP-5 ... proved that a market for minicomputers did exist"
is:
 "Data-processing computers have accomplished for mathematicians what the wheel did for transportation"
 "Very reliable data was obtained with ..."
 "A PDP-5 computer was used very successfully aboard Evergreen for ..."
all of which described the same PDP-5 used by the United States Coast Guard.
The principal designer of the PDP-5 was the young engineer Edson de Castro who went on later to found Data General.

Hardware 
By contrast with the 4-cabinet PDP-1, the minimum configuration of the PDP-5 was a single 19-inch cabinet with "150 printed circuit board modules holding over 900 transistors."  Additional cabinets were required to house many peripheral devices.

The minimum configuration weighed about .

The machine was offered with from 1,024 to 32,768 12-bit words of core memory.  Addressing more than 4,096 words of memory required the addition of a Type 154 Memory Extension Control unit (in modern terms, a memory management unit); this allowed adding additional Type 155 4,096 word core memory modules.

Instruction set 
Of the 12 bits in each word, exactly 3 were used for instruction op-codes.

The PDP-5's instruction set was later expanded in its successor, the PDP-8.  The biggest change was that, in the PDP-5, the program counter was stored in memory location zero, while on PDP-8 computers, it was a register inside the CPU.  Another significant change was that microcoded instructions on the PDP-5 could not combine incrementing and clearing the accumulator, while these could be combined on the PDP-8. This allowed loading of many small constants in a single instruction on the PDP-8. The PDP-5 was one of the first computer series with more than 1,000 built.

Software 
DEC provided an editor, an assembler, a FORTRAN II Compiler and
DDT (a debugger).

Marketplace 
With a base price of $27,000 and designed for those not in need of the 18-bit PDP-4, yet having "applications needing solutions too complicated to be solved efficiently by modules systems" the PDP-5, when introduced in 1963, came at a time when the minicomputer market was gaining a foothold.

Photos 
 PDP-5 computer, including Teletype Model 33 ASR
 PDP-5 from Ed Thelen's collection
 Front panel of a PDP-5

Notes

References 

DEC minicomputers
12-bit computers
Transistorized computers
Computer-related introductions in 1963