178 BEGINNING OF THE MINICOMPUTER

Figure 4. The LINC-8.

only 1 Kword or 2 Kwords of primary memory available, two LINC tapes, and one CRT. By bounding the system to a single configuration, it was possible to provide a complete computing environment including software and to provide for convenient interchange of user software.

As indicated in Chapter 6, discussions with Foxboro Corporation in the fall of 1961 led to the design, using many LLNC ideas, of a 12-bit digital controller called the DC-12. Instead of building the DC-12, DEC built the 18-bit PDP-4 and sold one to Atomic Energy of Canada Limited. AECL used the PDP-4 for a reactor control computer system at Chalk River, an application requiring an elaborate analog monitoring system as a front-end. To reduce the complexity of the analog system, a special front-end computer was needed. The Wes Clark

Figure 5. The LINC-12.

10-bit L-l design was considered but rejected because the encoded analog values required words longer than 10 bits, and because the size and complexity of the program seemed too great for such a small computer. After visiting Chalk River in the winter of 1962, DEC engineers decided that a 12-bit design based on the DC-12 would be excellent for such a front end in PDP-4 process control applications. The instruction set for the new machine, the PDP-5, was specified in detail by Alan Kotok and Gordon Bell, and the logic design was carried out by Edson DeCastro, the applications engineer responsible for building the analog front end at Chalk River.

The intent of the design was to simplify the system so that it would take no longer to design the PDP-5 than it had taken to design the analog front end that it would be replacing. The machine used the standard modules developed for the PDP-4, including the concept of bit-slice