Chapter 39 ½ Implementation and Performance Evaluation of the PDP-1 1 Family 669

them to read from and write to memory and peripheral devices. The control unit provides all the signals necessary to evoke the appropriate operations in the data paths and Unibus interface. All PDP-11's have comparable data-path and control unit implementations that allow them to be contrasted in a uniform way. In this section a basis for comparing these machines shall be established and used to characterize them.

3.1.1 Data Paths. An archetype may be constructed from which the data paths of all midrange PDP-11's differ but minimally. This archetype is diagrammed in Fig. 3. All major registers and processing elements, as well as the links and switches which interconnect them, are indicated. The data-path illustrations for individual implementations are shown in Figs. 5 through 7. These figures are laid out in a common format to encourage comparison. Note that with very few exceptions all data paths are 16 bits wide (the PDP-11 word size).

The heart of the data paths is the arithmetic logic unit or ALU, through which all data circulate and where most of the processing actually takes place. Among the operations performed by the ALU are addition, subtraction, l's and 2's complementation, and logical ANDing and ORing.