304 Part 4 The instruction-set processor level: special-function processors
Section 1 Processors to control terminals and secondary memories
Another approach to the design of a P.display is based on a P.microprogram which is shared among many T.displays (Rose, 1967]. Yet another alternative, which has not yet been tried, is to incorporate a Pio (P.display) as a special mode in a conventional Pc. Thus the P would interpret either conventional Pc instructions or P.display instructions.
P-display is the interpreter for the output of pictures or graphics. The 338 utilizes data space efficiently simply because the data are long variable-length strings (word vectors). The instruction requires almost no space to specify the data operations and addresses; data are interpreted directly or immediately in the instruction rather than via instruction addresses.
Another feature which allows a program to be efficiently encoded is the stack mechanism for storing subroutine linkages. Subroutines in P.displays are actually programs which form part of a more complete picture. Subroutines are actually subpictures. Although the stack mechanism allows for recursive picture calls, the stack is used principally to save space and to allow multiple T.displays to use common picture programs.
A problem in the 338 which is common to all multi-P structures is intercommunication among the P's. Pc is the controlling P, as is the case with most Pc-Pio structures. The P('338) has no trap to itself but relies on an interrupt signal to Pc. The Pc processes both tasks which P.display might process, given an interrupt system, and other tasks beyond P.display's capability.
A clock should be built into the 338. The brightness or intensity of a picture is determined both electronically (see the mode instructions for controlling intensity) and by the rate at which the pictures are repeated. A clock would allow the time when pictures are started or drawn to be specified; thus the intensity would be independent of picture length.
The 338 requires more hardware than a simpler Pc. However, a large amount of this hardware is used to control the generation of characters and lines. The lines (vectors) are drawn using a DDS (Digital Differential Analyzer) technique. Perhaps one-half of the registers could be eliminated if the 338 were not a P. A simpler alternative was constructed about a similar computer, the PDP-9, by Bell Telephone Laboratories and DEC, using the approach of making the display only a K.
A more elaborate Pc interrupt system with reduced overhead time would enable Pc to take on the specialized program control functions in the 338. Such a scheme might pass the program or instruction counter parameter directly from P.display to Pc. In this way, Pc or P.display would alternatively process part of a single instruction stream, depending on the task.
Despite the problems of this early P.display, it has a sophistication which successors appear to be following.