THE PDP-8 AND OTHER 12-BIT COMPUTERS 201

development of DEC's 12-bit computers in chronological order must stop here. However, during the development of the main line of 12- bit computers. some interesting systems based on DEC 12-bit processors have been developed, both by DEC and by others. Among these are the DEC 338 Display Computer, the cache- based PDP-8, and the PDP-14 Programmable Controller (a 1-bit machine similar in its instruction set to the PDP-8 and using Omnibus packaging concepts).

The 338 display, a variant of the PDP-8, is interesting for its historical importance [Bell and Newell, 1971: Chapter 25]. It was one of the earliest display processor-based computers - if not possibly the first. The problem of displaying data on a cathode ray tube clearly shows how the application need drives a complete change in hardware in order to interpret the necessary data-type (in this case, a graphic picture).

The 338 display idea was extended and applied to the displays used with the PDP-9, PDP-15, and the PDP-11 series. Although the 338 had the right general capabilities, it did not have the refinements of later display processors for the PDP-15 and PDP-11 (GT40 and GT60).

An observation that display and other specialized processors evolve in a fashion called the "wheel of reincarnation" [Myer and Sutherland, 1968] is diagrammed in Figure 30. As the figure shows, the process starts with a very simple basic design - here, to have graphics picture output for a computer. The trajectory around the wheel follows:

Position 1: Point-plotting. The computer includes a single instruction display controller which can plot a picture on a point-by-point basis under command of the central processor. For most displays, except storage scopes, the processor can barely calculate the next point fast enough to keep the display refreshed. Hence, the system is processor bound, and the display may be idle. The original PDP-l display is typical of this position, and a display of this type is offered on most DEC minicomputers.

Position 2: Vector-plotting. By adding the ability to plot lines (i.e., vectors), a single instruction to the display processor will free some of the processor and begin to keep all but the fastest display busy.

Position 3: Character-plotting and alphanumeric plotting. With the realization that characters are a major part of what is displayed, commands to display a character are added, further freeing the processor. Many of the point-plotting displays were extended to have character generation capability.

Position 4: General figure and character display. In reality, a picture does not consist of just characters and vectors; each element of the picture is actually a string of characters and a set of closed or open polygons to be displayed starting at a particular point. By providing the control display with a Direct Memory Access channel, the display can fetch each string of text and generate polygons without involving the central processor.

Position 5: Display processors. With the ability to put up sub-pictures with no processor intervention, it is easy for the whole picture to be displayed by linking the elements together in some fashion. This merely requires "jump" and "subroutine" call instructions so that common picture elements do not have to be re-defined. The 338 and other display processors have roughly this capability.

Position 6: Integrated display and central processor. Now, all the data paths and states are present for a fully general purpose processor so that the central processor need never be called on again. This requires a slightly more general purpose interpreter. By minor perturbations, the processor design can be refined in such a way as to execute the same instruction set as the original host computer because the cost of incompatibility is too great. Two processors require two compilers, diagnostics, manuals, and support for use. This state provides the same capability as that shown in Position 1.