Chapter 25 The DEC 338 display computer 307
Instructions and their interpretation in P(display)
Two instruction-set types are interpreted in the P.display: Data State. in which instructions specify display information; and Control State, in which instructions specify program control information (e.g., jumps, modes, etc.). A state diagram for the interpretation process is given in Fig. 1.
Data-state instructions
There are seven instructions (which DEC calls modes) that can be executed while P.display is in data state. The instructions (modes) are really substates of data state. The instructions (actually more like data) are interpreted for the mode. When all the data-mode instructions have been interpreted, an escape instruction returns the P.display to control state. A control instruction is issued to select a mode and simultaneously place the display n data state.
Increment mode. This mode is used to draw curves and alphanumeric characters and other small symbols. Two instructions are stored per word. An instruction will cause the beam position to be moved one, two, or three times, in 0.010-inch increments, in one of eight directions. Direction 0 is to the right, direction 1 is up and to the right, etc.