PDP-9/339 Graphics Subprogram Package

This document describes the PDP-9/339 Graphics Subprogram Package, a software toolkit designed to simplify the use of the Digital Equipment Corporation's Type 339 Programmed Buffered Display System. Intended for users programming in FORTRAN IV or Macro-9, it abstracts complex hardware interactions, allowing users to define, link, display, and modify graphic elements easily.

The document covers:

• System Description: The 339 is an on-line system comprising a CRT, control logic, light pen, pushbutton control box, and an interface to a PDP-9 computer via Direct Memory Access (DMA). This allows time-shared operation and independent program execution for the PDP-9.
• Man/Machine Interaction: Users interact with the display using a light pen (for pointing and drawing) and a pushbutton control box (for generating program interrupts).

• Software Operations: The package is structured around three types of routines:

  • Main Display File Routines: Build the primary display file that orchestrates the overall image.
  • Subpicture Routines: Generate modular, self-contained display files for specific graphic elements (e.g., lines, text, graphs), allowing dynamic manipulation and reuse. These include subroutines for drawing lines (LINE), displaying text (TEXT), linking subpictures (COPY), setting display parameters like scale, intensity, blink, and light pen sensitivity (PRAMTR), plotting data (GRAPH), and controlling subpicture visibility (BLANK/UNBLNK).
  • Input Routines: Enable program control based on user actions with the light pen (LTPN, TRACK) and pushbuttons (PBTN).

• Display Device Characteristics: The 339 system uses a 16-inch CRT offering a programmable drawing area equivalent to 75x75 inches, despite a physical display of 9 3/8 x 9 3/8 inches. Display characteristics such as scale, intensity, blink, and light pen function are programmable.

• Relocatable Display Files: Subroutines like DYSET and DYLINK facilitate storing and reinstating display files to different memory locations by converting subpicture calls to symbolic forms and then resolving them.
• System I/O Device Handler: Chapter 3 details the device handler, which manages direct communication between the PDP-9 and the 339 hardware, handling interrupts and providing macros for initialization (.INIT), reading display registers (.READ), writing display commands (.WRITE), and synchronizing operations (.WAIT, .WAITR, .CLOSE).
• Appendix: Includes sample FORTRAN IV programs demonstrating the use of the graphics subroutines for creating displays like a "house" image and tracking.