VP15A Graphics Software PDP-15

This document describes the VP15A Graphics Software for the PDP-15 computer, designed to simplify programming the VP15A Storage Tube Display System by abstracting hardware details. The software package includes an I/O device handler (VPA), subprograms callable by FORTRAN IV or MACRO-15, and an ASCII text line editing program (EDITVP).

The VP15A system is an on-line point plotting graphics system using a VT01 Storage Tube Display Scope. The display surface is a 1024x1024 dot matrix (8-1/4" x 6-3/8") where images are formed by an electron beam. It supports two operating modes: Store Mode, which retains points for up to 15 minutes without refreshing, and Non-store Mode, offering faster plotting but requiring a refresh rate of at least 30 times per second. Controls include ERASE and VIEW buttons. For programming, the system offers a 512x512 dot matrix capability (a software abstraction of the hardware's 1024x1024) to optimize memory, with no loss in visual resolution. A "Scale Factor" determines the spacing between intensified dots, and display dimensions are defined using Basic and Display Raster Units.

The VPA Device Handler manages communication between the PDP-15 and the display, supporting various data modes. These modes include ASCII for character display (5x7 matrix, ~1700 cps) and Dump Modes for plotting points using 9-bit coordinates. Key functions include .INIT to initialize the display, .WRITE for output (supporting different ASCII and point-plotting modes), .WAIT and .WAITR for I/O buffer management, .CLOSE to terminate file actions, and .FSTAT to check file status. Specific printing rules apply for text display, and certain system macros are ignored or deemed illegal.

The VP15A Subprogram Package provides routines for common graphics tasks: INIT for display setup, VECTOR for line drawing, POINT for plotting individual points, and TEXT for displaying text strings. The NUVAL function aids in packing 9-bit coordinate pairs and addressing FORTRAN's handling of sign bits. FORTRAN WRITE statements can also be used for direct text output.

Finally, the VP15A Text Editor (EDITVP) is a modified System Editor that uses the VP15A display to show text being created or edited. It features TV ON/OFF commands to control display visibility and visually marks modified lines by drawing a dashed line through the original and showing the corrected line below.