The 8 by 8 Display

Abstract

This paper describes a display system designed to make the recording and rearrangement of bits in a frame-buffer display system convenient and rapid. The advantage of frame-buffer displays is that because the intensity of each pixel can be specified independently, any picture can be displayed. The disadvantage is that a great many bits in the frame-buffer memory must be changed to make major changes in the picture. The 8 by 8 display described in this paper gets its name from the fact that in a single memory cycle it can access any 8 by 8 square of pixels. Internal shifters and special memory addressing circuits are provided to make the access independent of word boundaries in the memory. Pixel manipulation functions are included to process the 64 pixels thus accessed, mask them, overwrite them with new information, or combine them logically with other pixels. The resulting data can be stored into any other 8 by 8 square of pixels in a subsequent memory cycle. Looping mechanisms implemented in microcode provide RasterOp functions that transfer information from any rectangular area of the display to any other area with or without intervening pixelmodification operations. The prototype system is able to copy the entire 768 by 1024 array of the display in 52 ms, or two frame times. The ability to rearrange data quickly has proved to be an asset for character generation, line drawing, and picture construction, as well as for scrolling and other rearrangements of material already displayed on the screen. A simple model is developed to compare the performance of the 8 by 8 memory system with conventional frame-buffer organization. Execution traces of Smalltalk display programs are applied to the model to obtain figures of merit for different hardware organizations. © 1983, ACM. All rights reserved.