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Prevention of Screen Distortion on a Color Display Without Disrupting a Communications Session

IP.com Disclosure Number: IPCOM000039124D
Original Publication Date: 1987-Apr-01
Included in the Prior Art Database: 2005-Feb-01

Publishing Venue

IBM

Related People

Authors:
Bartek, BA Lucas, JC Pierschalla, BR [+details]

Abstract

A method is described for displaying data on a color display without distortion and without impacting communications. The IBM color graphics adapter card (CGA) has no hardware to resolve refresh buffer contention. If the microprocessor reads or writes data to the refresh buffer while a refresh cycle is occurring, the screen will become distorted. This distortion is similar to the snowy display of some TV sets and is widely known as "green lightning". The CGA does provide status bits indicating if a refresh cycle is occurring. Software can check these bits to determine when to access the refresh buffer. A method for resolving the problem of "green lightning" is to wait for the end of a horizontal retrace cycle.