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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
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Bartek, BA: AUTHOR [+3]

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.

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

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. Then inhibit interrupts and check for the start of a horizontal retrace cycle and store one character in the refresh buffer. The problem with this approach is that communications interrupts would be missed while the refresh buffer was being filled due to the duration of interrupts being inhibited for the retrace poll loop. A better solution is to calculate the longest period of time that interrupts could be disabled without loss of communications interrupts. This value was named the threshold value. The inhibited poll loop was timed and never allowed to exceed the threshold value. The vertical retrace cycle was used as the trigger for write operations. A vertical ...