Compaq Computer Corporation, Inc.

Compag Computer Corporation

Stephen B Preston

Paul B Wood

Thomas M Albers

William Caldwell Crosswy

Gary A Stimac

James S Flannigan

A graphics subsystem, including a video digital-to-analog converter, is disclosed. A high speed oscillator generates a pixel clock signal at the frequency at which pixels are to be displayed. Included in the video DAC is a frequency divider which presents an output clock signal having a period which is a multiple of the pixel clock signal, the multiple corresponding to the level of multiplexing of pixel data to be provided by the video DAC; this multiple can equal unity. The video controller in the system receives the output clock signal, and generates clock signals to control the serial port of the frame memory, and also to control the latching of the first stage in the video DAC. The first stage latch in the video DAC latches in the multiple pixel data from the frame memory, and the multiplexer in the video DAC presents the data to the color palette RAM, or around the color palette RAM in true-color non-multiplexed mode, according to the pixel clock signal. Highlighted pixels in a cursor are displayed by inverting the output of the color palette RAM at cursor locations, for example by way of an exclusive-OR function of the color palette RAM output and a bit corresponding to the comparison of the display location and the desired cursor location. Inversion of the output of the color palette RAM results in higher contrast pixels within the cursor.

Circuitry and method for high visibility cursor generation in a graphics

A virtual DOS monitor uses the paging hardware of a processor such as the Intel 80386 microprocessor in conjunction with its Virtual-8086 mode of operation to emulate expanded memory using extended memory. Support for application programs which access expanded memory is thereby provided without the need for additional memory boards or other hardware.

Software emulation of bank-switched memory using a virtual DOS monitor

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Spring, TX, United States of America

Stephen B Preston