Assembly Language: Video Character Interface

This section has a set of code examples for writing character data to a PC in 80 character by 25 line mode. The first routine will check the video mode, and if it is a standard mode, the code returns with a no carry condition and starts using the standard video BIOS routines to perform the requested functions. These routines provide many code entry points where other programs may call in. Before using any of the video routines, the program must call the reset_video routine to initialize and make ready the other video calls. Some of the video display routines will respond to video display escape codes. Most of the escape codes deal with standard cursor control functions. There is an example of table selection jumping code.

Most computers use a raster graphics video system. The data is sent to the monitor as rows from left to right that are arranged from top to bottom. The first data bit sent goes to the upper left corner and the last data bit in a frame goes to the lower right corner. When addressing video RAM, the memory may be divided into rows that can be divided into columns. A fundamental understanding of this system is necessary when translating a row and column position into the actual video RAM address for a character or pixel location.

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