Just think that each byte of video segment (beginning from 0xA000) represent 4 pixels on the screen, and you can set 1,2,3 or 4 pixels at one time (but each time you need to access VGA I/O ports to select them).
Try to search for XLib - very good X-Mode library with many features.
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FlyFire/CodeX
http://codexorg.webjump.com