Video Memory Under Win98/2000

Hello All ,
In my VXd / Sys i want to modify Video Memory …

1]How can I obtain Video Mem pointer from VXD/SYS ?
2] Is it neccessary to write a graphics/video driver for that ??(I think no
,right!)
3]What kernel mode API’s I can use to read/get/write video mem. ???
thanx in advance…