I am just shaking my head at how weak you understand of modern OS seem or that you seems not to understand how the OS will assign and protect memories spaces that programs and processes can have access to depending on their priority and classification.
That the OS will oversee the read and write to memory locations and in the case of video memory it is normally address by programs using OS service calls not directly.
I any case you do not seems to know that your own OS Linux protected it kernel memory from being access or change by a user process for example?
With similar behavior is build into all versions of windows from XP forward.
That malware often try to get around such limitation and it is a constant battle between malware and systems protection both build in and add on protection.
Here is a starting point for you to be able to understand your own OS better.