How does the bottom of the computer access the video card?

Address http://www.zhihu.com/question/20722310

It mentions a memory address of something, b8000h to C7FFFH, by calculation, the size of this address interval is about 3.5M.

The computer WINDOWS10 64-bit integrated video card, display the memory address space is C0000000-C03FFFFF, and B0000000-BFFFFFFF, the size is almost 4m+255m, the same text to the memory address space is larger.

Personal understanding of the cause of this result:

Whether it is the shared memory of the integrated graphics card or the independent video memory of the discrete graphics card, it is necessary to create an area in the address space for the CPU to write the data that will be displayed (the resolution of the display and the color information of each pixel).

However, this address space can not occupy the left and right computer memory address space, or the system will not be able to obtain memory, it can not be run, so the system in the memory address space and the system can use the address space to make a trade-off.

If the physical memory is small, only to meet the basic memory, if the physical memory is large, the system also has a larger display information, more points, so that you can save the multi-frame display data, display a bit more consistent.

If the system is a 32-bit system, video memory is not more than 4G, even with the use of independent video memory, the size of the video memory exceeds 4G, the final allocation of memory address space, probably only about 1G. When the system reaches 64 bits, it can take advantage of the larger memory.

It seems that some of the video memory will be divided into two parts, part of the image information is about to be displayed, part of the display material needs to be processed information. Because the memory size commonly required, with the display resolution and the number of colors displayed, such as 2160X1440*32/8, memory size is about 12M, the above calculation of the memory size of more than 12M, it should be the memory of the footage to save it.

How does the bottom of the computer access the video card?