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How is display memory decoded on a VGA?
This is controlled by the RAM Enablefield. If display memory decoding is disabled, then the VGA hardware ignores writes to its address space. The address range that the VGA hardware decodes is based upon the Memory Map Selectfield. The following table shows the address ranges in absolute 32-bit form decoded for each value of this field:
What does the write mode do on a VGA?
Writing to Display Memory. The VGA has four write modes, selected by the Write Mode field. This controls how the write operation and host data affect the display memory. The VGA, depending on the Write Mode field performs up to five distinct operations before the write affects display memory.
What are the features of a VGA display?
The VGA hardware contains hardware that can perform bit manipulation on data and allow the host to operate on all four display planes in a single operation. These features are fairly straightforward, yet complicated enough that most VGA programmers choose to ignore them.
Why is there no memory access for VGA?
Because the VGA was designed for 8 and 16 bit bus systems, and due to the way the Intel chips handle memory accesses, it is impossible for the host CPU to access the bit planes directly, instead relying on I/O registers to make part of the memory accessible.
Why is there a buffer on my video card?
In addition, the card may buffer one location of video memory in the chipset, making it appear that there is RAM at an address where there is none present, so you may have to read or write to a second location to clear the buffer.
What is the base address for video memory in MS-DOS?
The base address for the video memory in MS-DOS is 0xB8000. I am trying to write to this address using debug.exe, but I am getting an error: You need to use a segment and offset, 0xB8000 can’t be represented directly in 16 bits: