All About The Video RAM Memory
A video adapter installed in your computer system implements a segment of your system’s initial megabyte of memory to store character information or graphics for display, although this generally is utilized or operated when in standard VGA mode.
Keep in mind that although the current video card may have 256MB or more of onboard memory, just 128KB of this memory seems accessible to the computer system in the video RAM section. The remainder of the memory is available just by the video processor (on the video card) directly, or by your system processor through a memory aperture located close to the 4GB top of the system address space. Since different cards may configure this aperture variously, it is wise to refer to the technical documentation for your chipset or card for additional information. A majority of motherboard ROM possesses BIOS Setup options for managing the video card memory aperture (used principally with AGP video cards), although you are encountering some sort of difficulties associated with the video card, it is advisable to keep those options at their default settings.
Basically, when in standard VGA mode like when running in Windows safe mode or when at a DOS prompt, your computer’s processor can immediately gain access of up to 128KB of the video RAM from address A0000BFFFFh. Also, every current video card possess onboard BIOS usually addressed at C0000C7FFFh that is a section of the memory space set aside for adapter card BIOS. Basically, the higher the color capabilities and resolution of the video adaptor, the more system memory the video adapter utilizes, although, that added memory (past 128KB) is not generally accessible by the computer’s processor. In its place, the system informs the video chip what must be presented, and the video chip produces the picture by placing data exactly into the video RAM on the card.
In the typical system-memory map, an overall of 128KB is set aside for use by the video card to deposit presently displayed information when in standard VGA modes. Thus, the reserved video memory is situated in segments B000 and A000. The video adapter ROM employs extra upper memory room in segment C000. Still with the recent multiple monitor element in Windows 98 and later, just a single video card (the main video card) is in the memory map; all others employ no low system memory.
Note
The position of video adapter RAM is accountable for the well-known 640KB DOS conventional memory barrier. DOS can employ every accessible adjacent memory in the initial megabyte of memory up to the point where the video adapter RAM is encountered. The utilization of early video adapters like the MDA and CGA, may allow DOS access to more than 640KB of system memory.