Video RAM Memory

by Kyle Duke.
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A video adapter installed in your system uses a portion of your system's first megabyte of memory to hold graphics or character information for display, but this typically is used or active only when in basic VGA mode.

Note that even though a modern video card can have 256MB or more of onboard memory, only 128KB of this memory appears available to the system in the video RAM area. The rest of the memory is accessible only by the video processor (on the video card) directly, or by your system processor via a memory aperture positioned near the 4GB top of the system address space. Because this aperture can be configured differently by various cards, you should consult the technical documentation for your card or video chipset for more information. Most motherboard ROM have BIOS Setup options for controlling the video card memory aperture (used primarily with AGP video cards), but unless you are experiencing some type of problem related to the video card, it is best to leave those options to their default settings.

When in basic VGA mode, such as when at a DOS prompt or when running in Windows safe mode, your processor can directly access up to 128KB of the video RAM from address A0000BFFFFh. All modern video cards also have onboard BIOS normally addressed at C0000C7FFFh, which is part of the memory space reserved for adapter card BIOS. Generally, the higher the resolution and color capabilities of the video adapter, the more system memory the video adapter uses, but again, that additional memory (past 128KB) is not usually accessible by the processor. Instead, the system tells the video chip what should be displayed, and the video chip generates the picture by putting data directly into the video RAM on the card.

In the standard system-memory map, a total of 128KB is reserved for use by the video card to store currently displayed information when in basic VGA modes. The reserved video memory is located in segments A000 and B000. The video adapter ROM uses additional upper memory space in segment C000. Even with the new multiple monitor feature in Windows 98 and later, only one video card (the primary video card) is in the memory map; all others use no low system memory.

Note

The location of video adapter RAM is responsible for the infamous 640KB DOS conventional memory barrier. DOS can use all available contiguous memory in the first megabyte of memory up to the point where the video adapter RAM is encountered. The use of ancient video adapters, such as the MDA and CGA, can enable DOS access to more than 640KB of system memory.

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