Description

This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).

21 How do I calculate how much VRAM/DRAM I need?

This discussion only deals with calculating the minimum amount of RAM you will require _on your video card_ and is not related to main system RAM. The following calculations will tell you the minimum amount of RAM necessary, but some video cards do not use all of their RAM for the frame buffer (area that stores screen information). In particular, some Windows accelerator cards use some of their memory to store font or other graphical information. As a result, some cards with 2 Megs of video memory will not be able to display the higher pixel addressabilities and colour depths that you might expect.

There are two things that have to be decided in order to determine how much video RAM is required for a given pixel addressability. The first is the screen addressability in pixels and the second is the colour depth in bits. Before you go out and purchase a video card and/or extra RAM, make sure that the card is capable of the pixel addressability and number of colours that you want. Often cards are advertised as 1280x1024 and up to 16.7 million colours, _not_ 1280x1024 _at_ 16.7 million colours.

Standard pixel addressabilities available are: 640x480, 800x600, 1024x768, 1280x1024 & 1600x1200

Less commonly, 1152x864 and 1600x1280 are supported.

For an idea of pixel addressabilities appropriate for your monitor, see "What pixel addressabilities are best for my monitor". Colour depth information is provided in "How does colour depth relate to the number of colours?".

To calculate the amount of video memory you need, simply multiply:

(horizontal addressability) * (vertical addressability) * (pixel depth)/8

So, for 1024x768 and 256 colours (that's 8 bit):

1024 * 768 * 8/8 = 786432 bytes i.e. a 1 Meg card will suffice

and for other configurations:

640x480x24 bit colour = 921600 (min. 1 Meg card)
800x600x16 bit colour = 960000 (min. 1 Meg card)
800x600x24 bit colour = 1440000 (min. 2 Meg card)
1024x768x16 bit colour = 1572864 (min. 2 Meg card)
1024x768x24 bit colour = 2359296 (min. 4 Meg card)
1280x1024x8 bit colour = 1310720 (min. 2 Meg card)
1280x1024x24 bit colour = 3932160 (min. 4 Meg card)
1600x1200x24 bit colour = 5760000 (min. 6 Meg card)

Note that many truecolour implementations (24 bit colour) use 32 bit long words. For these chipsets/modes you will have to use a pixel depth of 32 in the above calculation i.e. 24 bit colour may not be available at 1280x1024 with some 4 Meg cards.

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