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The Math

 

Here's how you determine how much RAM you need to run at a certain resolutions and bpp (bits per pixel).

Maximum resolution is actually determined by three things. The first is the amount of video RAM that you have. Second is the maximum resolution output capability of the graphics card, and third is the maximum resolution output capability of the monitor.

For now, we'll just cover the limitation of video RAM.

Take the resolution and bpp that you would like to run at. Let's use 1024x768 at 24bpp (16.7 million colors).

What the resolution means in the above example is 1024 pixels horizontally, and 768 pixels vertically. Multiply these two numbers together. 1024 * 768 is 786,432. This is the total number of pixels displayed on your screen. Multiply this number by the bpp. For our example, this would be 786,432 * 24 which is 18,874,368 bits. Convert this to bytes by dividing by 8 which gives us 2,359,296. Now, because 1MB is not 1,000,000 bits (it's actually 1,024,000 bits) we further divide our number by 1024 which gives us 2.3MB. This is how much display adapter RAM you would need to display 16.7 million colors (24bpp) at 1024x768.

notebutt.gif (1747 bytes)Remember, Video RAM is different than System RAM. This demarcation gets a little fuzzy when talking about AGP cards, but for now just take it at face value. I'll cover how AGP cards work in another article.



Let's do this without all of the words.

1024 * 768 = 786,432 pixels
786,432 * 24 = 18,874,368 bits
18,874,368 / 8 = 2,359,296 bytes
2,359,296 / 1024 = 2.3MB

Here's another example using my settings:

1600 * 1200 = 1,920,000 pixels
1,920,000 * 24 = 46,080,000 bits
46,080,000 / 8 = 5,760,000 bytes
5,760,000 / 1024 = 5.62MB

Because I have 8MB on my graphics adapter, I could probably bump this up to 32bpp. Let's see.

1600 * 1200 = 1,920,000 pixels
1,920,000 * 32 = 61,440,000 bits
61,440,000 / 8 = 7,680,000 bytes
7,680,000 / 1024 = 7.5MB

Look at that! I could run 32bpp if I wanted to. Remember, the higher you set your resolution and the greater your bpp, the more information (pixels and colors) your computer must keep track of. This results in something of a performance hit. Depending on how fast your computer and graphics card are, it may be unnoticeable.

Just in case you're interested, here's a conversion for bpp vs colors.

1bpp = 2 colors (MDA)
2bpp = 4 colors (CGA)
4bpp = 16 Colors (EGA)
8bpp = 256 colors (VGA)
16bpp = 65,536 colors
24bpp = 16,777,216 colors
32bpp = 4,294,967,296 colors


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