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By Root 1386 0

is to appreciate the beginnings and evolution of video. Video output to computers was around long before PCs were created. At the time PCs became popular, video was almost exclusively text-based, meaning that the only image the video card could place on the monitor was one of the 256 ASCII characters. These characters were made up of patterns of pixels that were stored in the system BIOS. When a program wanted to make a character, it talked to DOS or to the BIOS, which stored the image of that character in the video memory. The character then appeared on the screen.

The beauty of text video cards was that they were simple to use and cheap to make. The simplicity was based on the fact that only 256 characters existed, and no color choices were available—just monochrome text.

You could, however, choose to make the character bright, dim, normal, underlined, or blinking. Positioning the characters was easy, as space on the screen allowed for only 80 characters per row and 24 rows of characters.

Long ago, RAM was very expensive, so video card makers were interested in using the absolute least amount of RAM possible. Making a monochrome text video card was a great way to keep down RAM costs. Let’s consider this for a minute. First, the video RAM is where the contents of the screen are located. You need enough video RAM to hold all of the necessary information for a completely full screen. Each ASCII character needs eight bits (by definition), so a monitor with 80 characters/row and 24 rows will need

80 characters × 24 rows = 1920 characters = 15,360 bits or 1920 bytes

The video card would need less than 2000 bytes of memory, which isn’t much, not even in 1981 when the PC first came out. Now, be warned that I’m glossing over a few things—where you store the information about underlines, blinking, and so on. The bottom line is that the tiny amount of necessary RAM kept monochrome text video cards cheap.

Very early on in the life of PCs, a new type of video, called a graphics video card, was invented. It was quite similar to a text card. The text card, however, was limited to the 256 ASCII characters, whereas a graphics video card enabled programs to turn any pixel on the screen on or off. It was still monochrome, but programs could access any individual pixel, enabling much more creative control of the screen. Of course, it took more video RAM. The first graphics cards ran at 320 × 200 pixels. One bit was needed for each pixel (on or off), so

320 × 200 = 64,000 bits or 8000 bytes

That’s a lot more RAM than was needed for text, but it was still a pretty low amount of RAM—even in the old days. As resolutions increased, however, the amount of video RAM needed to store this information also increased.

After monochrome video was invented, moving into color for both text and graphics video cards was a relatively easy step. The only question was how to store color information for each character (text cards) or pixel (graphics cards). This was easy—just set aside a few more bits for each pixel or character. So now the question becomes, “How many bits do you set aside?” Well, that depends on how many colors you want. Basically, the number of colors determines the number of bits. For example, if you want four colors, you need 2 bits (2 bits per pixel). Then, you could do something like this

So if you set aside 2 bits, you could get four colors. If you want 16 colors, set aside 4 bits, which would make 16 different combinations. Nobody ever invented a text mode that used more than 16 colors, so let’s start thinking in terms of only graphics mode and bits per pixels. To get 256 colors, each pixel would have to be represented with 8 bits. In PCs, the number of colors—called the color depth—is always a power of 2: 4, 16, 256, 64 K, and so on. Note that as more colors are added, more video RAM is needed to store the information. Here are the most common color depths and the number of bits necessary to store the color information per pixel:

2 colors = 1 bit (mono)

4 colors = 2 bits

16 colors = 4 bits

256 colors = 8 bits

64 K colors