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By Root 1667 0

Using these character definitions, you can fit 25 lines of 40 characters each on the 320 x 200 video display, which (for example) is enough to fit an entire short poem by Amy Lowell:

A video display adapter must contain some RAM to store the contents of the display, and the microprocessor must be able to write data into this RAM to change the display's appearance. Most conveniently, this RAM is part of the microprocessor's normal memory space. How much RAM is required for a display adapter like the one I'm describing?

This isn't a simple question! The possible answers can range from 1 kilobyte to 192 kilobytes!

Let's start with the low estimate. One way to reduce the memory requirements of a video display adapter is to restrict the adapter to text only. We've already established that we can display 25 rows of 40 characters each, or a total of 1000 characters. The RAM on the video board need only store the 7-bit ASCII codes of those 1000 characters. That's 1000 7-bit values, which is approximately 1024 bytes, or 1 kilobyte.

Such a video adapter board must also include a character generator that contains the pixel patterns of all the ASCII characters, such as I illustrated earlier. This character generator is generally read-only memory, or ROM (pronounced rahm). A ROM is an integrated circuit manufactured so that a particular address always results in a particular data output. Unlike RAM, a ROM doesn't have any data input signals.

You can think of ROM as a circuit that converts one code to another. A ROM that stores 8 x 8 pixel patterns of 128 ASCII characters could have 7 address signals (for the ASCII codes) and 64 data output signals. The ROM thus converts a 7-bit ASCII code to a 64-bit code that defines the character's appearance. But 64 data output signals would make the chip quite large! It's more convenient to have 10 address signals and 8 output signals. Seven of the address signals specify the particular ASCII character. (These 7 address bits come from the data output of the RAM on the video board.) The other 3 address signals indicate the row. For example, address bits 000 indicate the top row and 111 indicate the bottom row. The 8 output bits are the eight pixels of each row.

For example, suppose the ASCII code is 41h. That's a capital A. There are eight rows of 8 bits each. This table shows the 10-bit address (a space separates the ASCII code from the row code) and the data output signals for a capital A:

Address

Data Output

1000001 000

00110000

1000001 001

01111000

1000001 010

11001100

1000001 011

11001100

1000001 100

11111100

1000001 101

11001100

1000001 110

11001100

1000001 111

00000000

Do you see the A drawn with 1s against a background of 0s?

A video display adapter that displays text only must also have logic for a cursor. The cursor is the little underline that indicates where the next character you type on the keyboard will appear on the display. The character row and column position of the cursor is usually stored in two 8-bit registers on the video board that the microprocessor can write values into.

If the video adapter board is not restricted to text only, it's referred to as a graphics board. By writing into the RAM on a graphics video board, a microprocessor can draw pictures, including text in a multitude of sizes and styles. Graphics video boards require more memory than text-only boards. A graphics video board that displays 320 pixels across by 200 pixels down has 64,000 pixels. If each pixel corresponds to one bit of RAM, such a board requires 64,000 bits of RAM, or 8000 bytes. This, however, is the rock-bottom minimum. A correspondence of 1 bit to 1 pixel allows the use of only two colors—for instance, black and white. A 0 bit might correspond to a black pixel, and a 1 bit might correspond to a white pixel.

Black-and-white televisions display more than just black and white, of course. They're also capable of displaying many shades of gray. To display shades of gray from a graphics board, it's common for each pixel to correspond to an entire byte