CHAPTER 9

VIDEO SYSTEMS

This covers the following services offered by BIOS INT 10H:

The common (or once common) video adapters include MDA (monochrome display adapter), CGA (color graphics adapter), EGA (enhanced graphics adapter), and VGA (video graphics array). The VGA and its SVGA successors replace the CGA and EGA video adapters.

The basic components of a video system are monitor, Video Display Area, video BIOS, and Video Controller.

Monitor:

The monitor's screen consists of a group of closely-spaced horizontal lines know as the raster. Each line contains hundreds of points called pixels (picture elements) which consist of three consists of three luminescent phosphors dots for each of the primary additive colors red, green, and blue.

Video Display Area:

Data is sent to the video display area in RAM from a program by either an INT operation or by directly transferring it into the area. The following are the beginning segment addresses for major video modes:

•A000: [0] Used for font descriptor when in text mode and for high-resolution graphics for video modes 0DH through 13H.

•B000:[0-] Monochrome text for mode 07H.

•B800:[0] Text and graphics for modes 00H and 06H.

The video circuitry continuously scan the data in the Video Area and refreshes the screen accordingly.

This data can be either text where each character is represented by two bytes one for the character and one for its attribute or it can be graphics data which determines the color of the pixel.

The Video Display Area allows for data to be stored in pages. In text mode there can be 8 pages numbered page 0 through page 7 with page 0 being the default display page. Since each character in text mode requires two bytes one page requires 4K bytes of memory (80X25X2).

Video Controller:

The video controller does the following:

•Generates horizontal and vertical timing signals

•Maintains and increments a counter that indicates the current location in the Video Display Area

•Handles the size and location of the cursor and the selects the page to be displayed

Video BIOS:

The video BIOS acts as an interface to the video adapter. It contains routines for setting the cursor position and size and displaying characters and pixels. The Video RAM BIOS supports two Video Data Areas:

1. 40: [49H] contains such data as current mode, number of columns, and size of the Video Display area.

2. 40:[84H] contains such data as number of rows and character height.

VIDEO MODES

The video mode determines such factors as text or graphics, color or monochrome, screen resolution, and the number of colors.

BIOS INT 10H is used to initialize the mode, for the currently executing program or to switch between text and graphics. Setting the mode also clears the screen.

The following are some of the available text modes:

ATTRIBUTES

The attribute byte in text mode determines the characteristics of each displayed character.

The attribute byte has the following format:

The background can display one of eight colors and the foreground characters can display one of 16 colors. Blinking and intensity apply only to the foreground, although you can us INT 10H function 10H to override the blinking feature and enable the foreground to display 16 colors.

BIOS INT 10H OPERATIONS

INT 10H Function 00H: Set Video Mode

Sets the video mode and also clears the screen (this can be overridden).

AH contains 00H and AL contains the color

INT 10H Function 01H: Set Cursor Size

Sets the cursor size on the Video Controller.

set AH = 01H

set CH (bits 4-0) = top of cursor (start scan line)

set CL (bits 4-0) = bottom of cursor (end scan line)

INT 10H Function 02H: Set Cursor Position

Set the cursor anywhere on the screen whether in graphics or text mode.

set AH = 02H

set BH = page number (0 is the default)

set DH = row and DL = column

INT 10H Function 03H: Return Cursor Status

set AH = 03H

set BH = page number

AX and BX are unchanged and returns these values:

CH = Starting scan line DH = Row

CL = Ending scan line DL = Column

INT 10H Function 05H: Select Active Page

set AH = 05H

set AL = page number

INT 10H Function 06H: Scroll Up Screen

set AH = 06H

set AL = number of rows (00H for full screen)

BH = attribute or pixel value

CX = starting row:column

DX = ending row:column

The operation returns no values: Here's a standard approach for scrolling up on line:

1. Initialize ROW variable to 0

2. Display a line and advance the cursor to the next line.

3. Text whether ROW is near the bottom of the screen (CMP ROW,22)

4. If yes, scroll one line, use ROW to set the cursor, and clear ROW to 00H.

5. If no, increment ROW (INC ROW).

INT 10H Function 07H: Scroll Down Screen

set AH = 07H

set AL = Number of rows (00H for full screen)

set BH = Attribute or pixel value

set CX = Starting row:column

set DX = Ending row:column

INT 10H Function 08H: Read Character and Attribute at Cursor

set AH = 08H

set BH = page number

In text mode, AL contains the character and AH its attribute.

In graphics mode, 00H for a non-ASCII character.

INT 10H Function 09H: Display Character and Attribute at Cursor

set AH = 09H

set AL = ASCII character

set BH = Page number

set BL = Attribute or pixel value

set CX = count (how many times to repeat character in AL)

This operation does not advance the cursor or respond to the Bell, Carriage Return, Line Feed, or Tab Characters: instead, it attempts to display them as ASCII characters. In text mode, when the display exceeds the rightmost column, the operation automatically continues the display on the next row at column 0. For graphics mode, use BL for defining the foreground color. If bit 7 is 0, the defined color replaces present pixel color: if bit 7 is 1, the defined color is combined (XORed) with them.

INT 10H Function 0AH: Display Character at Cursor

Same as 09H, except instead of setting the attribute it uses the current value.

INT 10H Function 0BH: Set Color Palette

set AH = 0BH

set BH = 00H : Select the background color, where BL contains the color value in bits 0-3

(any of 16 colors)

set BH = 01H: Select the palette for graphics, where BL contains the palettel (0 or 1)

INT 10H Function 0CH: Write Pixel Dot

set AH = 0CH

AL = color of pixel

BH = page number

CX = column

DX = row

Maximum row or column size depends upon the graphics resolution.

INT 10H Function 0DH: Read Pixel Dot

set AH = 0DH

set BH = page number

set CX = column

set DX = row

AL contains the returned pixel color

INT 10H Function 0EH: Display in Teletype Mode

INT 10H Function 0FH: Get Current Video Mode

set AH = 0FH

AL = current video mode

BH = active video page

Figure 9-2 (See Text)

INT 10H Function 10H: Access Palette Registers and Video DAC

set AH = 10H

Subfunction 00H:

Update Palette Register (changes the color displayed by any of 16

Palette registers)

set BH = color value

set BL = 00 - 0FH (palette register number)

Subfunction 01H:

Update Border Color

set BH = new border color

Subfunction 03H:

Select Background Intensity (enable or disable blinking attribute)

set BL = 00H (disable) or 01H (enable)

Subfunction 07H:

Read Palette Register ( returns the color code stored in any of the 16

Palette registers)

BH = returned color code

Subfunction 08H:

Read Overscan Register (returns the color code currently in the

Overscan (border) register)

BH = returned color code

Subfunction 09H: Read Table of Palette Register Values

Subfunction 10H: Update DAC Color Register

Subfunction 12H: Update Block of DAC register

Subfunction 15H: Read Video DAC Color Register

Subfunction 17H: Read Block of DAC Registers

Subfunction 1BH: Perform gRay Scaling on DAC Registers

INT 10H Function 11H: Access Character Generator

INT 10H Function 12H: Select Alternative Video Routine

INT 10H Function 13H: Display Character String

INT 10H Function 1AH: Video Display Combination

INT 10H Function 1BH: Return Video BIOS Information

INT 10H Function 1CH: Save or Restore Video State