Chapter 11 Graphics
- Table of Contents
- 11.1 Text Mode
- 11.2 VGA Mode 13h Graphics
- 11.3 Interrupt 10h Video Reference
- 11.4 PCX Graphic File Format
This chapter describes the various common graphics modes and how to use them in assembly language, as well as how to implement various algorithms for drawing lines and other images on the graphics screen.
11.1 Text Mode
11.1.1 Displaying Text on the Screen
11.1.1.1 INT 10h Functions
One way to display text on the screen quickly is to use the BIOS interrupt 10h functions. See the INT 10h function list elsewhere for a complete description of these functions. A brief list of the more useful functions is given here:
11.1.1.2 Library Routines
In addition to these interrupts, the following subroutines for displaying characters are available through the LIB291 library file:
- dspout
-
This subroutine prints the character found in DL to the screen, at the current cursor position. The character must be in ASCII. The contents of DL are preserved upon return. The cursor is advanced after the write.
- dspmsg
-
This subroutine will print to the screen, starting at the current cursor position, a string of ASCII characters. DX must contain an offset from DS to the location of the first character to be printed. In addition, the string must end with an ASCII dollar sign ($), and hence may not contain one. (The dollar sign is the text delimiter). The contents of DX are preserved upon return.
11.1.1.3 Direct Writes to Video Memory
A third method for displaying text involves accessing the video memory directly. The contents of the text screen are stored in memory beginning at address B8000h. Each character on the text display comprises one word in memory, the meaning of which is shown in the following figure:
The attribute byte gives information about the color of that particular character on the screen (discussed in the following section). The character byte is simply the 8-bit ASCII code for the character at that position.
The screen is divided into rows and columns, with the upper-left character position usually referred to as row 0 and column 0. The first row of the screen is stored first beginning with the first column (row 0, column 0), then the next row, and so on. Thus, an 80x25 text screen requires
to store the entire screen.
The following code fragment illustrates an example of how to access the video memory. This code fragment will change the top row of characters to yellow "A"s on a blue background. Note the use of segment register ES to access the memory at absolute address B8000h.
mov bx, 0 ; Begin pointer at row 0 and column 0
mov ax, 0B800h ; Set up ES to point to video segment
mov es, ax
mov cx, 80 ; Counter for number of columns in top row
.lp:
mov [es:bx], 1E41h ; Yellow 'A' on blue background
add bx, 2 ; Update pointer to next character position
loop .lp ; Do 80 characters
11.1.2 Attributes
11.1.2.1 The Attribute Byte
The attribute byte specifies the colors for the character and its background. The table below describes the format for the attribute byte. You can write a character and its attribute to the display using Interrupt 10h, Function 09h (see Section 11.3.8), or by writing directly to video memory. The structure of the attribute is shown in the following figure:
11.1.2.2 Color Palette
| 0 -- Black | 8 -- Dark Gray |
| 1 -- Blue | 9 -- Light Blue |
| 2 -- Green | 10 -- Light Green |
| 3 -- Cyan | 11 -- Light Cyan |
| 4 -- Red | 12 -- Light Red |
| 5 -- Magenta | 13 -- Light Magenta |
| 6 -- Brown | 14 -- Yellow |
| 7 -- Light Gray | 15 -- White |
After power up the attributes for the entire screen are set to (00000111b), the attribute for a light gray character and a black background, with no blinking.