ICS312 Set 17
Introduction to Video Programming
BIOS Level Video Control (INT 10H) –
Display Modes
• Monitors can display both text and graphics and have different techniques and
memory requirements for each. Consequently, video adapters have two display
modes: text and graphics.
• In text mode, the screen is divided into columns and rows, typically 80
columns by 25 rows, and a character is displayed at each screen position
(character cell).
• In graphics mode, the screen is again divided into columns and rows, and
each screen position is called a pixel (short for picture element). A picture can
be displayed by specifying the color of each pixel on the screen.
If one uses e.g. 64k bit color and Unidec
coding, it takes 4 bytes to represent a
single screen character.
For the sake of simplicity, we will consider
text examples using 16 bit color with ascii
coding
and of the many graphics mode we will
consider (an easy to use) one that employs
200 rows and 320 columns
BIOS Level Video Control (INT 10H) –
Text Modes
mode
3
Description
80 x 25
16 bit color text
BIOS Level Video Control (INT 10H) –
Graphics Modes
Mode(hex) Description
13
320 x 200
256 color
Video Display Addresses
B800:0000h
Standard display address
for text mode
Text Mode Programming
• Positions on the screen are referenced using
(column, row) coordinates
• The upper left corner has coordinates (0,0)
• For an 80 x 25 display, the rows are 0-24
and the columns are 0-79
Text Mode Programming (Cont.1)
Table of some 80 x 25 screen positions
Position
Decimal
Hexadecimal
Upper left corner
Lower left corner
Upper right corner
Lower right corner
Center screen
(0,0)
(0,24)
(79,0)
(79,24)
(39,12)
(0,0)
(0,18)
(4F,0)
(4F,18)
(27,C)
Text Mode Programming (Cont.2)
• The character displayed at a screen position is specified
by the contents of a WORD in the display memory. So
it requires 80*25 words to specify a page, ie 4K bytes.
• The low byte of the word contains the character's ASCII
code; the high byte contains its attribute
• Attribute tells how the character will be displayed (its
color, whether it's blinking, underlined, and so on)
Display Pages
• Graphics adapters can store several screens of text data
• To fully use the display memory, it is divided into display
pages
• One display page can hold the data for one screen
• The pages are numbered starting with 0; the number of
pages available depends on the adapter and the display
mode selected. We will only use page 0
The Attribute Byte
16 bit Color Display
Attribute Byte:
Bit#
7
6
Attr
BL
R
5
G
4
B
3
IN
2
R
1
G
0
B
Attributes:
Bit #
Attribute
0-2
3
4-6
7
character color (foreground color)
intensity
background color
blinking
The Attribute Byte (Cont.)
• To display a red character on a blue background, the attribute byte
would be:
0001 0100 = 14h
• If the attribute byte is: 0011 0101 = 35h
Uses blue + green (cyan) in the background and red+blue (magenta)
in the foreground, so the character displayed would be magenta on
a cyan background
• If the intensity bit (bit 3) is 1, the foreground color is lightened
(brightened)
• If the blinking bit (bit 7) is 1, the character turns on and off
Sixteen Color Text Display
Basic
I R G
0 0 0
0 0 0
0 0 1
0 0 1
0 1 0
0 1 0
0 1 1
0 1 1
Colors
B
Color
0
black
1
blue
0
green
1
cyan
0
red
1
magenta
0
brown
1
lt. grey
I
1
1
1
1
1
1
1
1
R
0
0
0
0
1
1
1
1
G
0
0
1
1
0
0
1
1
Bright Colors
B
Color
0
dark gray
1
light blue
0
light green
1
light cyan
0
light red
1
lt magenta
0
yellow
1
white
foreground colors can be either basic or bright
background colors can only be basic colors
The first 16 colors in graphics modes
that employ large numbers of colors
are the same as those on the
preceeding slide
A Demonstration
To display a character with a given
attribute at any screen position, store
the character and attribute at the
corresponding word in the active display
page. The following program fills the
color screen with red "A"s on a blue
background.
TITLE SCREEN DISPLAY_1
.MODEL SMALL
.STACK 100H
.CODE
A Demonstration (Cont.1)
MAIN PROC
; set ES to active display page
MOV AX, 0B800H
; color active display page
MOV ES, AX
MOV DI, 0
; initialize DI
MOV CX, 2000
; 80x25 = 2000 words
CLD
; forward dir. for string fns
; fill active display page
MOV AH, 14H
; red color on blue background
MOV AL, 'A'
REP STOSW
; STOSW is one of the string-handling functions.
; It writes the word in AX repetitively CX times at the
; location specified by ES:DI
A Demonstration (Cont.2)
MAIN
MOV AH, 4CH
INT 21H
ENDP
END MAIN
After the program is run, the screen positions
retain these attributes (in DOS mode) unless
another program changes it, or the computer is
reset (or the window is closed).
INT 10h Video Functions – 00h
00h: Set Video Mode. Selects the video mode
and clears the screen automatically. Two
examples of video mode:
Input:
AH = 0
AL = 3; mode 3, giving 80 cols x 25 rows color text
INT 10h Video Functions – 02h
02h: Set Cursor Position.
to specified position.
Input:
AH
DH
DL
BH
=
=
=
=
Move cursor
2
row (0-24)
col (0-79 for 80x25 display)
video page number (usually 0)
INT 10h Video Functions – 06h
Scroll the Screen or a Window Up
Input:
AH = 6
AL = number of lines to scroll
(0 => whole screen)
BH = attribute for blank lines
CL, CH = column,row for upper left corner
DL, DH = column,row for lower right window
INT 10h Video Functions –
06h(Cont.1)
•Scrolling the screen up one line means moving each
display line UP one row and inserting a blank line at
the bottom of the screen
•The previous top row disappears from the screen
•The whole screen or any rectangular area (window)
may be scrolled
•AL contains the number of lines to scroll
•If AL = 0, all the lines are scrolled and this clears
the screen or window
INT 10h Video Functions – 06h
(Cont.2)
Example: Clear the screen to blank for
the 80x25 display.
MOV AH, 6
; scroll up function
MOV AL, 0
; clear entire screen
MOV CX, 0
; upper left corner is (0,0)
MOV DL, 4FH ; lower rt. corner is (4Fh, 18H) ie (79,24)
MOV DH, 18H
MOV BH, 7
; normal video attribute, a white
; foreground on a black background
INT 10H
INT 10h Video Functions – 09h
•Write character and attribute. Display any
ASCII character at current position and set
desired attribute. Can be used in graphics
or video mode.
Input:
AH = 09
AL = ASCII character code
BH = video page number (usually 0)
BL = attribute to be used.
CX = repetition count
INT 10h Video Functions – 09h
(Cont.1)
• Cursor does not move
• If AL contains the ASCII code of a control
character, a control function is not
performed
--- a display symbol is shown instead
INT 10h Video Functions – 0Ah
0Ah:Write character. Display any ASCII character
at current position without changing the current
attribute. Can be used in text or video mode.
Input:
AH = 0Ah
AL = ASCII character code
BH = video page number (usually 0)
CX = repetition count
SUMMARY
fn
set cursor 02 dl,dh=col,row bh=0
scroll up
06 cl,ch
bh=attrib al= amount
dl,dh
write char 09 al = char
& attrib.
bh=0
cx=count
bl=attrib
write char 0A al = char
bh=0
cx=count
A Comprehensive Example
A demonstration of several of the INT 10h
functions. This program:
1. Sets the display mode to 3 (80x25 16 bit color text)
2. Clears a window with upper left corner at
column 26,row 8 (1Ah, 08h), and lower right corner at
column 52, row 16 (34h, 10h) to cyan on red.
3. Moves the cursor to column 39, row 12 (0Ch, 27h).
4. Prints a blinking, cyan "A" at that cursor position.
A Comprehensive Example (Cont.1)
TITLE SCREEN DISPLAY_2
; red screen with blinking cyan 'A' in middle of screen
.MODEL SMALL
.STACK 100H
.CODE
MAIN PROC
; set video mode
MOV AH, 0
; select mode function
MOV AL, 3
; 80x25 color text
INT 10H
A Comprehensive Example (Cont.2)
; clear window to red
MOV AH, 6
; scroll up function
MOV CX, 081AH ; upper left corner (1Ah, 08h)
MOV DX, 1034H ; lower right corner (34h, 10h)
MOV BH, 43H
; cyan chars on red background
MOV AL, 0
; scroll all lines
INT 10H
; move cursor
MOV AH, 2
; move cursor function
MOV DX, 0C27H ; center of screen (27H, 0Ch)
MOV BH, 0
; page 0
INT 10H
A Comprehensive Example (Cont.3)
; display character with attribute
MOV AH, 09
; display character function
MOV BH, 0
; page 0
MOV BL, 0C3H
; blinking cyan char (code 03H),
; on a red background (code 0cH)
MOV CX, 1
; display one character
MOV AL, 'A'
; character is 'A'
INT 10H
; dos exit
MOV AH, 4CH
INT 21H
MAIN ENDP
END MAIN