The document describes how a raster scan display system works with a video controller. The video controller retrieves intensity values from a frame buffer area of memory and displays them on the screen line by line at a refresh rate of 50 times per second. It uses registers to store pixel coordinates and accesses the frame buffer to display the pixels. For color displays, it uses a lookup table to store RGB values and only needs to access the table index from the frame buffer for each pixel.

1. RASTER SCAN
DISPLAY SYSTEM
WITH
VIDEO CONTOLLER
1

2. 2

3. • In raster scan displays a special area of
memory is dedicated to Graphics only.
This memory area is called Frame Buffer.
• It holds the set of intensity values for all
the screen points.
• The video controller retrieves the stored
intensity values from frame buffer and
displays them on the screen one row (scan
line) at a time , typically 50 times per
second.
3

4. Raster display system
Architecture
4

5. Raster Scan Systems
• In addition to the central processing unit
(CPU), a special processor, called the video
controller or display controller, is used
to control the operation of the display device.

6. Video Controller
• A fixed area of the system memory is reserved
for the frame buffer, and the video controller
is given direct access to the frame buffer
memory.

7. Video Controller
• Frame buffer location, and the
corresponding screen positions, are
referenced in Cartesian coordinates.
x
y

8. Video Controller
• Scan lines are then labeled from ymax at
the top of the screen to 0 at the bottom.
Along each scan line, screen pixel
positions are labeled from 0 to xmax.
Line Scan
y max
x max x
y Line Scan
y max
x max

9. Simple Organization of the Video Controller
• Two registers are used to store the coordinates
of the screen pixels.
Raster Scan Generator
x Register y Register
Memory Address
Frame Buffer Intensity
The Basic
refresh
operation of
the video
controller.

10. • In color displays, 24 bits per pixel are commonly used,
where 8 bits represent 256 levels for each color.
• It is necessary to read 24-bits for each pixel from frame
buffer. This is very time consuming.
• To avoid this video controller uses Look Up Table (LUT)
to store many entries of pixel values in RGB format.
• With this facility , now it is necessary to only read index
to the Look Up Table from the frame buffer for each
pixel.
• The specified entry in the Look Up Table is then used to
control the intensity or color of the CRT.
10

11. 11
Color Map Look-Up Tables
Extends the number of colors that can be displayed by a given number of bit-planes.
0
1
0
0
0
0
1
1
67
100110100001
0
67
255
1001 1010 0001
R G B
RED
GREEN
BLUE
Pixel displayed
at x', y'
Pixel in
bit map
at x', y'
0 x
0
y
x
max
max
y
Bit map Look-up table Display
Video look-up table organization: each table entry is a 12 bit per entry.
A pixel with value 67 is displayed on the screen with the red electron gun
at 9/15 (binary 1001) of maximum, green at 10/15, and the blue is 1/15.

12. Raster Scan Display
System
with
Display Controller

13. Raster Scan Display
Controller/Processor
• A raster system containing a separate display
processor (graphics controller, display coprocessor)
• The purpose of the DP is to free the CPU from the
graphics chores.

14. Display processor
• Generation various line styles
(dashed, dotted, or solid)
• Displaying color areas
• Performing certain transformation
and manipulation on display
objects.

15. 15

16. Random Scan Systems

17. Random Scan System
• Graphic commands are translated by the
graphics package into a display file stored in
the system memory.
• This file is then accessed by the display
processor unit (DPU)(graphic controller) to
refresh the screen.