Graphic displays
wpc

Introduction
In this lecture I will be covering the topics of
CRT displays
Text mode displays
Graphics displays
Colour map tables

Overview
Computers currently rely mainly on Cathode Ray Tube displays and drive these with display cards that can operate either in character mapped or bit mapped modes.
We will look at the display characeristics and at the display card characteristics

CRT
Physical Characteristics
How it works
Scan control flyback and synchronisation
Mono and colour displays
Connector details

Physical details

Scanning

Colour display

Line and frame scan

Control inputs to monitor

Text Mode Displays
Most basic type of computer display
Oriented to displaying only character data
One character in buffer memory per character on the screen.

Text display

Use of the RAMs
RAM A has a 80 by 25 array of bytes corresponding to the text array. One to one mapp from these to screen characters
RAM B has a 256 by 16 array of bytes encoding the appearance of the characters
Both are dual ported, accessible by CPU or display hardware

Contents of RAM B

Ram addresses
RAM A
5 bits of vertical address from control logic
7 bits of horizontal address, from control logic
RAM B
8 bit of character code from RAM A
4 bit of vertical address from control logic

Counters
Row counter
9 bits long
Bottom 4 bits encode row within character
Top 5 bits encode which character row
Column counter
10 bits long
Bottom 3 bits encode column within character
Top 7 bits encode character column within row.

Advantages
Uses little memory no longer important
Easy to program for text I/O
Fast to update
Disadvantages
Can not handle variable width fonts
Can not readily handle graphics

Graphics display (VGA)

Colour map display controller

Colour Plane display controller

Adjacent colour displays controller

Summary
Images depend on raster scan displays
Display controllers use 2d array of elements to generate image
These are either pixels or character cells

Where to Get More Information
Other training sessions
List books, articles, electronic sources
Consulting services, other sources