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Controller PCB
3-4
3.2
CRT DISPLAY CONTROL
This adapter connects the microprocessor with a 12-inch monochrome or color
display. At the heart of this computer-graphics system is the ,uPD7220 Graphic
Display Controller (GDC), an intelligent LSI microprocessor that carries out the
high-speed and repetitive duties required to generate the raster display and manage
the display memory. The GDC duties include
• Generating the basic video raster timing (including sync and blanking
signals)
• Video-display-memory modification and data moves
• Calculating display-memory addresses.
Some characteristics of the CRT-control design are
• Display buffer is independent of system memory
• An 80-character by 25-line screen (2000 characters)
• A 26th line reserved for system status information
• Direct drive output
• An 8-dot by 19-dot character box
• A 7 -dot by II-dot character
• 8-dot by 16-dot special programmable characters.
A character generator supplies the video process logic with the information neces-
sary for displaying the characters. Additionally, a special-character generator
contains the fonts for user-programmable characters; these are 8 by 16 characters in
8 by 19 character boxes.
In the display-control adapter is a character-code buffer memory that stores charac-
ter codes or special-character codes (each character has 1 of 256 codes in RAM or
250 codes in ROM, of which 6 in ROM are not assigned) and an attribute-code
buffer memory that stores character attributes (each character is associated with
one or more of eight attributes). Figure 3-3 shows the functional relationships
between the principal components of CRT control.
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