Description
of
Basic Operations
In the alphanumeric mode the adapter fetches character and attribute
information from its display buffer. The starting address
of
the display
buffer
is
programmable through the 6845, but it must be an even
address. The character codes and attributes are then displayed
according to their relative position
in
the buffer.
DISPLAY
BUFFER
(EVEN)
Starting
Address
CHAR
CODE
'A'
(Example
of
a 40x25
screen)
ATTRIBUTE
A B
CHAR
CODE
'B'
ATTRIBUTE
X
Video
Screen
CHAR
CODE
'X'
Last
Address
1000
ATTRIBUTE
The
CPU
and the display control unit have equal access to the display
buffer during all the operating modes except high resolution alpha-
numeric. During this mode, the
CPU
should access the display buffer
during the vertical retrace time (if not, then the display will be affected
with random patterns as the
CPU
is
using the display buffer). The
characters are displayed from a prestored "character generator" which
contains the dot patterns
of
all the displayable characters.
In the graphics mode the displayed dots and colors are also fetched
from the display buffer (up
to
16K bytes). In the Color/Graphics Mode
Section, the bit configuration for each graphics mode is explained.
2-54
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