Modes
of
Operation
The mode control register determines
the
mode
of
operation. The
following
is
a list
of
availabe RAM
access
modes.
* Alphanumeric
Mode
80
x 25
40
x 25
* Graphics
Mode
640
x 1
320
x 2
Alphanumeric
Mode
When
the
HP45981A
is
set in
the
alphanumeric mode, words
(attribute byte
+ character byte) are fetched
from
the
memory
starting at
the
offset programmed into
the
6845
CRT
controller.
This address
is
a
word
pointer and will always be used
to
fetch
from
even byte boundaries. Words are fetched contiguously
from
memory.
For
an
80
x 25 display, 2000 words (8 pages) are
fetched.
For
a
40
x 25 display,
1000
words (16 pages) are fetched.
The character code
is
a byte
with
the
ASCII
code
for
the
character.
The attribute byte
is
defined
as:
07
06
05
04
03
~
02
01
DO
W-J
Foreground color
(R-G-B)
Foreground Intensity
Background color
(R-G-B)
Background intensity/Blink
If underline enable
is
set, an underline will be generated
for
any
character
with
a foreground color
of
001, and
the
color
of
the
character will be forced
to
111
. If underline enable
is
not
set
the
character will be displayed
as
blue
or
a gray level.
88 Multi-Mode Video Adapter
Card
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