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166
Upon
system reset, data is written into video RAM starting at
address SDOOO, and when more than 50 lines of data
are
written
that
is,
when
data
has been
written into
the area from SD000
through
SD7CF
-
SD028
through
SD7F7
become the actual video RAM
area.
When
more
data is written and
one
line scrolled, the
area from
SD050-SD7FF
becomes the video RAM area, followed by the
area from SDO0O-SD020.
Thus, the
video
RAM
is used in an anchored
configuration. Figure 3.4 shows the relationship between video
RAM
and the display for the
2 K
byte
video RAM area
when its first K
byte, its middle
K
byte,
or
its
last K
byte is displayed
on
the CRT
screen.
However, in this example, the
actual video RAM
area
capable of displaying data on the CRT
screen
is the
2000
bytes starting at $D1
EO.
SDOOO
SD1E0
SD7FF
Actual display area
SDOOO
.
Lower
right
of the
SD1AF
SD1E0
—
Upper left
of the
screen
SD7FF
Actual display
area
Lower
right
of
the
screen
Upper left
of the
screen
Condition
when the
screen
is rolled
Intermediate
state.
down all the way.
Condition when the
screen
is rolled
up all the way.
Figure
3.4
Relationship between video RAM and the area actually displayed.
Memory
mapped
I/O
address
SE200-SE2FF
are used for
rolling the
display up and down.
When
a memory read
instruction is executed against
address SE200 (such as LD
A,
(E200H)), the CRT
display is rolled all the way down.
When such an instruction is executed against address SE2FF, the
CRT display
is rolled all the way up. The lower bytes
of these addresses from OOH-FFH can be freely used
to roll the
screen
up
or
down in 8 -character units.
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