(3) Mapping RAM
Outline
* The mapping
RAM
is
used
to
specify
the
location
and
area occupied by each window on
the
display screen.
* The mapping
RAM
exists
on
the
GDC's memory map,
as
shown
in
the
above figure (can
not
accessed from
the
CPU).
* The mapping
RAM
is
used in correlation with
the
display screen,
as
illustrated below (for
640
x
400
dot
display):
bi t 0 1 2 3 4 5 6 7
30000H
3007F
3008
OH
Not
used.
Column
Map
Row
Map
lO24Worns
128Words
Fig.
71
Column
length
bi
t
AdO
I.
Window A
·1
399
1023
Mapping RAM access
CRT
1
0 0
1
0 0
2
0
3
0
I ,
, I
, ,
-,"
,~
.,.'
---
0
1
0
0
I
,
I
I
,
,
/
'"
1 0
/
/
,
/
0
,
1
I
0
I
0 1 0
0
0 I 0
0 0
0 0 1
I 1
I I
I I I
I I I
I I I
/ / I
/ I /
/ / /
'"
",I'
/ /
".
/ "
" / /
,
,/
".
""
.....
."..,./
/~~
,-
/
'"
,-
,-
..-
,/
,-
,-
/
,/
/
/
/
/
0
0
0
0
0
1
0
0
0
Column
Map
RAM
The
four
corners
of
each
window
are specified.
!-"""""""I.....c....-'-.....,..:;....,::;~
______
L
__
---
bi
t AdO
4
5
6
7
0
0
0
0
o 1 0
0
o 0
1
0
1 0
0
1
, I
I
0
o 0 1 0
0
1 0 0
0
1
0
1
00'
,
0
'
,
1
Row
length
WindowB
Fig.
72
* Mapping information
is
divided
into
horizontal
and
vertical
components
when
written
into
the
mapping
RAM,
as
shown above.
*
As
each window
is
located using 4 bits, no more
than
16
bits are ever set
at
anyone
time.
Configuration
of
WOC
and
mapping RAM
I-o--'--r-I
ADO
(Al
~
AD
15
Data
Mappirg
WDC
Address
RAM
2XCCLK
GOC
BLANK
RAS
Mapping
Data
RAM
Address
Write
Fig. 73
-60-
127
0
0
Row
Map
RAM
0
0
* Initial information
is
written into
the
mapping
RAM
by
using
the
GDC's drawing feature.
The
Fig, 73 provides
the
WDC, mapping RAM, and
GDC configuration. The
CS
of
the
mapping
RAM
is
always
low,
and
is
read when windows are displayed. When writing
data
into
the
mapping RAM,
the
WDC
controls
the
buffer
(A)
and
RAM
write signal
to
let
the
GDC write
the
neces-
sary data.
Fig.
70
shows
the
WDC
block diagram. The
WDC
latches
address information (for displaying, drawing and refreshing)
transferred from
the
GDC,
and
modifies it according
to
the
contents
of
the
VMA
or
mapping RAM, if needed.
The
WDC
knows
the
locations of windows from bit inform-
ation
read
out
of
the
mapping RAM, latches
the
display
address transferred from
the
GDC, adds VMA
to
the
latched display address, and uses
the
resulting address as the
actual display address
on
the
screen. However, when
the
GDC
is
blanking (for drawing
or
refreshing)
or
if
the
priority register
value
is
zero,
the
WDC
directly
outputs
the
display address which has been transferred from
the
GDC.
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