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86 BIS
ATTRIBITE
f-------'
ME\IOR\
REFRESH
1 - - - - '
MEMOR\
(,G 81'S
('O'<TROL
('IR('\IT
Figure 3-3 CRT Display Control Block Diagram
3.2.1
Display Buffer Memory
ATTRIBITf:
REGISTER
ROM
('G
16 BIT
SillFT
REGISTER
There are 2000 positions on the display screen, and each position is associated with
3 bytes (24 bits) of display buffer memory. Two of the three bytes of data indicate
the character code or special-character code for an individual display position. A' l'
in the character-set bit (Bit 7) indicates that the character comes from the special (or
user programmed) character set (see Figure 3-4).
15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
0
0
I
0
I
0
I
0
I
0
0
I
0
I
I..
y
)
L..
CHARACTER-SET BIT
CHARACTER CODE
15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
11
I
0
I
0
I
0
I
0
I
0
0
I
0
I
\
)
L.CHARACTER-SET BIT
y
SPECIAL-CHARACTER CODE
Figure 3-4 Character-Code Representation in the Character Code Buffer Memory
Controller PCB
3-5
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