Programming Considerations
Programming the 6845 CRT Controller
The following table summarizes the 6845 Internal
Data
Registers and
their functions and parameters.
For
the IBM Monochrome Display,
r--.., the values in the table must be programmed into the 6845 to insure
proper initialization of the device.
Table
2.
6845
INITIALIZATION
PARAMETERS
REGISTER
REGISTER
PROGRAM
80x25
#
FILE
UNIT
MONOCHROME
RO
HORIZONTAL
TOTAL
CHARACTERS
61H
Rl
HORIZONTAL
OISPLAYED
CHARACTERS
50H
R2
HSYNC
POSITION
CHARACTERS
52H
R3
HSYNC
WIDTH
CHARACTERS
FH
R4
VERTICAL
TOTAL
CHAR
ROWS
19H
R5
VTOTAL
ADJUST
SCAN
LINE
6H
R6
VERTICAL
DISPLAYED
CHAR
ROW
19H
R7
VSYNC
POSITION
CHAR
ROW
19H
RS
INTERLACE
MODE
---
02
R9
MAX
SCAN
LINE
ADDRESS
SCAN
LINE
DH
R10
CURSOR
START
SCAN
LINE
BH
Rll
CURSOR
END
SCAN
LINE
CH
R12
START
ADDRESS
(H)
---
DOH
R13
START
ADDRESS
(L)
---
DOH
R14
CURSOR
(H)
---
DOH
R15
CURSOR
(L)
---
DOH
R16
RESERVED
---
---
R17
RESERVED
---
---
Sequence
of
Events
The first command issued to this attachment must be to output to
PORT
3B8, hex 01, to set high resolution mode.
If
the high resolution
mode
is
not set, an infinite
CPU
wait-state will occur!
Memory Requirements
The attachment has
4K
bytes
of
memory which
is
used for the display
buffer. The memory supports one screen
of
25
rows
of
80 characters,
plus a character attribute for each display character.
No
parity
is
provided on the memory.
No
system Read/Write memory is required
for the monochrome adapter portion. The display buffer starts
at
address
'BOOOO'.
2-41
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