IBM 5150 PC Technical Reference (6025005, August, 1981) (PDF)

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. Ifthe 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