ee161aba@sdcc18.ucsd.EDU (David L. Smith) (01/09/87)
Several people have been looking for these parameters, so here are the
parameters used on my hercules clone:
Graphics pages
B0000-B7FFFh page 1
B8000-BFFFFh page 2
I/O ports
03B4 - 6845 Index Register
03B5 - 6845 Data Register
03B8 - Display Mode Control Port
03BA - Display Status Port
6845 Registers
To access these, load the 6845 Index Register with the register number you wish
to access and then do something to the 6845 Data Register
0 - Total chars per row including SYNC less 1
1 - Number of visible chars per row
2 - Position of the first character during SYNC, less 1
3 - Number of characters during SYNC per row, less 1
4 - Number of rows less 1 including the row during vertical retrace
5 - Number of scans (row fractions) in addition to total number of rows
6 - Number of visible rows
7 - Row number to begin the retrace less 1. (This will always last 16 scans)
8 - Always set to 2 (magic cookie)
9 - Number of scans per row, less 1
10 - First scan where the cursor will overlay a character
11 - last scan where the cursor will overlay a character
12 - Always set to 0
13 - Always set to 0
14 - Offset of the cursor position in the display buffer high
15 - Offset low
16 - Offset when light pen tripped high
17 - Offset low
(I have no better explanations for the 6845 registers, either find the chip
data or just follow the "standard settings" given further on)
Display Control Port (03B8)
Bit Option
0 Unused
1 0 = text, 1= graphics (*BE SURE TO SET 6845 PARAMETERS when changing)
2 Unused
3 0 = Blank the screen, 1 turns it on
4 Unused
5 0 = No blinking, 1 = blinking on
6 Unused
7 0 = Use page 0, 1 = page 1
Display Status Port
Bit Condition
0 0 = Normal character, 1=SYNC (screen is temporarily blanked)
1,2 Unused
3 0 = dots off, 1= dots on
4,5,6 Unused
7 0 = Vertical retrace (screen is temporarily blanked) 0=active display
Configuration Switch (03BF) (Some cards may not have this)
Bit Option
0 0 = inhibit graphics mode, 1 = allow graphics mode to be set
1 0 = Mask page 1 (B8000-BFFFF) out of memory map and inhibit it's
selection. 1 = Put back in memory map and allow selection
2-7 Unused
The text page occupies 4K, from B0000 - B0FFF
To place a character, use the formula
character offset = 160*(line-1)+2*(column-1)
attribute offset = 160*(line-1)+2*(column-1)+1
(1<=line<=25) (1<column<80)
Attributes are set as follows:
Bit 7 is the blink bit, bit 3 is the bold bit
B000I000 - Don't display
B000I001 - Underline
B000I111 - normal
B111I000 - reverse video
Graphics
To place a dot in the graphics screen, use
Byte offset = 2000h * (y mod 4) + 90 * int(y/4) + int (x/8)
Bit position in byte is 7 - (x mod 8)
(0 < x < 719) (0 < y < 347)
Standard settings for 6845 register 0 - 11
Text
61, 50, 52, 0f, 19, 06, 19, 19, 02, 0d, 0b, 0c
Graphics
35, 2d, 2e, 07, 5b, 02, 57, 57, 02, 03, 00 ,00
To flip modes, it is suggested to blank the screen, set the mode, set the
parameters, turn the screen back on.
--
Cheers,
David L. Smith
{ucbvax, ihnp4}!sdcsvax!sdcc18!ee161aba
"Consistency is the hobgoblin of little minds"