bradley@druxq.UUCP (DavidsonBC) (08/06/85)
This is a different way to control the monitor to remove the fuzz or
snow. The snow is caused by the video chip and the CPU trying to
access the same section of memory at the same time, basically your
trying to write to the video buffer as the video chip is trying to
read memory. The snow usally appears on the color monitor more than
on a monichrome monitor.
Below is the C and assembler code needed to display a screen without snow.
A few notes on the code that follows:
The code works for the AT&T 6300, IBM and an other
compatible computer.
The dos has a few storage locations that this code uses;
1.) 40:63h is where the video port address is saved,
3B4h for mono and 3D4h for color.
2.) 40:65h contains the current mode of the video.
The basic way to avoid the snow is to avoid contension for the
video buffer, the most reliable way to do this is to disable
the video while writting to the video buffer.
While the video is disabled you can only write a maximum of 265
characters before the user will notice any flicker in the screen
so the maximum you write should be 240 characters of three lines
of characters and attributes.
The way this module works is by having a global pointer indexing
to the [0][0] position of an internal memory buffer used to
build the screens. This means that the program can change the
current screen without having to re-build screens, just change the
pointer to another buffer and call the module to display it.
This technique of disabling the video only needs to be used for
color since moving 3 lines at a time seems to avoid snow on a
monochrome monitor.
The DeSmet C module for moving an entire buffer to the screen.
global variables used:
screen_buffer character pointer to the home corner of the source buffer.
screen_page integer for which page video page to move the source to.
screen_cols integer for number of columns in a line.
Other routines:
peek() passed segment and offset and returns the value stored
in memory at that location. Not standard in the DeSmet
library, Source not provided.
_showds() Standard in the DeSmet library. Returns the current
segment where all the data is stored.
color_off() Not Standard in the DeSmet library. Disables the color video.
color_on() Not Standard in the DeSmet library. Enables the color video.
_lmove() Standard in the DeSmet library. Simply moves the number of
bytes from the source segment:offset to the destination
segment:offset.
to_screen(video_page)
int video_page;
{
int video,video_seg,ds_reg,color,size,loop;
int *screen_ptr; /* a char and attribute pointer. */
color = (peek(0x40,0x63) & 0x40); /* 0 = mono 1 = color */
size = (3 * screen_cols) << 1; /* 3 lines of bytes */
video = 25 * screen_cols * page; /* offset to start of video page. */
video_seg = (color != 0) ? 0xB800 : 0xB000;
ds_reg = _showds();
screen_ptr = screen_buffer; /* copy pointer so we do not change
the global pointer. */
for (loop = 0; loop < 25; loop += 3)
{
if (color != 0) /* remove this if snow on mono */
color_off();
_lmove(size,screen_ptr,ds_reg,video,video_seg);
if (color != 0) /* remove this if snow on mono */
color_on();
screen_ptr += size;
video += size;
}
} /* end of to_screen() */
The in-line assembler for color_off();
color_off() /* disables the video signal */
{
#asm /* assembler to follow */
push ES
mov ax,40
mov ES,ax ; ES points to DOS data seg
mov dx,ES:[63h] ; base register for video chip
add dx,6h ; status register
color_off_loop:
in al,dx ; read status of retrace.
test al,8h
jz color_off_loop ; wait for retrace
mov al,ES:[65h] ; current mode register for video chip
and al,0F7h ; disable video bit
sub dx,2 ; dx points to mode register now.
out dx,al ; video disabled
pop ES
# /* end of in-line assembler */
}
The in-line assembler for color_on();
color_on() /* enable video */
{
#asm
push ES
mov ax,40h
mov ES,40h ; ES points to DOS data area
mov al,ES:[65] ; video mode
mov dx,ES:[63] ; base register for video chip
add dx,4 ; dx points to mode register
out dx,al ; video enabled again
pop ES
#
}
Sorry that this article was so long but snow free screens on any type
of monitor is not a simple process. If you need some of the other
source for functions or have any questions post an article or send me mail
I will try to find the source in my library or answer any questions about
the 6300 or compatibles.
Brad Davidson @ A T & T Information Systems Laboratories, Denver
x 1607
druxq!bradleyludemann@ubc-cs.UUCP (Peter Ludemann) (08/09/85)
In article <1884@druxq.UUCP> bradley@druxq.UUCP (DavidsonBC) writes: >This is a different way to control the monitor to remove the fuzz or >snow. The snow is caused by the video chip and the CPU trying to >access the same section of memory at the same time ... > > The basic way to avoid the snow is to avoid contension for the > video buffer, the most reliable way to do this is to disable > the video while writting to the video buffer. There's a much easier way. One of the bits in a port for the video controller tells you whether or not the monitor is doing retrace. Just loop busily while the bit says retrace isn't happening and then write your bit of the screen. I managed to write about 100 characters without any snow but I suspect more are possible. I don't have the code handy, but if anyone's interested, I'll look it up and post it. But if you've got the IBM Tech Reference handy, it shouldn't be hard to figure out. One thing: my own IBM-"compatible" doens't have the snow problem and leaves the port looking as if retrace never happens. The scren writing program ran at least twice as fast as on the IBM-PC. And both were much faster than using BIOS. ubc-vision!ubc-cs!ludemann.UUCP