Constant, The "snow" on a CGA monitor is caused by two devices accessing the video memory at the same time (the CPU and the video processing chip (I used to know the part number).
There is a register on the card which can be read by a program and which indicates the state of the video processor at any given moment. Among other things it indicates when the electron gun in the monitor is in one of its retrace cycles. The bits needed to draw the screen image are only accessed on the "forward" sweep, while the gun is "off" during the retrace. By monitoring these register bits, it is possible to write to the video memory (which holds what's being displayed) during one of the retrace sweeps, thus eliminating the interference (snow) caused by two different processors clocked at different speeds both executing a read cycle on the same memory at the same time. This is entirely a software solution. Each individual program is responsible for good manners when writing to the video RAM. (Incidentally, this problem only occurred because the BIOS was so slow in writing to the screen that programmers had to bypass BIOS to get any performance.) ~~Garry [EMAIL PROTECTED] Original Message: ----------------- From: Constant Brouerius van Nidek [EMAIL PROTECTED] Date: Mon, 1 Oct 2001 19:04:11 +0700 To: [EMAIL PROTECTED] Subject: [SURVPC] snow on a CGA monitor >From years ago I remember that some programs had a possibility to suppress the so called snow on CGA monitors. I can't remember if it was done with the programs or with commands in DOS. Can somebody help me with the suppression of the CGA color monitor which I use on a XT ? Thanks in advance -------------------------------------------------------------------- mail2web - Check your email from the web at http://mail2web.com/ . To unsubscribe from SURVPC send a message to [EMAIL PROTECTED] with unsubscribe SURVPC in the body of the message. Also, trim this footer from any quoted replies. More info can be found at; http://www.softcon.com/archives/SURVPC.html
