First post, by VileR
- Rank
- l33t
Other than the usual things you can expect in such a situation (such as some monitors losing sync) - could this also result in garbage/unexpected *content* from VRAM being displayed?
Not that this is a very useful thing to do... but I've seen this happen on the internal plasma display of a PS/2 Model P70 (while the machine's owner and I were trying to figure out some *other* puzzling behavior with the P70's video subsystem). One of our tests involved setting plain old 80x25 text mode - clearing video RAM in the process - then increasing Vertical Display End. When this exceeded Vertical Retrace Start, the screen suddenly filled up with garbage content.
I'm curious to know whether this is common VGA behavior, or just another quirk of the P70. I realize that the start of vertical retrace is 'special' in some respects, e.g. it's when the (latched) start address takes effect for the next frame; also, there are certain situations in which the VGA has no business reading video RAM, and the vertical retrace period might be one of those.
But that wouldn't fully explain actual content being displayed when VRAM has just been cleared by a mode set. I suppose it may somehow be the font data from plane 2 (misinterpreted as character/attribute pairs), but how and why is still a puzzle.
If anyone feels like trying a quick test with TWEAK program (http://ftp.lip6.fr/pub/pc/garbo/pc/screen/tweak16b.zip), you can:
- hit TAB for the text mode test, then Enter to see what it looks like with the default values
- change "Vert. disp. enable end" to 9c or greater, then run the test again
- does the screen now show the same test pattern, or some other garbage? Is the display stable, or does it keep changing?