jdm1@eds1.UUCP (Jon McCown) (11/28/90)
I just installed 286 Xenix (Tandy labeled Microsoft product) and have found
a bizarre problem to wit:
System: Herc Clone Mono Video, Mono Monitor, 286-12, 2MB Ram, MFM disk etc...
Once the screen fills and gets to the bottom line (ie it should scroll up),
the screen scrolls, but no further text is output. (invisible text? :-)
After a while of pounding return, text will begin to appear at about column 40,
but it is not 'current' (seems to match what should have appeared earlier).
My guess is that the invisible text is being scrolled up through the video
memory above the visible area. I have seen a similar problem with PC/IX
where displayed lines 'reappear' at about column 40 from time to time.
Is this a known problem (ie got a fix?). My apologies for running an old
release of Xenix, but the price was right.
- Jon
--
J.D. McCown - RCSG Director - Senate of Pennsylvania
psuvax1!eds1!jdm1 (this space intentionally "Your lupins or your life!"
jdm1@eds1.eds.com filled with this text) - Dennis Moorearyeh@eddie.mit.edu (Aryeh M. Weiss) (11/29/90)
In article <626@eds1.UUCP> jdm1@eds1.UUCP (Jon McCown) writes: > >Once the screen fills and gets to the bottom line (ie it should scroll up), >the screen scrolls, but no further text is output. (invisible text? :-) >After a while of pounding return, text will begin to appear at about column 40, This is an old old problem involving a fundamental incompatibility between the orig IBM monochrome and more sophisticated graphics cards. The IBM card had exactly 4KB of memory and the video processor address automatically wrapped around. I.e., you could set the CRTC start address to a location in the middle of the page, the data at this loc would be at the top of the screen, continue til it got to the end of the 4K page, then start displaying from location 0. This way you got a smooth scroll: you just use the 4K page as a circular buffer, and increment the CRTC start register to scroll the screen. New mono cards have >4K and do not wrap in this manner. Unfortunately your OS is assuming the former behavior, but since it never writes into video memory locations >4K and the video memory does not address wrap you end up with a screen that is scrolling uninitialized data onto your screen! New Xenix OS's handle these cards correctly. Now, the original Hercules mono graphics card had a compatibility mode which did do the address wrap. You might try to find a genuine Herc card, not a clone, or not be a cheapskate and get a new OS. --