286 Xenix Hercules Scrolling Problem (help!)

[Aryeh M. Weiss]

In article <626 at eds1.UUCP> jdm1 at 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.

--