Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!evax!cs4344af
From: cs4344af@evax.arl.utexas.edu (Fuzzy Fox)
Newsgroups: comp.sys.cbm
Subject: Re: Mouse driver for the C-128 in 80 columns (in text mode)
Message-ID: <1990Nov26.020347.13092@evax.arl.utexas.edu>
Date: 26 Nov 90 02:03:47 GMT
References: <1990Nov23.043410.12818@vlsi.polymtl.ca>
Organization: Computer Science Engineering Univ. of Texas at Arlington
Lines: 17

In article <1990Nov23.043410.12818@vlsi.polymtl.ca> if18@vlsi.polymtl.ca writes:
>In 40 columns, this problem could be solved by generating a interrupt
>with the raster beam (when it's outside the screen). 

This is a neat idea, and would seem to work, since the 40 and 80 column
video are both refreshed at 60 Hz.  However, I don't think the beams are
synchronized (that is, they don't start their vertical refreshes at the
same time), so if you refreshed at line 0 of the 40 column screen, it
might constantly refresh at line 97 on the 80 column screen, and so on.

>Can I used bit 6 of address $d600 (in read mode) to know if the raster
>beam is outside the screen , or is there an internal VDC register that
>could be used ??

You should use $D600, bit 6 to determine if the video is being
refreshed, and when it is, then do your changes to the character info.
Not the most efficient way to use CPU time, but it works.