Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ukma!husc6!husc7!byrd
From: byrd@husc7.HARVARD.EDU (John Byrd)
Newsgroups: comp.sys.cbm
Subject: Re: Machine Language Smooth Scrolling Problem
Keywords: machine language, scrolling, C64
Message-ID: <2823@husc6.harvard.edu>
Date: 11 Oct 89 08:02:18 GMT
References: <2317@jarthur.Claremont.EDU> <136@cbmtor.UUCP>
Sender: news@husc6.harvard.edu
Reply-To: byrd@husc7.UUCP (John Byrd)
Organization: Harvard University Science Center   Cambridge, MA
Lines: 45

There have been a number of messages requesting help on "smooth scrolling"
for the 64 mode, and Jim Butterfield posted a very intelligent solution
to this problem.  However, his solution may have been a little difficult to
understand for novice programmers.  I hope that I can explain the smooth-
scroll idea here in more of lay-programmers' terms.

Locations $D011 and $D016 in the 64 contain the smooth-scroll information in
the lower 3 bits of each of the registers.  All the information on the screen
may be slid up to 7 pixels (dots) to the right and 7 pixels down.  So how does
a programmer achieve the sliding, scrolling effect popular in games?

When the screen picture has been slid to its maximum distance, e.g. when a
register contains $111 in its lower 3 bits, all the characters on the screen
must be moved via a software routine, and new characters must be added on to
one side of the screen.  At this point the register may be reset to $000.  The
effect is that of shifting the picture one pixel.  This procedure may be
reversed and used on the other register to facilitate scrolling in any 
direction.

The programmer must watch for "flicker" when designing and writing this
type of sliding routine.  If the raster (present line which is being drawn on
the television screen) happens to be on the screen when the screen is being
moved via the software routine, the screen will flicker.  This can be
resolved by checking location $D012.  The raster is on the screen only when
the value of this location is between 51 and 251, AND when the high bit of
location $D011 is OFF.  Otherwise it is okay to do the software scroll.

Jim Butterfield suggested that TWO separate screens should be maintained.  In 
essence, display one screen while preparing the other for viewing; then, 
when the raster is off the screen, swap the viewed screen and the hidden
screen.  This swap can generally be done with a few POKES and PEEKS, although
the BASIC language is inadvisable for this type of programming due to
its lack of speed.  Speed of processing is essential for raster techniques;
the raster makes a pass of the screen every 1/60th of a second.

I hope that this message gives a better understanding of scrolling
techniques.

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John Byrd
byrd@husc7.bitnet
Q-Link: John Byrd

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