Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!unmvax!ddnvx1!Path-Lost
From: GALETTI@uservx.afwl.af.mil
Newsgroups: comp.sys.amiga
Subject: Re: ddfstrt ddfstop diwstrt diwstop
Message-ID: <11827@uservx.afwl.af.mil>
Date: 14 Sep 90 19:56:00 GMT
References: <6336@castle.ed.ac.uk>
Distribution: comp
Lines: 41

In article <6336@castle.ed.ac.uk>, aiajms@castle.ed.ac.uk (-=Andy=-) writes:
> Could someone explain the above copper registers. I know that they are
> for screen addressing but the book I have which explains how they work
> is not very clear.
> 
> 
> 
> Any help appreciated.
> 
> Thanks
> 
> -=Andy=-

DIWSTOP and DIWSTRT specify the horizontal and vertical position of the beam 
where the raster display begins and stops.  Typically a raster doesn't cover
the whole screen; there's a background-colored border around the raster because
most monitors can't display the entire screen.  DIWSTRT defines the top-left
corner of the displayed raster and DIWSTOP defines the bottom-right corner.

Once you decide how big of a screen you want, you then need to tell the copper
where to start and where to stop DMA access to the screen's data.  To quote
from the "Amiga System Programmer's Guide:"
	
	"The data must be read from the bit planes at the right times
	 so that the pixels appear on the screen at the desired time."

In other words, you have to tell the copper when to start fetching data so by
the time the copper reads and processes the data and then outputs it to the 
video signal, the video electron beam is tracing the desired part of the
screen.

DDFSTRT (Display Data Fetch STaRT) and DDFSTOP allow you to specify when the
bitplane DMA begins and ends so you can line the accessed data up with the
borders you defined with DIWSTOP and DIWSTRT.

Does this help?  I highly recommend the "Amiga System Programmer's Guide" to
learn about these things.  For the most part, it's pretty straightforward, and
for the parts that it's not straighforward, a little experimentation usually
clarifies things.

-Ralph Galetti