Path: utzoo!mnetor!uunet!mcvax!cernvax!ethz!forty2!poole
From: poole@forty2.UUCP (Simon Poole)
Newsgroups: comp.sys.atari.st
Subject: Re: ST hardware scrolling
Message-ID: <238@forty2.UUCP>
Date: 29 Apr 88 16:32:18 GMT
Reply-To: poole@forty2.UUCP (Simon Poole)
Organization: Exp. Physics University Zuerich
Lines: 36

In article <4650@watdcsu.waterloo.edu> magore@watdcsu.waterloo.edu (Mike Gore) writes
>My 2 cents to do 'hardware' scrolling would be to do it partly 
>in software and hardware. If we were to use the horizontal interrupt
>we could maintain a linked list of scan lines - or even a group of     
>scan lines. This could be run as a interrupt driven display process that
>would then be a set up as a display list of sorts.  In order to scroll the
>entry link for the beginning of the display would only need to be changed. If
>a group of scan lines were used to make up a single character line interrupt
>Overhead could be reduced and character I/O would be slightly easier. The  
>disadvantage to this method would be the waisted space at the end of each line
>[ required to fall on 256 byte offsets ] and interrupt overhead.
             
>        Comments ???                                                          

If I understand you correctly, you want to change the current value
of the display position counter on the fly in the HBI, well this
is a very neat idea I had about two years ago , if possible it would
allow some really nice effects like overlaying screens etc.. So what
about writing to Atari and ask them to make the register writable in 
the next revision of the ST................

(BTW if you where thinking about actually copying the scanlines, a
simple calculation shows (for mono screen):

  1 Scanline = 80 Bytes = 20 Longs
  1 move.l ~ 20 Cycles ~ 2 uS
  20 * 2 uS = 40 uS (Time to copy a scanline)
  400 * 40 uS = 16'000 uS = 16 mS (Total time used in one vertical scan)
  70 * 16 mS = 1120 mS = 1.1 S (Total time used for 70 screen refreshes)

  and as you know 1.1s > 1s........)

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