Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!ux1.cso.uiuc.edu!tank!eecae!abaa!esker
From: esker@abaa.uucp (Lawrence Esker)
Newsgroups: comp.sys.amiga
Subject: Re: Larry's ghost fix works....why?
Message-ID: <4202@abaa.UUCP>
Date: 20 Oct 89 17:12:17 GMT
References: <1648@nigel.udel.EDU> <8722@ardent.UUCP>
Reply-To: esker@abaa.UUCP (Lawrence Esker)
Organization: Allen Bradley
Lines: 39

In article <8722@ardent.UUCP> rap@peck.ardent.com (Rob Peck) writes:
>In article <1648@nigel.udel.EDU> ant@cmr.ncsl.nist.gov (John Antonishek) writes:
>>lphillips@lpami.wimsey.bc.ca (Larry Phillips) writes:
>>>Try moving your screen to the right with the Preferences program.
>>This fixed my ghost problems, but will someone please tell me why it works?
>
>Amiga displays sprites by doing direct memory access during explicitly
>reserved memory clock cycles near the start of the time that the leftmost
>edge of the screen can be displayed.  The designers of the Amiga allowed
>the screen to by dynamically moved left or right by adjusting the position
>at which the screen DMA begins.  The Amiga Hardware manual shows a chart
>that defines the DMA time slots, showing that the first time slots
>are assigned to memory refresh, and audio, the disk, serial, the
>sprite come next and finally the display DMA. 
>
>Rob Peck

This brings an interesting question.  From the above desription, the Amiga
preloads the sprite data and then multiplexes the preload data with the
bitmap data being read on the fly.  When the time comes to display the sprite,
the multiplexer ignores the bitmap data and uses the preloaded sprite data.

Assuming the above to be true, why didn't the design multiplex the bitmap
current address with a sprite address generator?  When the time for the sprite
arrives, the chip ram address would just change.  This would avoid the use
of preload data buffers, indirectly allowing sprites to be any width.  Would
also free the time slots allocated to the sprites.

Too bad Jay Miner does not read this group.  I would really like to pick his
brain about a lot of Amiga custom chip decisions.
--
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