Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!cs.utexas.edu!asuvax!noao!arizona!dave
From: dave@cs.arizona.edu (David P. Schaumann)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Bitplanes - good or bad
Keywords: writing a pixel
Message-ID: <196@caslon.cs.arizona.edu>
Date: 29 Mar 90 22:48:19 GMT
References: <5917@tekig5.PEN.TEK.COM> <133675@sun.Eng.Sun.COM>
Organization: U of Arizona CS Dept, Tucson
Lines: 51

In article <133675@sun.Eng.Sun.COM>, cmcmanis@stpeter.Sun.COM (Chuck McManis) writes:
| [ ... ]
| One, the blitter wants to work on one bitplane, in part because 
| logical operations on multiple bit pixels are tougher. So to 
| enhance the speed of the operation, you can enhance the parallelism.
| This is sometimes referred to as the "Blitter per Bitplane" approach.
| Given four blitters, drawing into four bitplanes and sharing all
| registers except the source/destination pointers in common. 
| Operating in this mode would be slightly more than 4 times faster
| than the current scheme. The slightly more comes from the fact
| that in the current system there are some register reload times
| which are common in our mythical multiblitter. 
| 
| [...]
| So far everything looks wonderful, but...
| 
| And you knew it was coming. How would this multiheaded blitter talk
| to memory ? Now the Amiga blitter is a "Word" blitter and not a true
| "Bit" blitter. What it does is read in a word, (16 bits) operate on
| it and write it back out, possibly shifted left or right. The speed
| comes from the fact that the memory cycle of the blitter looks something
| like : (psuedo timing diagram)
| 
| [ ... ]
| 
| The only way to utilize the massive parallelism of 4 blitters (or N
| blitters) is if they can all read data from memory _at the same time_.
| Hmmm, bad news eh?

Not necessarily.

First, assume: blitter operations are independant.  That is, given a task
  for the blitters, the order they perform their sub-tasks is unimportant.

Second, give the Amiga interleaved memory, and start the blitters on staggered
  words in memory.

I realize there would be a problem in the case where the memory is not 
contiguous, but I think this would occur rarely enough that you would still
get nearly a 4x speed up (assuming 4 blitters.)  Of course, all this would
cost $$$, but you knew that, didn't you?

| 
| --Chuck McManis
| uucp: {anywhere}!sun!cmcmanis   BIX: <none|   Internet: cmcmanis@Eng.Sun.COM
| These opinions are my own and no one elses, but you knew that didn't you.
| "If it didn't have bones in it, it wouldn't be crunchy now would it?!"


Dave Schaumann		| "Constable Parrot ate one of those!"
dave@cs.arizona.edu	|  ;)