Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!ogicse!qiclab!techbook!waynekn
From: waynekn@techbook.com (Wayne Knapp)
Newsgroups: comp.sys.amiga.advocacy
Subject: Re: Blitter vs. 040 (was: Computer Architecture question
Message-ID: <1991May13.004217.23406@techbook.com>
Date: 13 May 91 00:42:17 GMT
References: <1991May9.070349.15151@neon.Stanford.EDU> <3496.tnews@templar.actrix.gen.nz> <vbcH!4p@cs.psu.edu> <1991May10.180908.29565@convex.com>
Organization: TECHbooks of Beaverton Oregon - Public Access Unix
Lines: 60

swarren@convex.com (Steve Warren) writes:


>In the A3000, the 68030 can update display memory faster than the blitter can.
>This does not mean that there is no advantage to using the blitter (and the
>blitter does not loose by *that* much).  The fact is, if the blitter is used
>to manage the display and the '030 is able to stay out of display memory, then
>the '030 will be able to "multiprocess" in parallel with the blitter.  So if
>you have calculations that need to be performed then, it is advantageous to use
>the blitter to offload graphic manipulations from the CPU.


Two three things wrong with this.  

   1) The blitter is seldom used for animation.  ANIM players use the CPU,
      always.   The blitter is often used for brush moving, but not for real
      animation.

   2) Even the a 68000 can scream past the blitter, depending on the blit.
      For simple large blits, the blitter is much faster.  For very complex
      smaller blits a 68000 routine can be written that is much faster than
      the blitter.  I done it many times in my code.

      There is a common trade off that can be made between memory used, nature
      of the code and speed of the code.  The blitter is very general and do to
      that nature can be hard pressed to complete with specail purpose code. 
      By using all the 68000 registers and extra banks of memory it is possible
      to reduce the number of memory accesses required in a very complex blit.
      (like, restoring the background from a cookie cut blit)

      Now there are some real constraints, if you are working on something that
      requires a LOT of barrel shifting, there is no way a 68000 can keep up 
      with a blitter.  Maybe a 68030, but not a 68000.

   3) Everyone is always shouting about how the CPU and Blitter can run at the
      same time.  But in real life, it rarely happens.  There are several
      reasons for this.  
      a) The blitter requires CPU to adjust it's registers between bitplanes.
      b) Most blits are short in time, too short for the operations system to
         allow the CPU much chance to run.
      c) It is very hard and complex to pull off.
      Now, it can be done, and I'm sure someone has some has.  But not as a 
      general rule, and certainly not something you get for free.

Now I realize the need to have a blitter in the Amiga, but not for speed in
the higher-end Amigas, but for BACKWARD compatibility.  As for speed, es. in
68020+ Amigas I think the blitter is VASTLY overrated.  

Anyway that is MHO.  Since I make my living from writing real-time graphics/
animation software, I suppect I used the blitter about 10,000 more than most
people. 

                                                     Wayne Knapp
                                                       Consultanting Assoc.
                                                         Hash Enterprises

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