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From: melling@cs.psu.edu (Michael D Mellinger)
Subject: Re: Blitter vs. 040 (was: Computer Architecture question
In-Reply-To: caw@miroc.Chi.IL.US's message of 15 May 91 00: 56:18 GMT
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Date: Wed, 15 May 1991 08:46:56 GMT
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In article <caw.0521@miroc.Chi.IL.US> caw@miroc.Chi.IL.US (Christopher A. Wichura) writes:

   Talking about dense!  You just don't get it, do you?  Using the blitter in
   parallel, even if it might be a bit slower than an 040 running balls out
   doing all your graphical work, does not necessarily make the 040 faster.  You
   could start the blitter on a job and then have the 030 calculate the next job
   while the blitter is working.  When the blitter is done the next job can be
   started instantly. With the 040 you would finish the job and then have to
   prepare the next in a synchronous manner.

Hmmm.  1+1 != 3.  You could have the 68040 do the graphical work then
do the calculations then do the graphical work all in the same time
that it took the blitter to do the the graphical work and the 68030 to
do the calculations.  In other words, the 68040 gets more work done in
the same period of time.

   The other thing to consider is that the Amiga is a multitasking machine.
   Yes, the 040 might be able to render stuff faster than the blitter (this is a
   point which I feel really depends on the application), it has to stop doing
   other things and actually do the graphics.  Consider what would happen if all
   the screen updates on the Amiga's workbench were done by CPU and not the
   blitter.  Say I'm running a ray tracer in the background.  If I move a
   window, or a cli window scrolls, updating the screen is going to steal CPU
   time from the ray tracer.  Using the blitter, the display might not be
   updated as fast as it possibly could, but the CPU cost to do it is near nil
   because the blitter is doing the grunt work and thus the ray tracer doesn't
   get locked out as long.

So the CPU has to stop and update the screen.  What's the big deal?
You have 2*N mips in your machine and N of those mips are only capable
of bit manipulation, while the other N mips can be used for your ray
tracer.  Drop an 040 in the A3000 and you now have 3*N mips for
calculations and you still only have have N mips for bit
manipulations.

N = 68030, of course.


   >So, the blitter is stealing from the faster chip, the 68030?

   Only in CHIP memory, of which there is a relatively small amount compared to
   FAST memory.  Outside of things that HAVE to be in CHIP, the Amiga will use
   FAST memory first by default.  Thus, most program code is loaded into FAST
   memory, and any number crunching type program (such as a ray tracer or spread
   sheet) will have its data storage in FAST memory as well.  So it doesn't
   matter if the blitter is stealing cycles from the CPU's access to CHIP
   memory.

Assuming you have 1 meg. Amiga.  When you exand the Amiga to 1 meg. is
512K chip RAM and 512 FAST ram?  

   Again, this depends on the application.  For general usage, I'd much rather
   have the blitter used as it is now.  It helps the system AS A WHOLE run
   faster.  Now, I'm no multimedia guy, but for some apps I can see you would
   want to run an 040 balls out for extra rendering speed.  That's a choice the
   designed of the software will have to make.

Are people still writing directly to the hardware on the Amiga?
Dedicated graphics hardware is nice but if all your software depends
on specific hardware then you are not going to be able to take
advantage of future advances.  If you think the 68040 is fast wait
until MIPS/Compaq/Microsoft drop their R4000 bomb(looks like next
year).  It's going to be almost three times faster than the 68040.  On
the Amiga that's N mips dedicated to graphics and 9*N mips dedicated
to calculations.  

-Mike

