Re: [MiNT] Re[2]: GEM boost

[<jan.thomas@bt.com>]

You are absolutely correct. 

Its the bus that lets everything down.

Supervidel will put much of this issue to bed.



-----Original Message-----
From: Evan Langlois [mailto:Evan@CoolRunningConcepts.com]
Sent: Sat 16/07/2005 09:28
To: Thomas,J,Jan,XJM5A C
Cc: mint@fishpool.com
Subject: Re: [MiNT] Re[2]: GEM boost
 
On Sat, 2005-07-16 at 08:51 +0100, jan.thomas@bt.com wrote:
> >But even now I'm used to a 2x1.8GHz G5, my TT still doesn't feel sluggish or obsolete at all. As long as I don't use it to browse the web. ;o)
> 
> Yeah, odd that. My TT is crisp at only 32mhz, but my 80/25mhz CT60'd Falcon is painfully slow screen-wise.

Sound to me like bus contension.  Doesn't the TT have a 32Mhz bus?  I
know its a 32 bit wide bus, and the Falcon is 16 bit wide, so right away
there is half the bus gone.  So, 25Mhz * 16 bit = 50MB/s.  32Mhz * 32
bit = 128MB/s.   Considering that screen draws are memory intensive by
definition, you are limited by the bus speed and not as much by overall
CPU speed, and further limited in that you have to share the bus with
the video chip (unlike a TT that can at least store code and data in TT
RAM).

Its just like those shared-video-memory i810 PC boards.  If you have a
screen thats 1024x768 at 32 bits/pixel, then the screen is 3MB of data.
Now if you have a 75Hz refresh, your video chip is sucking 3MB * 75
times/second = 225MB/s from the memory bus!   Now you know why PCs have
to put so much processing and RAM on the video card, and shared memory
systems seem to crawl.

I'm running about 1600x1200, 32 bit, 70hz .. 525MB/s.  Luckily the ATI
can do that - pretty sure the card uses dual-ported VRAM or something.