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

[Evan Langlois <Evan@CoolRunningConcepts.com>]

On Sat, 2005-07-16 at 11:12 +0200, Johan Klockars wrote:
> Actually, I seem to recall the TT's RAM bus being 64 bit wide for the 
> video system and the Falcon's 32 bit (the CPU's both having half that), 
> but I may be wrong there.

Nah, you may be thinking of the Jaguar.  I remember everyone bitching
about the 16 bit bus of the Falcon when it was released.  And was it
16Mhz or 25Mhz?  

I wonder if my NeXT could run MiNT.  I've had it running Linux and I
also have a Mac OS emulator for it.  Running Atari software on the NeXT
would be slick, even if its only a 25Mhz 040, it should do ok.

> But, the CPU's have far worse bandwidth to video RAM than you might think.
> A 100 MHz CT60 with a 25 MHz Falcon bus gets something like 6 Mbyte/s 
> for the CPU when reading from ST RAM and 10 MByte/s when writing 
> (numbers from files in the KRONOS distribution). My AB040 Falcon gets 
> even less, at something like 4/6 Mbyte/s, IIRC.

Considering the stock 030 can't do a bus access every clock cycle (not
that I know of anyway), Atari didn't have a reason not to interleave bus
cycles with video and DNS devices.   I wouldn't be surprised if the CPU
only gets 1/2 or 1/4 of the bus cycles.

> It's funny, though, that if that measly sub-10 Mbyte/s bandwidth is used 
> to communicate with a reasonable graphics card, even the Falcon will 
> feel really speedy. (Granted, you don't want to go blitting large 
> amounts of data between normal RAM and the card, but that can usually be 
> avoided.)

As long as your demos are 320x200 x4bpp, you only have about 32K per
screen.   The video would need just under 2MB/s for screen refresh at
60Hz.  Full screen video 30 fps means writing under 1MB/s.

For 320x200x32bpp the numbers are 256K/screen, 15MB/s at 60Hz, and half
that for the CPU to write the data at 30fps.  Of course I'm guessing the
demos aren't going to be writing the entire screen every time at a full
30 fps - likely around 20, which would be about 5MB/s.

> Dual ported RAM hasn't been used in main stream graphics card for a very 
> long time. It costs too much and there's really no reason for it with 
> today's bandwidths. Top end modern graphics cards are up somewhere in 
> the 30-60 Gbyte/s range now.

Yeah your right, I wasn't sure what memory they are using these days.
Found out its just DDR.   Its an ATI 9200SE - the last one with full
open-source 2D and OpenGL drivers available.  The same card is also
available as an embedded low-power graphics chip - maybe some day
someone will wire it to a Coldfire 4e  :)