From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.hardware,comp.sys.amiga.advocacy
Subject: Re: Atari Falcon (wasn't: Amiga Falcon)
Date: 6 Apr 92 20:59:38 GMT
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA

In article <1992Apr4.055110.4562@elroy.jpl.nasa.gov> hyc@hanauma.jpl.nasa.gov (Howard Chu) writes:
>In article <30033@cbmvax.commodore.com> daveh@cbmvax.commodore.com (Dave Haynie) writes:
>>In article <torrie.702204984@Xenon.Stanford.EDU> torrie@cs.stanford.edu (Evan Torrie) writes:
>>>hyc@hanauma.jpl.nasa.gov (Howard Chu) writes:

>>>"On the Macintosh IIfx, burst mode reads are only done when the data
>>>is not located in the internal 256 byte and 32K Static RAM Cache. When
>>>the Macintosh IIfx reads from memory, the first access takes 6 clock
>>>cycles. The next 3 reads are performed at 3 clock cycles each.  This
>>>is referred to as 6 3 3 3 burst reads."

>>That's not too bad for a 40MHz '030 using cheap DRAM and a plain old everyday
>>ordinary memory design.  That's a total of 450ns per burst.  The A3000 does
>>its burst in 440ns.

>Where's the time going? The TT and SST using fast RAM do 4/1/1/1 bursts.
>TT at 32MHz, SST at anywhere from 25 to 50MHz.

The hell they do.  The TT (I dunno anything about any SST) ran a 32MHz 
processor connected to a 16MHz memory design (in fact, the original TTs had
this hacked in as a tower, though eventually they put the circuitry on
the motherboard).  Even if they had managed 4/1/1/1 at 16MHz, that would
translate to 8/2/2/2 at 32MHz.  A 2-clock burst is about the best you can
do with a single bank of 80ns DRAM at 32MHz, unless you go to nybble mode
DRAM (most vendors don't, because you can then only upgrade in 4MB chunks;
some of the GVP boards used this, as did the NeXT '030 system).  However,
8 clocks is downright pitiful for a 32MHz system, which is why the TT runs
most benchmarks slower than the A3000, even though it has, on paper, a
faster clock.

>I find that hard to believe, given that the 68030 doesn't do burst writes.

Well, it's nice to challenge your belief system every now and again.  When 
done correctly, this results in a thing called wisdom.

>>That's the reason for the wacky giant SIMMs in the IIfx.  They basically 
>>latch writes on each SIMM module.  

>Not bad... With a bit of interleaving, that possibility can be reduced
>pretty dramatically. 

Yeah, it's not a bad scheme if you use it effectively.  Controlling it at
the DRAM tends to make it cheaper than building it in as a system function,
but a single longword write latch isn't going to help in more advanced 
systems like '040 machines, which is undoubtedly why Apple went back to
traditional SIMMs in the Quadras.  However, the idea of localizing enhancements
in DRAMs is far from dead -- check out the new "self-caching" DRAMs for an
example of a far more interesting application of the idea.

>  -- Howard Chu @ Jet Propulsion Laboratory, Pasadena, CA


-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      BIX: hazy
 "I used to be disgusted, now I try to be amused" - Elvis Costello
 
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