Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!apple!rutgers!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Static vs. static column vs. dynamic vs. ???
Message-ID: <15762@cbmvax.commodore.com>
Date: 9 Nov 90 17:31:31 GMT
References: <15571@brahms.udel.edu>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Distribution: na
Organization: Commodore, West Chester, PA
Lines: 75

In article <15571@brahms.udel.edu> don@brahms.udel.edu (Donald R Lloyd) writes:

>		Why is DRAM slower than SRAM?  I can't see how having to 
>		refresh it would cause any tremendous slowdown.

The refresh, while certainly a small slowdown, it's the main reason for the
difference.  Perhaps the most important reason DRAM is slower than SRAM is
the fact that, historically, DRAM has a muliplexed address bus.  This has a
tremendous savings factor in pin count -- each time you add an address line,
you can quadruple the size of your memory.  

To access an SRAM, you provide an address to it, and one access time later,
the data is valid at the output pins.  Simple, straight forward.  To access
a DRAM, you first drive the Row address.  Next, clock that into the DRAM
by dropping the RAS* line (Row Address Strobe).  Next, switch in the Column
address, and then drop the CAS*  line (Column Address Strobe).  Now, based
on the worst of the column access time or the remaining row access time, data
will be valid on the output pins.

>		What is static column RAM?  I've been told it's the same as 
>		static RAM, but that doesn't seem right (otherwise the 3000 
>		would be a good bit faster and a good bit more expensive).  
>		Is it the equivalent of what Tannenbaum calls pseudo-static?

No, the pseudo-static RAM is kind of a weird puppy.  Static column RAM is 
DRAM that play a trick to go a bit faster.  You start out with a Row address
and RAS* strobe, just like before.  Then give it a Column address, but instead
of a CAS* strobe, you basically have a CS* line, which acts more like a 
chip select and buffer enable then a strobe.  Once CS* is low, you can vary
your column address, and you'll get data out corresponding to that column,
with the previously latched row, one column access time later.  Essentially,
once you've locked in the row address, the part acts like a static RAM in
for the column address.  And it's fast, too; typical SCRAM parts may have a
row access time of 80ns-100ns, but a column access time of 35ns-50ns.  The
A3000 uses this feature to run the 68030 burst cycles, where the first access
(in which the row address is clocked in) takes 5 clocks, but the three
additional burst cycles only take two clocks each.

>		What about VRAM?  Somehow specially designed for video memory, 
>		or just some marketer's way to make the static RAM on a video 
>		card sound superior?

VRAM is a DRAM specially modified in a way that's useful to video displays. It
has a sequential port as well as the normal random access port.  Using a 
special cycle, you can dump an entire row's worth of data into a large shift
register in one cycle.  The part can then continue normal cycles, while
this data is shifted out very quickly.  Since video displays typically access
lots of data in sequence, rather than requiring random access, this is perfect
to support such displays.  

>		While I'm at it, why are ROMs inherently so slow?

You can get fast ROMs, but, like static RAM, you generally have your choice
between BIG and FAST.  If you're reading a normal program, like perhaps
the Amiga's ROM Kernel, out of ROM, ROM density and cost are more important
than speed.  If the ROM costs too much, you might as well eliminate it 
completely and run out of RAM.  In general, Speed == Money.  There are very
small and fast ROMs available, though they're generally used for control
functions (more like a PAL), rather than main program store.  Same deal with
SRAM; the larger ones may be used for main program space, but they're not
so fast, the smaller, faster parts are typically used for cache or similar
kinds of things.

>		Why is there air?

Why not?  Seems like a good idea to me.

>		How many licks does it take to get to the center of a Tootsie 
>		Pop?

Three.
-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
	Standing on the shoulders of giants leaves me cold	-REM