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From: johnhlee@flute.cs.cornell.edu (John H. Lee)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Static vs. static column vs. dynamic vs. ???
Message-ID: <48149@cornell.UUCP>
Date: 9 Nov 90 20:07:11 GMT
References: <15571@brahms.udel.edu>
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Reply-To: johnhlee@cs.cornell.edu (John H. Lee)
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Organization: Cornell Univ. CS Dept, Ithaca NY
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In article <15571@brahms.udel.edu> don@brahms.udel.edu (Donald R Lloyd) writes:
>	Questions:
>		Why is DRAM slower than SRAM?  I can't see how having to
>		refresh it would cause any tremendous slowdown.

Two things:  DRAM design and refresh.  RAMs are arranged in a matrix with
memory cells set in rows, each row sharing the same sense line.  The
capacitance of the sense line is non-significant.  In a SRAM, each memory
cell actively drives its sense line, so access is fast, but the cell is large
as it requires at least 4 transistors (to form a flip-flop.)  In a DRAM, each
memory cell is a capacitor and a gate transistor with only the stored charged
in the miniscule capacitor driving the sense line, so access is slower, but
each cell can be made much smaller.  Since no capacitor is perfect, periodic
refresh cycles are needed to recharge the capacitors and the refresh cycles
times are comparable to a normal access.

A DRAM also requires a "recovery period" call pre-charge that ensures that
the amps on the sense lines can detect the miniscule capacitor charges.
This period is on the order of an access and is the reason why a DRAM can
be accessed only half as fast as its access time.

>		What is static column RAM?

Static column RAMs support a faster access method.  A normal access requires
that the address be clocked into two phases: row address with RAS (Row Address
Strobe) then column address with CAS (Column Address Strobe.)  Each strobe
has a minimum hold time.  Now a static column RAM permits both strobes to
be held active and the column address to change.  The RAM can provide
valid data much faster since the row address is already latched and row is
already accessed.

> 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.  I believe psuedo-static RAMs only implement hidden refresh within the
chip.  The two concepts are not the same.

>		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?

VRAMs are usually high speed RAMs with special features to make the video
system simpler, like built-in shift registers and dual access-ports.

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

ROMs aren't inherently slow.  However, depending on their design, ROMs may
have a largely passive structure being essentially a huge, dense matrix of
wires with cross-links and the capacitance of the lines affect speed.  In
trade journals I regularly see advertised ROMs that are as fast as high-speed
RAMs.

>		Why is there air?

Because we need something to blow the smog out of L.A. (not that it works
all the time, but engineering considerations and funding impose limitations.)

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

Three.

>		Where's my free 68040 upgrade for the 3000 I just ordered?

I don't know.  I already got mine.  :-)

>	Thanks for any info you can provide.

You're welcome.

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	John Lee		Internet: johnhlee@cs.cornell.edu
The above opinions of those of the user, and not of this machine.