7 Jul
2001
7 Jul
'01
12:53 p.m.
SRAMs were smaller in size, and simpler in architecture, though the elemental
memory structures were larger. The fast SRAMs were built for access time, at
the cost of power, mostly, aside from the die size. DRAMs were built for
capacity (number of bits) and density (pounds of silicon per bit) which
translated into cost. Slower SRAMs were built for simplicity of operation, as
there was always a market for devices that could be applied without a large
hardware overhead. DRAMs were built for the applications that justified a
larger overhead since there was a demand for large memory size in a limited
power, space and cost budget. SRAMs in small arrays were much cheaper as well
as being simpler.
Dick
----- Original Message -----
From: "Vance Dereksen" <vance(a)ikickass.org>
To: <classiccmp(a)classiccmp.org>
Sent: Saturday, July 07, 2001 5:20 PM
Subject: Re: 2116 and other old memory chips
Is that also why static RAM is so much faster?
Peace... Sridhar
On Sat, 7 Jul 2001, Tony Duell wrote:
This
answers most of my questions, thanks. One further question. What
is the difference between "static" ram and "dynamic" ram, since both
are "volatile" ram? Or, putting it another way, what is "static"
about
"static" ram, since it changes as well?
OK...
Static RAM is made from flip-flops. Which means that when you write data
to it, you set some flip flops one way (for '1's) and some the other way
(for '0's). And the flip-flops remain in that state until either you turn
the power off or you write some other data to it.
Dynamic RAM is made from capacitors. Generally, a '1' is represented by a
charge on a capacitor, a '0' is a discharged capacitor. Writing data to
DRAM involves charging some capacitors and discharging others.
The problem is that all electronics has leakages. So the capacitors
discharge themselves (in a few milliseconds). So you have to read each
location periodically and write the data back again to 'refresh' the
charge on the capacitors. Most (all?) DRAMs have internal circuitry to
help with this (e.g. accessing a particular location will refresh a
number of locations -- perhaps all those with the same pattern in the
high-order half of the address). But you still have to do something -- on
'classic' DRAMs, you have to cycle the address inputs and apply RAS/
pulses (or at least that's the normal way). If you don't, you lose data.
That's the difference. The contents of static RAM are maintained as long
as power is applied. Dynamic RAM needs to be refreshed or it will forget.
-tony