Answer Posted / rajesh pagi
Your computer probably uses both static RAM and dynamic RAM
at the same time, but it uses them for different reasons
because of the cost difference between the two types. If you
understand how dynamic RAM and static RAM chips work inside,
it is easy to see why the cost difference is there, and you
can also understand the names.
Dynamic RAM is the most common type of memory in use today.
Inside a dynamic RAM chip, each memory cell holds one bit of
information and is made up of two parts: a transistor and a
capacitor. These are, of course, extremely small transistors
and capacitors so that millions of them can fit on a single
memory chip. The capacitor holds the bit of information -- a
0 or a 1 (see How Bits and Bytes Work for information on
bits). The transistor acts as a switch that lets the control
circuitry on the memory chip read the capacitor or change
its state.
A capacitor is like a small bucket that is able to store
electrons. To store a 1 in the memory cell, the bucket is
filled with electrons. To store a 0, it is emptied. The
problem with the capacitor's bucket is that it has a leak.
In a matter of a few milliseconds a full bucket becomes
empty. Therefore, for dynamic memory to work, either the CPU
or the memory controller has to come along and recharge all
of the capacitors holding a 1 before they discharge. To do
this, the memory controller reads the memory and then writes
it right back. This refresh operation happens automatically
thousands of times per second.
This refresh operation is where dynamic RAM gets its name.
Dynamic RAM has to be dynamically refreshed all of the time
or it forgets what it is holding. The downside of all of
this refreshing is that it takes time and slows down the
memory.
Static RAM uses a completely different technology. In static
RAM, a form of flip-flop holds each bit of memory (see How
Boolean Gates Work for detail on flip-flops). A flip-flop
for a memory cell takes 4 or 6 transistors along with some
wiring, but never has to be refreshed. This makes static RAM
significantly faster than dynamic RAM. However, because it
has more parts, a static memory cell takes a lot more space
on a chip than a dynamic memory cell. Therefore you get less
memory per chip, and that makes static RAM a lot more expensive.
So static RAM is fast and expensive, and dynamic RAM is less
expensive and slower. Therefore static RAM is used to create
the CPU's speed-sensitive cache, while dynamic RAM forms the
larger system RAM space.
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