How RAM Works

Random access memory (RAM) is the best known form of computer
memory. RAM is considered "random access" because you can
access any memory cell directly if you know the row and
column that intersect at that cell.

The opposite of RAM is serial access memory (SAM). SAM stores
data as a series of memory cells that can only be accessed
sequentially (like a cassette tape). If the data is not in
the current location, each memory cell is checked until the
needed data is found. SAM works very well for memory buffers,
where the data is normally stored in the order in which it
will be used (a good example is the texture buffer memory on
a video card). RAM data, on the other hand, can be accessed
in any order.

Similar to a microprocessor, a memory chip is an integrated
circuit (IC) made of millions of transistors and capacitors.
In the most common form of computer memory, dynamic random
access memory (DRAM), a transistor and a capacitor are paired
to create a memory cell, which represents a single bit of
data. 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.

Memory Cells and DRAM

Memory cells are etched onto a silicon wafer in an array of
columns (bitlines) and rows (wordlines). The intersection of
a bitline and wordline constitutes the address of the memory
cell.

DRAM works by sending a charge through the appropriate column
(CAS) to activate the transistor at each bit in the column.
When writing, the row lines contain the state the capacitor
should take on. When reading, the sense-amplifier determines
the level of charge in the capacitor. If it is more than 50
percent, it reads it as a 1; otherwise it reads it as a 0.
The counter tracks the refresh sequence based on which rows
have been accessed in what order. The length of time necessary
to do all this is so short that it is expressed in nanoseconds
(billionths of a second). A memory chip rating of 70ns means
that it takes 70 nanoseconds to completely read and recharge
each cell.

Memory cells alone would be worthless without some way to get
information in and out of them. So the memory cells have
a whole support infrastructure of other specialized circuits.
These circuits perform functions such as:

* Identifying each row and column (row address select and
column address select)
* Keeping track of the refresh sequence (counter)
* Reading and restoring the signal from a cell (sense
amplifier)
* Telling a cell whether it should take a charge or not
(write enable)

Other functions of the memory controller include a series of
tasks that include identifying the type, speed and amount of
memory and checking for errors.

Static RAM

Static RAM uses a completely different technology. In static
RAM, a form of flip-flop holds each bit of memory.
A flip-flop for a memory cell takes four or six 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 up 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.

Static RAM is fast and expensive, and dynamic RAM is less
expensive and slower. So static RAM is used to create the
CPU's speed-sensitive cache, while dynamic RAM forms the
larger system RAM space.

Memory chips in desktop computers originally used a pin
configuration called dual inline package (DIP). This pin
configuration could be soldered into holes on the computer's
motherboard or plugged into a socket that was soldered on the
motherboard. This method worked fine when computers typically
operated on a couple of megabytes or less of RAM, but as the
need for memory grew, the number of chips needing space on
the motherboard increased.

The solution was to place the memory chips, along with all of
the support components, on a separate printed circuit board
(PCB) that could then be plugged into a special connector
(memory bank) on the motherboard. Most of these chips use
a small outline J-lead (SOJ) pin configuration, but quite
a few manufacturers use the thin small outline package (TSOP)
configuration as well. The key difference between these newer
pin types and the original DIP configuration is that SOJ and
TSOP chips are surface-mounted to the PCB. In other words,
the pins are soldered directly to the surface of the board,
not inserted in holes or sockets.

Memory chips are normally only available as part of a card
called a module. You've probably seen memory listed as 8x32
or 4x16. These numbers represent the number of the chips
multiplied by the capacity of each individual chip, which is
measured in megabits (Mb), or one million bits. Take the
result and divide it by eight to get the number of megabytes
on that module. For example, 4x32 means that the module has
four 32-megabit chips. Multiply 4 by 32 and you get 128
megabits. Since we know that a byte has 8 bits, we need to
divide our result of 128 by 8. Our result is 16 megabytes!