Inside your
computer,RAM takes the form of separate microchip modules that plug into slots
in the computer’s motherboard. These slots connect through a line (bus) or set
of electrical paths to the computer’s processor. Before you turn on a
computer,its RAM is a blank slate. As you start and use your computer, the
operating system files,applications, and any data currently being used by the
processor are written to and stored in RAM so the processor can access them
quickly.
How is this data written to and stored in RAM? In the most
common from of RAM,dynamic random access memory (DRAM), transistors (in this
case,acting as swithces) and a capacitor (as a data storage
element) create a memory cell, which
represents a single bit of data.
Memory cells are etched onto a silicon wafer in a series
of columns (bitlines) and rows (wordlines), known as an array. The intersection of a
column and row constitutes the address of the memory cell
9Figure 4-46). Each memory cell has a unique address that can be found by
counting across columns and then counting down by row. The address of a
character consists of a series of memory cell addresses put together. Most DRAM
chips actually have arrays that are 16 rows deep.
To write data to RAM,the processor sends the memory
controller the address of a memory cell in which to store data. The memory controller organizes
the reuest and sends the column and row address in an eletrical charge along
the appropriate address linesm which are very thin electrical lines
etched into the RAM chip. This causes the transistors along those address lines
to close.
These transistors act as a switch to control the flow of
electrical current in an either closed or open circuit. While the transistors
are closed,the software sends bursts of electricity along selected data lines.
When the electrical charge traveling down the data line reaches an address line
where a transistor is closed,the charge flows through the closed transistor and
charges the capacitor.
A capacitor works as electronic storage that holds and
electrical charge. Each charged capacitor along the address line represents a 1
bit. An uncharged capacitor represents a 0 bit. The combination of 1s and 0s
from eight data lines forms a single byte of data.
The capacitors used in dynamic RAM, however, lose their
electrical charge. The processor or memory controller continuously has to
recharge all of the capacitors holding a charge (a 1 bit) before the capacitors
discharges. During this refresh operation,which happesn
automatically thousands of times per second,the memory controller reads memory
and then immediately rewrites it. This refresh operation is what gives dynamic
RAM its name. Dynamic RAM has to be refreshed continually, or it loses the
charges that represent bits of data. A specialized circuit called a counter
tracks the refresh sequence to ensue that all of the rows are refreshed.
The process of reading data from RAM uses a similar, but
reverse,series of steps. When the processor gets the next intruction it is to
perform, the instruction may contain the address of a memory cell from which to
read data. This address is sent to the memory controller sends the column and
row address in an electrical charge down the appropriate address lines.
This electrical charge causes the transistors along the
address line to close. At every point along address line where a capacitor is
holding a charge,the capacitor discharges through the circuit created by the
closed transistors, sending electrical charges along the data lines.
A specialized circuit called a sense amplifier datermines
and amplifies the level of charge in the capacitor. A capacitor charge over a
certain voltage level represents the binary value 1;a capacitor charge below
that level represents a 0. The sensed and amplified value is sent back down the
address line to the processor.
As long as a computer is running,data continuously is
being written to and read from RAM. As soon as you shut down a computer,RAM
loses its data. The next time you turn on a computer, operating system files
and other data are again loaded into RAM and the read/write process stars all
over.
A few articles about The Genius of Memory : Transistors,
Capacitors, and Electricity, Hopefully Usefull