When you think about it, it's amazing how many different
types of electronic memory you encounter in daily life. Many
of them have become an integral part of our vocabulary:
You already know that the computer in
front of you has memory. What you may not know is that most of
the electronic items you use every day have some form of
memory also. Here are just a few examples of the many items
that use memory:
Memory Basics Although memory is technically
any form of electronic storage, it is used most often to
identify fast, temporary forms of storage. If your computer's
CPU
had to constantly access the hard
drive to retrieve every piece of data it needs, it would
operate very slowly. When the information is kept in memory,
the CPU can access it much more quickly. Most forms of memory
are intended to store data temporarily.
As you can see in the diagram above, the CPU accesses
memory according to a distinct hierarchy. Whether it comes
from permanent storage (the hard drive) or input (the keyboard),
most data goes in random access memory (RAM) first.
The CPU then stores pieces of data it will need to access,
often in a cache, and maintains certain special
instructions in the register. We'll talk about cache
and registers later.
All of the components in your computer, such as the CPU,
the hard drive and the operating
system, work together as a team, and memory is one of the
most essential parts of this team. From the moment you turn
your computer on until the time you shut it down, your CPU is
constantly using memory. Let's take a look at a typical
scenario:
You turn the computer on.
The computer loads data from read-only memory (ROM) and
performs a power-on self-test (POST) to make sure all
the major components are functioning properly. As part of
this test, the memory controller checks all of the
memory addresses with a quick read/write operation to
ensure that there are no errors in the memory chips.
Read/write means that data is written to a bit and
then read from that bit.
The computer loads the basic input/output system
(BIOS)
from ROM. The BIOS provides the most basic information about
storage devices, boot sequence, security, Plug and
Play (auto device recognition) capability and a few
other items.
The computer loads the operating system (OS) from
the hard drive into the system's RAM. Generally, the
critical parts of the operating
system are maintained in RAM as long as the computer is
on. This allows the CPU to have immediate access to the
operating system, which enhances the performance and
functionality of the overall system.
When you open an application, it is loaded into
RAM. To
conserve RAM usage, many applications load only the
essential parts of the program initially and then load other
pieces as needed.
After an application is loaded, any files that
are opened for use in that application are loaded into RAM.
When you save a file and close the
application, the file is written to the specified storage
device, and then it and the application are purged from RAM.
In the list above, every time something is loaded or
opened, it is placed into RAM. This simply means that it has
been put in the computer's temporary storage area so
that the CPU can access that information more easily. The CPU
requests the data it needs from RAM, processes it and writes
new data back to RAM in a continuous cycle. In most
computers, this shuffling of data between the CPU and RAM
happens millions of times every second. When an application is
closed, it and any accompanying files are usually
purged (deleted) from RAM to make room for new data. If
the changed files are not saved to a permanent storage device
before being purged, they are lost.
The Need for Speed One common question about
desktop computers that comes up all the time is, "Why does a
computer need so many memory systems?" A typical computer has:
Why so many? The answer to this
question can teach you a lot about memory!
Fast, powerful CPUs need quick and easy access to large
amounts of data in order to maximize their performance. If the
CPU cannot get to the data it needs, it literally stops and
waits for it. Modern CPUs running at speeds of about 1
gigahertz can consume massive amounts of data --
potentially billions of bytes per
second. The problem that computer designers face is that
memory that can keep up with a 1-gigahertz CPU is extremely
expensive -- much more expensive than anyone can afford
in large quantities.
Computer designers have solved the cost problem by
"tiering" memory -- using expensive memory in small
quantities and then backing it up with larger quantities of
less expensive memory.
The cheapest form of read/write memory in wide use today is
the hard
disk. Hard disks provide large quantities of
inexpensive, permanent storage. You can buy hard disk space
for pennies per megabyte, but it can take a good bit of time
(approaching a second) to read a megabyte off a hard disk.
Because storage space on a hard disk is so cheap and
plentiful, it forms the final stage of a CPUs memory
hierarchy, called virtual
memory.
The next level of the hierarchy is RAM. We discuss
RAM in detail in How RAM Works,
but several points about RAM are important here.
The bit size of a CPU tells you how many bytes of
information it can access from RAM at the same time. For
example, a 16-bit CPU can process 2 bytes at a time (1 byte =
8 bits, so 16 bits = 2 bytes), and a 64-bit CPU can process 8
bytes at a time.
Megahertz (MHz) is a measure of a CPU's processing
speed, or clock cycle, in millions per second. So, a
32-bit 800-MHz Pentium III can potentially process 4 bytes
simultaneously, 800 million times per second (possibly more
based on pipelining)! The goal of the memory system is to meet
those requirements.
A computer's system RAM alone is not fast enough to match
the speed of the CPU. That is why you need a cache (see
the next section). However, the faster RAM is, the better.
Most chips today operate with a cycle rate of 50 to 70
nanoseconds. The read/write speed is typically a function of
the type of RAM used, such as DRAM, SDRAM, RAMBUS. We will
talk about these various types of memory later.
System RAM speed is controlled by bus width and
bus speed. Bus width refers to the number of bits that
can be sent to the CPU simultaneously, and bus speed refers to
the number of times a group of bits can be sent each second. A
bus cycle occurs every time data travels from memory to
the CPU. For example, a 100-MHz 32-bit bus is theoretically
capable of sending 4 bytes (32 bits divided by 8 = 4 bytes) of
data to the CPU 100 million times per second, while a 66-MHz
16-bit bus can send 2 bytes of data 66 million times per
second. If you do the math, you'll find that simply changing
the bus width from 16 bits to 32 bits and the speed from 66
MHz to 100 MHz in our example allows for three times as much
data (400 million bytes versus 132 million bytes) to pass
through to the CPU every second.
In reality, RAM doesn't usually operate at optimum speed.
Latency changes the equation radically. Latency refers
to the number of clock cycles needed to read a bit of
information. For example, RAM rated at 100 MHz is capable of
sending a bit in 0.00000001 seconds, but may take 0.00000005
seconds to start the read process for the first bit. To
compensate for latency, CPUs uses a special technique called
burst mode.
Burst mode depends on the expectation that data requested
by the CPU will be stored in sequential memory cells.
The memory controller anticipates that whatever the CPU is
working on will continue to come from this same series of
memory addresses, so it reads several consecutive bits of data
together. This means that only the first bit is subject to the
full effect of latency; reading successive bits takes
significantly less time. The rated burst mode of memory
is normally expressed as four numbers separated by dashes. The
first number tells you the number of clock cycles needed to
begin a read operation; the second, third and fourth numbers
tell you how many cycles are needed to read each consecutive
bit in the row, also known as the wordline. For
example: 5-1-1-1 tells you that it takes five cycles to read
the first bit and one cycle for each bit after that.
Obviously, the lower these numbers are, the better the
performance of the memory.
Burst mode is often used in conjunction with
pipelining, another means of minimizing the effects of
latency. Pipelining organizes data retrieval into a sort of
assembly-line process. The memory controller simultaneously
reads one or more words from memory, sends the current word or
words to the CPU and writes one or more words to memory cells.
Used together, burst mode and pipelining can dramatically
reduce the lag caused by latency.
So why wouldn't you buy the fastest, widest memory you can
get? The speed and width of the memory's bus should match the
system's bus. You can use memory designed to work at 100 MHz
in a 66-MHz system, but it will run at the 66-MHz speed of the
bus so there is no advantage, and 32-bit memory won't fit on a
16-bit bus.
Cache and Registers Even with a wide and
fast bus, it still takes longer for data to get from the
memory card to the CPU than it takes for the CPU to actually
process the data. Caches are designed to alleviate this
bottleneck by making the data used most often by the CPU
instantly available. This is accomplished by building a small
amount of memory, known as primary or level 1
cache, right into the CPU. Level 1 cache is very small,
normally ranging between 2 kilobytes (KB) and 64 KB.
The secondary or level 2 cache typically
resides on a memory card located near the CPU. The level 2
cache has a direct connection to the CPU. A dedicated
integrated circuit on the motherboard,
the L2 controller, regulates the use of the level 2
cache by the CPU. Depending on the CPU, the size of the level
2 cache ranges from 256 KB to 2 megabytes (MB). In most
systems, data needed by the CPU is accessed from the cache
approximately 95 percent of the time, greatly reducing the
overhead needed when the CPU has to wait for data from the
main memory.
Some inexpensive systems dispense with the level 2 cache
altogether. Many high performance CPUs now have the level 2
cache actually built into the CPU chip itself. Therefore, the
size of the level 2 cache and whether it is onboard (on
the CPU) is a major determining factor in the performance of a
CPU. For more details on caching, see How Caching
Works.
A particular type of RAM, static
random access memory (SRAM), is used primarily for cache.
SRAM uses multiple transistors, typically four to six, for
each memory cell. It has an external
gate array known as a bistable multivibrator that
switches, or flip-flops,
between two states. This means that it does not have to be
continually refreshed like DRAM. Each cell will maintain its
data as long as it has power. Without the need for constant
refreshing, SRAM can operate extremely quickly. But the
complexity of each cell make it prohibitively expensive for
use as standard RAM.
The SRAM in the cache can be asynchronous or
synchronous. Synchronous SRAM is designed to exactly
match the speed of the CPU, while asynchronous is not. That
little bit of timing makes a difference in performance.
Matching the CPU's clock speed is a good thing, so always look
for synchronized SRAM. (For more information on the various
types of RAM, see How RAM
Works.)
The final step in memory is the registers. These are
memory cells built right into the CPU that contain specific
data needed by the CPU, particularly the arithmetic and
logic unit (ALU). An integral part of the CPU itself, they
are controlled directly by the compiler that sends information
for the CPU to process. See How
Microprocessors Work for details on registers.
Types of Memory Memory
can be split into two main categories: volatile and
nonvolatile. Volatile memory loses any data as soon as
the system is turned off; it requires constant power to remain
viable. Most types of RAM fall into this category.
Nonvolatile memory does not lose its data when the system
or device is turned off. A number of types of memory fall into
this category. The most familiar is ROM, but Flash
memory storage devices such as CompactFlash or
SmartMedia cards are also forms of nonvolatile memory. See the
links below for information on these types of memory.
