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:
Each of these devices uses different types of memory in different ways!
In this edition of
HowStuffWorks, you'll learn why there are so many different types
of memory and what all of the terms mean.
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
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
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
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
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
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 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
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
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
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
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
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
gate array known as a bistable multivibrator that switches, or
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
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
storage devices such as CompactFlash or SmartMedia cards are also forms of
nonvolatile memory. See the links below for information on these types of