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How Computer Memory Works
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:
RAM
ROM
Cache
Dynamic RAM
Static RAM
Flash memory
Memory sticks
Volatile memory
Virtual memory
Video memory
BIOS
SIMM
DIMM
EDO RAM
RAMBUS
DIP
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:
In this article
we will start with an overview that helps you understand the
different types of memory in your computer.
Memory Is
at the Heart of Your Computer
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 data are kept in memory,
the CPU can access data 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 each of these 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:
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You turn
the computer on.
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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.
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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. We discuss the BIOS in more detail in
How Flash Memory Works.
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The computer
loads the operating system (OS) from the hard drive
into the system’s RAM. Generally, the critical parts of
the OS are maintained in RAM as long as the computer is
on. This allows the CPU to have immediate access to the
OS, which enhances the performance and functionality of
the overall system.
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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.
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After an
application is loaded, any files that are opened for use
in that application are loaded into RAM.
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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 like 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 another
article in this series, 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 divided by
8 = 2), 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) of data to the
CPU one hundred million times per second while a 66 MHz 16-bit
bus can send 2 bytes of data sixty-six million times per second.
If you do the math, you’ll find that simply changing the bus
width from 16-bit to 32-bit 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 .00000001/second
but may take .00000005/second 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 must match the system’s bus.
You can’t use memory designed to work at 100 MHz in a 66 MHz
system or 32-bit memory with a 16-bit CPU.
Cache and
the 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 2K and 64K.
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 256K to 2MB. 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, please read 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 fast.
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.
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. Other articles in this
series will cover these types of memory.
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