ABOUT RAM
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.
The capacitor in a dynamic RAM memory cell is like a leaky bucket.
It needs to be refreshed periodically or it will discharge to 0.
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.
In this article, you'll learn all about what RAM is, what kind you should buy and how
to install it. See the next page to learn more about dynamic RAM and memory cels.
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 works differently from DRAM. We'll look at how in the next section.
Static RAM
Static RAM uses a completely different technology. In static RAM, a form of flip-flop
holds each bit of memory (see How Boolean Logic Works for details on flip-flops). 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. STypes of RAM
The following are some common types of RAM:
•SRAM: Static random access memory uses multiple transistors, typically four to six,
for each memory cell but doesn't have a capacitor in each cell. It is used primarily for
cache.
•DRAM: Dynamic random access memory has memory cells with a paired transistor
and capacitor requiring constant refreshing.
•FPM DRAM: Fast page mode dynamic random access memory was the original form
of DRAM. It waits through the entire process of locating a bit of data by column and
row and then reading the bit before it starts on the next bit. Maximum transfer rate
to L2 cache is approximately 176 MBps.
•EDO DRAM: Extended data-out dynamic random access memory does not wait for
all of the processing of the first bit before continuing to the next one. As soon as
the address of the first bit is located, EDO DRAM begins looking for the next bit. It
is about five percent faster than FPM. Maximum transfer rate to L2 cache is
approximately 264 MBps.
•SDRAM: Synchronous dynamic random access memory takes advantage of the burst
mode concept to greatly improve performance. It does this by staying on the row
containing the requested bit and moving rapidly through the columns, reading each
bit as it goes. The idea is that most of the time the data needed by the CPU will be
in sequence. SDRAM is about five percent faster than EDO RAM and is the most
common form in desktops today. Maximum transfer rate to L2 cache is approximately
528 MBps.
•DDR SDRAM: Double data rate synchronous dynamic RAM is just like SDRAM
except that is has higher bandwidth, meaning greater speed. Maximum transfer rate
to L2 cache is approximately 1,064 MBps (for DDR SDRAM 133 MHZ).
•RDRAM: Rambus dynamic random access memory is a radical departure from the
previous DRAM architecture. Designed by Rambus, RDRAM uses a Rambus in-line
memory module (RIMM), which is similar in size and pin configuration to a standard
DIMM. What makes RDRAM so different is its use of a special high-speed data bus
called the Rambus channel. RDRAM memory chips work in parallel to achieve a data
rate of 800 MHz, or 1,600 MBps. Since they operate at such high speeds, they
generate much more heat than other types of chips. To help dissipate the excess
heat Rambus chips are fitted with a heat spreader, which looks like a long thin wafer.
Just like there are smaller versions of DIMMs, there are also SO-RIMMs, designed for
notebook computers.
•Credit Card Memory: Credit card memory is a proprietary self-contained DRAM
memory module that plugs into a special slot for use in notebook computers.
•PCMCIA Memory Card: Another self-contained DRAM module for notebooks, cards
of this type are not proprietary and should work with any notebook computer whose
system bus matches the memory card's configuration.
•CMOS RAM: CMOS RAM is a term for the small amount of memory used by your
computer and some other devices to remember things like hard disk settings -- see
Why does my computer need a battery? for details. This memory uses a small battery
to provide it with the power it needs to maintain the memory contents.
•VRAM: VideoRAM, also known as multiport dynamic random access memory
(MPDRAM), is a type of RAM used specifically for video adapters or 3-D accelerators.
The "multiport" part comes from the fact that VRAM normally has two independent
access ports instead of one, allowing the CPU and graphics processor to access the
RAM simultaneously. VRAM is located on the graphics card and comes in a variety of
formats, many of which are proprietary. The amount of VRAM is a determining factor
in the resolution and color depth of the display. VRAM is also used to hold
graphics-specific information such as 3-D geometry data and texture maps. True
multiport VRAM tends to be expensive, so today, many graphics cards use SGRAM
(synchronous graphics RAM) instead. Performance is nearly the same, but SGRAM is
cheaper.ince we know that a byte has 8 bits, we need to divide our result of 128 by
8. Our result is 16 megabytes!
In the next section we'll look at some other common types of RAM.
Memory Modules
The type of board and connector used for RAM in desktop computers has evolved
over the past few years. The first types were proprietary, meaning that different
computer manufacturers developed memory boards that would only work with their
specific systems. Then came SIMM, which stands for single in-line memory module.
This memory board used a 30-pin connector and was about 3.5 x .75 inches in size
(about 9 x 2 cm). In most computers, you had to install SIMMs in pairs of equal
capacity and speed. This is because the width of the bus is more than a single SIMM.
For example, you would install two 8-megabyte (MB) SIMMs to get 16 megabytes
total RAM. Each SIMM could send 8 bits of data at one time, while the system bus
could handle 16 bits at a time. Later SIMM boards, slightly larger at 4.25 x 1 inch
(about 11 x 2.5 cm), used a 72-pin connector for increased bandwidth and allowed for
up to 256 MB of RAM.
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.
The capacitor in a dynamic RAM memory cell is like a leaky bucket.
It needs to be refreshed periodically or it will discharge to 0.
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.
In this article, you'll learn all about what RAM is, what kind you should buy and how
to install it. See the next page to learn more about dynamic RAM and memory cels.
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 works differently from DRAM. We'll look at how in the next section.
Static RAM
Static RAM uses a completely different technology. In static RAM, a form of flip-flop
holds each bit of memory (see How Boolean Logic Works for details on flip-flops). 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. STypes of RAM
The following are some common types of RAM:
•SRAM: Static random access memory uses multiple transistors, typically four to six,
for each memory cell but doesn't have a capacitor in each cell. It is used primarily for
cache.
•DRAM: Dynamic random access memory has memory cells with a paired transistor
and capacitor requiring constant refreshing.
•FPM DRAM: Fast page mode dynamic random access memory was the original form
of DRAM. It waits through the entire process of locating a bit of data by column and
row and then reading the bit before it starts on the next bit. Maximum transfer rate
to L2 cache is approximately 176 MBps.
•EDO DRAM: Extended data-out dynamic random access memory does not wait for
all of the processing of the first bit before continuing to the next one. As soon as
the address of the first bit is located, EDO DRAM begins looking for the next bit. It
is about five percent faster than FPM. Maximum transfer rate to L2 cache is
approximately 264 MBps.
•SDRAM: Synchronous dynamic random access memory takes advantage of the burst
mode concept to greatly improve performance. It does this by staying on the row
containing the requested bit and moving rapidly through the columns, reading each
bit as it goes. The idea is that most of the time the data needed by the CPU will be
in sequence. SDRAM is about five percent faster than EDO RAM and is the most
common form in desktops today. Maximum transfer rate to L2 cache is approximately
528 MBps.
•DDR SDRAM: Double data rate synchronous dynamic RAM is just like SDRAM
except that is has higher bandwidth, meaning greater speed. Maximum transfer rate
to L2 cache is approximately 1,064 MBps (for DDR SDRAM 133 MHZ).
•RDRAM: Rambus dynamic random access memory is a radical departure from the
previous DRAM architecture. Designed by Rambus, RDRAM uses a Rambus in-line
memory module (RIMM), which is similar in size and pin configuration to a standard
DIMM. What makes RDRAM so different is its use of a special high-speed data bus
called the Rambus channel. RDRAM memory chips work in parallel to achieve a data
rate of 800 MHz, or 1,600 MBps. Since they operate at such high speeds, they
generate much more heat than other types of chips. To help dissipate the excess
heat Rambus chips are fitted with a heat spreader, which looks like a long thin wafer.
Just like there are smaller versions of DIMMs, there are also SO-RIMMs, designed for
notebook computers.
•Credit Card Memory: Credit card memory is a proprietary self-contained DRAM
memory module that plugs into a special slot for use in notebook computers.
•PCMCIA Memory Card: Another self-contained DRAM module for notebooks, cards
of this type are not proprietary and should work with any notebook computer whose
system bus matches the memory card's configuration.
•CMOS RAM: CMOS RAM is a term for the small amount of memory used by your
computer and some other devices to remember things like hard disk settings -- see
Why does my computer need a battery? for details. This memory uses a small battery
to provide it with the power it needs to maintain the memory contents.
•VRAM: VideoRAM, also known as multiport dynamic random access memory
(MPDRAM), is a type of RAM used specifically for video adapters or 3-D accelerators.
The "multiport" part comes from the fact that VRAM normally has two independent
access ports instead of one, allowing the CPU and graphics processor to access the
RAM simultaneously. VRAM is located on the graphics card and comes in a variety of
formats, many of which are proprietary. The amount of VRAM is a determining factor
in the resolution and color depth of the display. VRAM is also used to hold
graphics-specific information such as 3-D geometry data and texture maps. True
multiport VRAM tends to be expensive, so today, many graphics cards use SGRAM
(synchronous graphics RAM) instead. Performance is nearly the same, but SGRAM is
cheaper.ince we know that a byte has 8 bits, we need to divide our result of 128 by
8. Our result is 16 megabytes!
In the next section we'll look at some other common types of RAM.
Memory Modules
The type of board and connector used for RAM in desktop computers has evolved
over the past few years. The first types were proprietary, meaning that different
computer manufacturers developed memory boards that would only work with their
specific systems. Then came SIMM, which stands for single in-line memory module.
This memory board used a 30-pin connector and was about 3.5 x .75 inches in size
(about 9 x 2 cm). In most computers, you had to install SIMMs in pairs of equal
capacity and speed. This is because the width of the bus is more than a single SIMM.
For example, you would install two 8-megabyte (MB) SIMMs to get 16 megabytes
total RAM. Each SIMM could send 8 bits of data at one time, while the system bus
could handle 16 bits at a time. Later SIMM boards, slightly larger at 4.25 x 1 inch
(about 11 x 2.5 cm), used a 72-pin connector for increased bandwidth and allowed for
up to 256 MB of RAM.
From the top: SIMM, DIMM and SODIMM memory modules
As processors grew in speed and bandwidth capability, the industry adopted a new
standard in dual in-line memory module (DIMM). With a whopping 168-pin or 184-pin
connector and a size of 5.4 x 1 inch (about 14 x 2.5 cm), DIMMs range in capacity
from 8 MB to 1 GB per module and can be installed singly instead of in pairs. Most
PC memory modules and the modules for the Mac G5 systems operate at 2.5 volts,
while older Mac G4 systems typically use 3.3 volts. Another standard, Rambus in-line
memory module (RIMM), is comparable in size and pin configuration to DIMM but
uses a special memory bus to greatly increase speed.
Many brands of notebook computers use proprietary memory modules, but several
manufacturers use RAM based on the small outline dual in-line memory module
(SODIMM) configuration. SODIMM cards are small, about 2 x 1 inch (5 x 2.5 cm), and
have 144 or 200 pins. Capacity ranges from 16 MB to 1 GB per module. To conserve
space, the Apple iMac desktop computer uses SODIMMs instead of the traditional
DIMMs. Sub-notebook computers use even smaller DIMMs, known as MicroDIMMs,
which have either 144 pins or 172 pins.
Most memory available today is highly reliable. Most systems simply have the
memory controller check for errors at start-up and rely on that. Memory chip with
built-in error-checking typically use a method known as parity to check for errors.
memory controller check for errors at start-up and rely on that. Memory chip with
built-in error-checking typically use a method known as parity to check for errors.
Parity chips have an extra bit for every 8 bits of data. The way parity works is simple.
Let's look at even parity first.
When the 8 bits in a byte receive data, the chip adds up the total number of 1s. If the
total number of 1s is odd, the parity bit is set to 1. If the total is even, the parity bit
is set to 0. When the data is read back out of the bits, the total is added up again
and compared to the parity bit. If the total is odd and the parity bit is 1, then the
data is assumed to be valid and is sent to the CPU. But if the total is odd and the
parity bit is 0, the chip knows that there is an error somewhere in the 8 bits and
dumps the data. Odd parity works the same way, but the parity bit is set to 1 when
the total number of 1s in the byte are even.
The problem with parity is that it discovers errors but does nothing to correct them.
If a byte of data does not match its parity bit, then the data are discarded and the
system tries again. Computers in critical positions need a higher level of fault
tolerance. High-end servers often have a form of error-checking known as
error-correction code (ECC). Like parity, ECC uses additional bits to monitor the data
in each byte. The difference is that ECC uses several bits for error checking -- how
many depends on the width of the bus -- instead of one. ECC memory uses a special
algorithm not only to detect single bit errors, but actually correct them as well. ECC
memory will also detect instances when more than one bit of data in a byte fails.
Such failures are very rare, and they are not correctable, even with ECC.
The majority of computers sold today use nonparity memory chips. These chips do
not provide any type of built-in error checking, but instead rely on the memory
controller for error detection.
