PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

5.2 Memory Access Methods

PC memory may use the following access
methods:

Asynchronous
DRAM


, which
was used in all PCs until the late 1990s, uses a window of fixed
minimum duration to determine when operations may occur. If the CPU
has transferred data while a window is open, and if a subsequent
clock cycle occurs while that window remains open, the CPU cannot
transfer additional data until the next window opens, thereby wasting
that clock cycle. Asynchronous operation forces the CPU to conform to
a fixed schedule for transferring data, rather than doing so whenever
it wishes. Asynchronous DRAM is available in the following types:

FPM was
commonly used on 486 and earlier systems, and may be installed in
early Pentium systems. FPM is not supported by recent chipsets.
Although you can migrate FPM DRAM from an old Socket 5 or Socket 7
system to a newer Socket 7 system, it is good for little else. You
may be able to install surplus FPM DRAM in your laser printer.

EDO, also sometimes called Hyper
Page Mode DRAM, is marginally faster than FPM, is still
available in all common package types, and was commonly installed on
new systems until late 1998. EDO DRAM now costs so much that it is
often less expensive to replace the existing motherboard, processor,
and memory with current products than to buy EDO memory. That said,
you may be able to upgrade the memory in an EDO-based system
economically. Many EDO-based systems can also use SDRAM DIMMs, which
are faster, much less expensive, and much more readily available. To
upgrade the memory in such systems, replace the existing EDO memory
with compatible SDRAM DIMMs.

BEDO slightly improved upon EDO, but
was inferior to SDRAM, which was introduced at about the same time,
and so never became popular. If you have a BEDO-based system, follow
the upgrade advice given for an EDO-based system.

All forms of asynchronous DRAM are now obsolete. Although
asynchronous DRAM is still available, it costs so much per megabyte
that it never makes sense to buy it. For example, in July 2003, the
price per megabyte of asynchronous DRAM SIMMs was five to 25 times
that of SDRAM DIMMs, depending on capacity and type.

A system or motherboard that accepts only asynchronous DRAM is too old to be upgraded economically. If you have such a system, we recommend you upgrade its memory only if you can salvage compatible memory from discarded systems. If you have a late-model Pentium or Pentium Pro system that also accepts SDRAM and is still performing useful service (perhaps as a fax server or other appliance), we suggest you pull all of the asynchronous DRAM and replace it with compatible SDRAM.

Synchronous DRAM
, also called SDRAM,
shares a common clock reference with the CPU. No window is needed
because the CPU and memory are slaved together, allowing the CPU to
transfer data to and from memory whenever it wishes to do so, instead
of requiring the CPU to await an arbitrary window. For links to
formal SDRAM standards, see the Intel SDRAM Specifications page at
http://developer.intel.com/technology/memory/pcsdram/spec/index.

SDRAM takes one of the following forms:

Ordinary SDRAM, sometimes called JEDEC SDRAM or PC66 SDRAM to
differentiate it from PC100 SDRAM and PC133 SDRAM. PC66 SDRAM was
formerly less expensive than PC100 or PC133 SDRAM, but as the price
of those faster variants declined to near that of PC66 SDRAM, the
demand for PC66 SDRAM plummeted. PC66 SDRAM is now hard to find and
may cost more than PC100 or PC133 SDRAM. Because PC133 SDRAM can be
used on nearly any system running a 133 MHz or slower FSB, buying
JEDEC SDRAM never makes sense, even for systems that run memory at 66
MHz. PC66 SDRAM salvaged from an older system can be used in any
system that runs a 66 MHz FSB, including those running older-model
Celeron or Pentium II processors.

SDRAM that complies with the Intel PC100 specification, and is rated
for use on a 100 MHz FSB. Like PC66 SDRAM, PC100 SDRAM is
obsolescent, and may actually cost more than faster PC133 SDRAM.

SDRAM that complies with the Intel PC133 specification, and is rated
for use on a 133 MHz FSB. PC133 SDRAM costs little or no more than
PC100 SDRAM, operates properlyalthough only at the lower
speedin nearly all systems designed to use PC66 or PC100
SDRAM, and is usually the best choice when you're
buying SDRAM memory, even for a 66 MHz or 100 MHz FSB system. PC133
SDRAM is commonly available in two variants which vary only in CAS
latency. CAS-3 PC133 SDRAM is the more common
form, and is what you will receive if you do not specify otherwise.
CAS-2 PC133 SDRAM has lower latency, is
therefore slightly faster in a motherboard that can take advantage of
it, and costs only a few cents more per megabyte.

PC133 SDRAM is obsolescent. Even entry-level systems now use some
variant of DDR-SDRAM, described in the next section, so PC133 SDRAM
is now useful only to upgrade older systems. In those systems, PC133
SDRAM provides memory throughput that is well matched to the
bandwidth of the processor.

Some memory packagers sell so-called PC166 SDRAM. In fact, there is no such standard, and these modules are used primarily by overclockers who run the FSB at 166 MHz rather than 133 MHz. We suggest that you avoid both running your FSB at a higher than intended speed and using PC166 SDRAM.

Double Data Rate SDRAM (DDR-SDRAM) doubles the amount of data
transferred per clock cycle, and thereby effectively doubles peak
memory bandwidth. DDR-SDRAM is an evolutionary improvement of
standard SDRAM, which is now sometimes called Single Data Rate SDRAM
or SDR-SDRAM to differentiate it. Because DDR-SDRAM costs essentially
the same to produce as SDR-SDRAM, it sells for about the same price.

The chips used to produce DDR-SDRAM DIMMs are named for their
operating speed. For example, 100 MHz chips are double-pumped to 200
MHz, and so are called PC200 (or DDR200) chips. (We use the
"PCxxx" speed nomenclature rather
than the "DDRxxx" nomenclature for
first generation DDR memory, although
"DDRxxx" is also commonly used.)
Similarly, chips that operate at 133 MHz are called PC266 chips,
those that operate at 166 MHz are called PC333 chips, and those that
operate at 200 MHz are called PC400 chips. (In fact, only PC200 and
PC266 are formal standards, although memory makers produce so-called
PC333 and PC400 chips based on de facto standards.)

Unlike SDR-SDRAM DIMMs, which are designated by their chip speeds,
DDR-SDRAM DIMMs are designated by their throughput. Their data path
is 64 bits (8 bytes) wide. So, for example, a DDR-SDRAM DIMM that
uses PC200 chips transfers 8 bytes 200 million times per second, for
a total throughput of 1,600 million bytes/second and is called a
PC1600 DIMM. Similarly, DDR-SDRAM DIMMs that use PC266 chips are
labeled PC2100, and those that use PC333 chips are labeled PC2700.
Based on chip speed, PC2700 modules are actually PC2667, but no one
uses that name. Some early DIMMs built with PC333 chips were labeled
PC2600, but all memory makers now use the PC2700 designation for
obvious reasons. DDR-SDRAM DIMMs that use PC400 chips are labeled
PC3200.

PC2700 DDR-SDRAM is now the dominant mainstream memory technology,
although the April 2003 introduction of the Intel 875P chipset has
kickstarted the market for PC3200 memory. All mainstream AMD and
Intel processors and chipsets now support PC2700 or faster DDR-SDRAM,
and there is little reason to choose any slower type of memory.
PC1600 DDR-SDRAM was economically obsoleted by mid-2002, when the
price of PC2100 memory fell to the same level as PC1600. By early
2003, the same fate befell PC2100 memory, as the price of PC2700
memory fell to PC2100 levels.

Used with chipsets such as the Intel 875P that are optimized for it, PC3200 memory is faster than PC2700 memory, but not as fast as one might expect. In a motherboard that matches processor bandwidth to PC3200 memory bandwidth, memory performance may increase by 5% to 8%, rather than the nominal 18.5%. In a motherboard that mismatches processor bandwidth to PC3200 bandwidth, the performance increase is much smaller, and may even be negative. For example, in early 2003 we used an ASUS A7N8X n Force2 motherboard with dual-channel DDR memory support to test Corsair CL2 PC3200 DIMMs against Crucial CL2.5 PC2700 DIMMs. We found that, although the PC3200 DIMMs did yield slightly faster benchmark numbers than PC2700 DIMMs, the practical advantage of PC3200 was nil. We expect PC3200 to be the fastest DDR-SDRAM that will ever be produced in volume. Mass-produced PC3200 modules push the electrical and mechanical limits of current technology. Some memory makers, notably Corsair, rigorously test chips to isolate those fast enough to operate as PC433 and use those chips to produce small runs of what amount to handcrafted PC3500 DIMMs. These PC3500 modules are very expensive and provide only minor throughput improvements relative to PC3200 modules.

By early 2003, the original DDR-SDRAM technology was fast approaching
its limits. As AMD and Intel transition to higher FSB speeds,
DDR-SDRAM will be hard-pressed to keep pace. Current DDR technology
tops out at PC3200. Dual-channel DDR chipsets using PC3200 memory
limit peak throughput to 6400 MB/s. That is sufficient for now, but
as FSB speeds increase from 400 MHz to 533 MHz, 800 MHz, and beyond,
even dual-channel DDR-SDRAM will be challenged to keep up with
increases in processor bandwidth.

The long-term solution is DDR-II SDRAM. DDR-II
incorporates a series of evolutionary improvements on DDR-I
technology, including increased performance and bandwidth, reduced
cost, lower power consumption, and improved packaging. DDR-II basic
device timing and page size are compatible with DDR-I, and because
the DDR-II command set is a superset of the DDR-I command set, a
DDR-II controller can also control DDR-I modules.

DDR-II DIMMs use a new 232-pin connector, and it is likely that DDR-I
modules will be produced with that connector to facilitate the
transition from DDR-I to DDR-II modules. DDR-II chips will initially
ship in DDR400 and DDR533 variants, which will be used to produce
PC3200 and PC4300 DDR-II DIMMs. We expect that DDR-II will eventually
be produced in DDR600, DDR667, and DDR800 variants, which will be
used to produce PC4800, PC5300, and PC6400 modules, respectively.

Although some high-performance video cards currently use DDR-II, we
do not expect DDR-II to become standard in desktop PCs until late
2004 or 2005. We think PC2700 DDR-I modules in single- and
dual-channel configurations will remain the standard until DDR-II
chipsets ship in volume in 2004. DDR-II will at first be used in
high-end systems, and will migrate to midrange and entry-level
systems throughout 2004 and 2005.

On the theory that if twice as fast is good, four times as fast must
be better, the QBM Alliance is developing Quad Band Memory (QBM),
sometimes called Quad Data Rate SDRAM (QDR-SDRAM). The QBM Alliance
roster boasts many second- and third-tier companies, including Acer
Laboratories, Acuid, Avant Technology, CST, Denali Software,
Integrated Circuit Systems, Kentron Technologies, Netlist, Peripheral
Enhancements, PNY Technologies, SiS, SiSoft, ST Microelectronics,
Terarecon, and VIA Technologies. Alas, the key first-tier
chipset/motherboard companiesnotably Intel and AMDare
not QBM Alliance members. Neither have major memory companies such as
Crucial/Micron, Kingston, or Samsung chosen to join the QBM Alliance.
Without real support from those companies, we don't
think the QBM Alliance can hope to establish a viable standard.

QBM is based on DDR-I technology, but quad-pumps rather than
double-pumps the data channel. Although we have been wrong before, we
expect QBM to fail for both technical and marketing reasons.
Technically, QBM offers little real advantage over dual-channel
PC2700 or PC3200 DDR-SDRAM, which is already widely supported by
chipsets for Intel and AMD processors. That means memory makers have
no incentive to produce yet another type of module that would sell in
relatively small numbers, making it difficult to recoup their startup
costs. From a marketing standpoint, QBM is almost doomed from the
start. At best, QBM will garner support from second- and third-tier
companies such as VIA Technologies and Kentron, which means that QBM
will be perceived by consumers as a second-rate solution. Meanwhile,
Intel and AMD will continue backing DDR-I and DDR-II, leaving only
scraps for QBM.

SDRAM uses separate address, control, and data busses, each with many
lines. Managing these wide parallel busses limits performance.
Protocol-based DRAM instead uses a narrow, very fast channel with
protocols that manage address, control, and data information. Rambus
RDRAM, a proprietary RAM standard developed jointly by Intel and
Rambus, is the sole surviving type of protocol-based RAM.

There are three types of Rambus memory, called Base Rambus,
Concurrent Rambus, and Direct Rambus. The first two are obsolescent,
and are used only in devices such as game consoles. All Rambus memory
used in PCs is Direct Rambus memory. RDRAM is available in four
speeds, designated PC600, PC700, PC800, and PC1066, although only
PC800 and PC1066 are used in current systems. As with DDR-SDRAM,
RDRAM modules are named according to their throughput, but with a
difference. RDRAM uses a 16-bit or 18-bit data path (versus 64-bit
for SDRAM) to transfer two bytes at a time. Accordingly, PC600 RDRAM
provides peak throughput of 1200 million bytes/second, PC700 1400
million bytes/second, PC800 1600 million bytes/second, and PC1066
2133 million bytes/second.

Until recently, Rambus RIMMs were supplied as 16-bit or 18-bit parts. In dual-channel RDRAM motherboards, those modules had to be installed in pairs, one per channel. PC1066 32/36-bit RDRAM modules are now available. These 32/36-bit RIMMs are in effect two RIMMs combined into a single package, and can be installed singly in dual-channel RDRAM systems.

In theory, then, it appears that PC800 RDRAM
matches the throughput of PC1600 DDR-SDRAM, and PC1066 RDRAM the
throughput of PC2100 DDR-SDRAM. In fact, that is true only if you are
considering peak throughput. In the real world, RDRAM provides higher
sustained throughput because it is more efficient than SDRAM in
typical applications. Whereas SDRAM efficiencies are in the 40% to
70% range, RDRAM efficiency is about 80%. Accordingly, PC800 RDRAM
might deliver sustained throughput of 1280 million bytes/second,
whereas PC1600 DDR-SDRAM delivers much less, and even PC2100
DDR-SDRAM may not be able to match the actual sustained throughput of
PC800 RDRAM.

On that basis, RDRAM might seem the better
choice, but that is seldom true for several reasons. First, the
throughput advantage of RDRAM is unrealized in most applications.
Although modern processors such as the Pentium 4 can in theory use
very wide memory bandwidths, in practice few applications require
more memory bandwidth than PC1600 DDR-SDRAM provides, let alone
PC2100, PC2700, or PC3200 DDR-SDRAM. Second, RDRAM typically costs
significantly more than DDR-SDRAM. Third, throughput is only one
aspect of memory performance. At least as important as throughput is

latency the time that elapses from
requesting data from memory until the memory begins delivering that
data. Despite the arguments of Rambus to the contrary, real-world
RDRAM implementations exhibit high latency. What's
worse is that RDRAM latency is cumulative. That is, with SDRAM,
latency is a property of the memory chips themselves and remains the
same regardless of the number of DIMMs installed in the system. With
RDRAM, installing additional memory modules increases latency
linearly. Not surprisingly, all the memory performance comparisons
that we have seen from Rambus are based on using one RDRAM module per
channel.

One can argue theory all day, of course, but
the simple fact is that in our experience Rambus RDRAM memory seldom
provides a significant performance advantage over SDRAM and may, in
fact, be slower in some applications than even PC133 SDR-SDRAM. We
suggest avoiding RDRAM memory entirely. In the past, we recommended
RDRAM for Pentium 4 systems for which memory performance was a very
high priority and the additional cost of RDRAM was not a deciding
factor. With the advent of dual-channel DDR-SDRAM systems, that
advice is now obsolete, because dual-channel PC2700 or PC3200 memory
outperforms RDRAM in every respect. Intel will soon discontinue all
of their RDRAM motherboards, making the point moot.

As of July 2003,
the memory landscape for PCs appears to be predictable for the next
couple of years. PC133 SDR-SDRAM and PC1600/PC2100 DDR-SDRAM are
useful only for upgrading older systems. PC2700 DDR-I SDRAM is the
current standard, although the Intel 875P- and 865-series chipsets
have made PC3200 DDR-SDRAM a mainstream technology. Inexpensive
systems use single-channel PC2700 DDR-SDRAM, and mainstream or higher
systems use dual-channel PC2700 or PC3200 DDR-SDRAM. This state of
affairs is likely to remain unchanged for the next year or more, so
PC2700 or PC3200 DDR-I SDRAM remains a
"safe" purchase.

As
we move into 2004, PC2700 and PC3200 DDR-I SDRAM will begin yielding
its position as the mainstream memory technology to dual-channel
PC3200 DDR-II SDRAM, at first in high-end systems and later in the
year in midrange systems. By late 2004, only entry-level PCs will use
PC2700 or PC3200 DDR-I SDRAM. Beginning in 2005, even inexpensive
systems will use PC3200 DDR-II SDRAM, and high-end systems will use
faster variants of DDR-II SDRAM.

Rambus RDRAM never
became a mainstream memory technology despite
Intel's efforts to push it. Those efforts were
particularly futile with Pentium III-class processors, which do not
require the additional bandwidth available with RDRAM. Those early
efforts to promote RDRAM failed miserably because people noticed that
despite the hype, RDRAM provided little or no performance benefit
relative to PC133 SDRAM with sixth-generation processors.

The advent of the bandwidth-hungry Pentium 4 processor should
have made the advantages of RDRAM compelling, but the issue of
relative memory performance has been overtaken by events.
Intel's contract with Rambus has expired, its
enthusiasm for RDRAM has faded, and it has now developed dual-channel
DDR-SDRAM chipsets that provide more throughput than RDRAM with
better latency and at a lower price. In our testing, RDRAM-based
Pentium 4 systems provide better memory performance than those that
use single-channel DDR-SDRAM, but worse performance than those that
use dual-channel DDR-SDRAM. That leaves RDRAM as an expensive
technology with no remaining market niche, and we expect it to fade
quickly.