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Publisher5.6 Memory Selection Guidelines
The following sections provide guidelines
for selecting memory for a new system or to upgrade an existing
system. Follow these guidelines as closely as possible to ensure the
memory you select functions optimally in your system.
5.6.1 Upgrading Older Systems
When
upgrading an older motherboard, it is sometimes impossible to match
the installed memory. Some motherboards have 30-pin and 72-pin SIMM
sockets, and many have both SIMM and DIMM sockets. If the installed
memory occupies all available sockets of one type, you may have to
add memory of a different type.If so, consult the manual to determine supported configurations. For
example, many 30/72-pin motherboards provide four 30-pin and two
72-pin sockets, but allow you to populate both 72-pin sockets only if
no memory is installed in the 30-pin sockets. Similarly, motherboards
with both SIMM and DIMM sockets may allow using a DIMM only if one or
more SIMM sockets are vacant or populated only with single-sided
SIMMs.Mixing memory types may degrade performance. Some chipsets run all
memory at the speed of the slowest module. Others, including the
430HX and 430VX, run each bank at optimum speed. Mixed-memory
configurations are nonstandard and best avoided. Memory is cheap
enough that it's often better to use all new memory.
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selecting memory:
- Verify your
motherboard can cache the full amount of RAM you install. For
example, the Intel 430TX chipset supports 256 MB of RAM, but caches
only 64 MB. Adding RAM beyond 64 MB actually decreases system
performance. Some motherboards have insufficient cache installed to
cache the full amount of cacheable RAM supported by the chipset. Some
of these have sockets that you can fill with additional cache. The
cacheable RAM area of Slot 1 systems is determined by the processor
itself. The L2 cache present on Pentium II CPUs can cache a maximum
of 512 MB of system memory. Modern socketed processors from Intel and
AMD can cache more main memory than can physically be installed.Purchase only name-brand memory,
especially if you overclock your system. Motherboards vary in their
tolerance for different brands of memory. Some motherboards accept
and use nearly any compatible memory, but others are reliable only
with some brands of memory. A name-brand memory module functions
properly on nearly any motherboard designed to accept that module. We
use only Crucial memory (http://www.crucial.com) and Kingston memory
(http://www.kingston.com) in our
systems, and have never had a problem with them.Don't attempt to save
money by mixing newer memory with older memory. For example, if you
buy two 32 MB PC133 SDRAM DIMMs for a system that currently has one
16 MB EDO DIMM installed, you may be tempted to use all three DIMMs
for a system total of 80 MB rather than 64 MB.
That's usually a mistake. Some systems accept mixed
EDO and SDRAM memory, but most systems will run the newer, faster
memory at the same speed as the older, slower memory. The same holds
true for upgrading newer systems. For example, if you buy two 256 MB
PC2700 DIMMs to upgrade a system that currently has one 128 MB PC1600
DIMM, settle for 512 MB of pure PC2700 memory rather than going for
640 MB of mixed-speed memory that will operate at only PC1600.Buy one larger-capacity module in
preference to two smaller ones of the same total capacity. This is
particularly important with DIMM modules. Most motherboards have only
two to four DIMM sockets, so buying smaller-capacity DIMMs may later
force you to replace existing memory when you want to add more memory
to the system.Memory modules and
sockets may use tin or gold contacts. Manufacturers often recommend
using modules with the same contact metal as the sockets. In theory,
mixing gold and tin can cause corrosion and other problems. In
practice, we've never seen this happen.
5.6.2 Upgrading Newer Systems
Newer systems use DIMM or RIMM modules exclusively. DIMM and RIMM
memory are available in many more variants than SIMM memory, so be
careful to buy DIMMs or RIMMs that match your requirements. Use these
guidelines when choosing DIMM or RIMM memory modules:
- For SDRAM DIMMs, get the proper voltage. All DDR-SDRAM DIMMs use
2.5V. Most SDR-SDRAM DIMMs use 3.3V. Some DIMMs, often called
Macintosh DIMMs, use 5V. Some early DIMM-based PC motherboards use 5V
DIMMs.DIMMs are available in EDO, SDR-SDRAM, and DDR-SDRAM. Some transition
motherboards support two typesEDO and SDR-SDRAM, or SDR-SDRAM
and DDR-SDRAM. If you have the choice of EDO or SDR-SDRAM, buy
SDR-SDRAM DIMMs for future flexibility. If your motherboard supports
both SDR-SDRAM and DDR-SDRAM, buy DDR-SDRAM.SDR-SDRAM and DDR-SDRAM DIMMs are available in
buffered (also called
registered) and unbuffered
versions. Some motherboards accept only buffered DIMMs, others accept
only unbuffered DIMMs, and still others will accept either buffered
or unbuffered DIMMs, but do not allow mixing types. Check the
motherboard or system maker's documentation or web
site to verify which type(s) of module your system accepts. If you
are upgrading an existing system that accepts both types, open the
lid to determine which type is currently installed. Even if the
motherboard accepts mixed buffered and unbuffered DIMMs, avoid mixing
types if possible.Buy the fastest-speed memory available, which usually costs little or
no more than slower versions. For SDR-SDRAM, buy nothing slower than
PC133 modules, which are backward-compatible with systems that use
PC100 or PC66 memory. For DDR-SDRAM, buy PC2700 or (better) PC3200
modules in preference to PC2100 modules. Like SDR-SDRAM modules,
faster DDR-SDRAM modules are generally backward-compatible with
motherboards designed to use slower modules. Verify backward
compatibility using the Crucial Memory Configurator (http://www.crucial.com).
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labeled "100 MHz" for true PC100
memory. PC100 memory supports 125 MHz (8 ns) operation, but 100 MHz
(10 ns) memory is usable at FSB speeds no higher than 83 MHz. Some
vendors label modules "LX" to
indicate they work with the 66 MHz FSB 440LX chipset, and
"BX" to indicate they work with the
100 MHz FSB 440BX chipset. "BX"
modules usually run at 100 MHz FSB, but are not
PC100 modules unless so labeled. Similarly, some vendors sell
"133 MHz" DIMMs, hoping buyers will
mistake them for PC133.
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Presence Detect (SPD), a serial EEPROM that stores DIMM
parametersincluding memory type, size, speed, voltage, number
of row and column addresses, and so forthwhich can be read by
the system BIOS at boot time and used to correctly configure memory.
Some motherboards won't boot with non-SPD memory.
Others boot, but with a warning message. Some vendors label SPD
modules "EEPROM," but a module so
labeled isn't necessarily SPD-compliant. Ask.Purchase modules with lower CAS
latency whenever possible. PC100 and PC133 DIMMs are readily
available in CAS2 or CAS3 (sometimes shown as CL2 or CL3). CAS2
modules provide a small performance boost relative to CAS3 modules.
More important, CAS2 modules have a bit more in reserve that allows
them to function more reliably in overclocked or heavily loaded
systems. CAS2 modules typically cost 2% or so more than CAS3. You may
find CAS1 modules, which ordinarily sell at a high enough premium
over CAS2 modules to make them a poor choice.When upgrading an older system, determine
whether it requires two-clock or four-clock modules. Recent PCs use
four-clock memory, which is now ubiquitous. If your system requires
two-clock memory, you'll have to ask for it. It may
be a special-order item, for which you will be charged a premium.
All 168-pin DIMMs, 184-pin
DIMMs, and 184-pin RIMMs use similar sockets, but both modules and
sockets are keyed to prevent using the wrong type of module. If there
is such a thing as a "standard"
SDR-SDRAM DIMM, it would be 3.3 volt, nonparity (x64), unbuffered,
PC100, CAS3. Similarly, a
"standard" DDR-SDRAM DIMM would be
2.5 volt, nonparity (x64), PC2700 or PC3200, CAS2.5.
5.6.3 SIMM Guidelines
Use
the following guidelines when choosing SIMM memory modules:
- Unless you are upgrading an older Socket 7 system that accepts only
72-pin SIMMs, buy only 168-pin DIMM memory. Many Socket 7
motherboards have both SIMM and DIMM sockets. Buying DIMMs for such a
board makes more sense than buying obsolete SIMMs. However, note that
some early boards support DIMMs only with very slow memory timings.SIMMs are available in FPM and EDO. If your motherboard supports
either type, buy EDO (assuming you can't scrounge it
somewhere). Some motherboards support both FPM and EDO, but do not
allow mixing types, either per bank or globally. Because FPM is
obsolescent and demand for it is small, FPM often costs more than
EDO. If FPM is installed and the motherboard does not allow mixing
FPM with EDO, consider removing FPM SIMMs and installing all EDO
SIMMs.Don't assume the motherboard supports large SIMMs.
Check the manufacturer's web site to determine the
supported SIMM capacities and memory configurations. Older
motherboards often support nothing larger than 16 MB SIMMs.
Conversely, don't assume the motherboard cannot
support larger SIMMs than those currently installed just because the
manual says so. For example, one of our technical reviewers from an
earlier edition has a Compaq system whose manual lists 16 MB SIMMs as
the largest supported. That was true when the manual was printed
because 32 MB SIMMs were not yet available. But that system uses 32
MB SIMMs without problems.Some motherboards, particularly those with four or more SIMM sockets,
restrict the number of double-sided SIMMs that can be used, or
restrict the total number of SIMM sides that can be used. If your
motherboard has such a limitation, verify that the combination of
currently installed SIMMs and the SIMMs you plan to install does not
exceed the limit.
5.6.4 Upgrading Memory in Pentium Pro and Pentium-Class Systems
Pentium Pro and P54C/P55C
Pentium-class systems, including those with an AMD K6 or higher, are
too old to be good upgrade candidates, but many such systems are
still used as firewalls and fax servers, and for other appliance
duties, where it may make sense to extend their useful lives for
another year or two by installing a memory upgrade. When upgrading
memory in a Pentium or Pentium Pro, note the following issues:
- Try to install the memory identical to that already installed. Early
Pentiums and most Pentium Pros use 72-pin SIMMs, but either type of
system may use DIMMs in addition to or instead of SIMMs. Existing
memory may be FPM, EDO, or SDRAM. Many systems can use different
memory typese.g., FPM or EDObut do not allow mixed
types. If you have a choice, install SDR-SDRAM DIMMs. Even PC66 DIMMs
work fine in systems this old, and most systems of this type yield
better memory performance with SDRAM than with older memory styles.Most such systems have nonparity memory installed. Some do not
support parity memory. Pentium Pro systems often have parity memory
installed, and may use ECC. Some systems support parity or nonparity
memory, but do not allow mixed types. Others accept parity or
nonparity RAM interchangeably, but disable parity if any nonparity
modules are installed. For SIMM-based systems, count chips to
determine memory type. For DIMM-based systems, locate the
identification number on a module and check the
manufacturer's documentation to determine its type.These systems use a 64-bit memory bus, and therefore require adding
72-pin SIMMs in pairs. DIMMs may be added individually.Most SIMM-based systems use 60 or 50 ns memory. Make sure new memory
is at least as fast as that already installed. Early DIMM-based
systems use FPM or EDO DIMMs, which you should match as closely as
possible. Later DIMM-based systems may use JEDEC (PC66) SDRAM. You
can use PC66 SDRAM, but it's better to buy PC133
SDRAM, which can be recycled later into systems that will themselves
be approaching their design lifetimes.
5.6.5 Upgrading Memory in Pentium II/III/4, Celeron, and Athlon Systems
These systems are excellent candidates
for memory upgrades. Early Pentium II systems often have only 16 MB
of RAM. In the price-sensitive consumer Celeron market, many early
systems shipped with only 16 MB, and some Celeron systems have been
sold with only 8 MB. Expanding memory to 128 MB or more is the most
cost-effective upgrade you can make. When upgrading memory in one of
these systems, note the following
issues:
- Some early Pentium II and Celeron systems use EDO SIMMs or DIMMs, but
most use 3.3 volt 168-pin unbuffered JEDEC SDR-SDRAM DIMMs.Conserve DIMM sockets. A few motherboards have four DIMM sockets,
most have three, and some low-end systems have only two. If you have
the choice, always install one larger DIMM rather than two smaller
ones that total the same amount of memory. Note, however, that older
systems may not recognize large-capacity DIMMs or those that use
128-megabit or larger memory chips. In that situation, a BIOS upgrade
may help, but the limitation is often hardcoded into the chipset.Most of these systems have nonparity memory installed, but can use
either parity or nonparity DIMMs interchangeably. Unless you plan to
install 512 MB or more, install nonparity DIMMs. We have been told
that when using very large amounts of memorymore than 512
MBmemory errors introduced by cosmic rays make it worthwhile
to pay the additional cost for parity/ECC memory and to accept the
small performance hit that using ECC produces.Memory must always be added in full banks. These systems use a 64-bit
memory bus, and therefore require adding 72-pin SIMMs in pairs. DIMMs
may be added individually.Most SIMM-based systems use 60 ns or 50 ns memory. Make sure memory
you add is at least as fast as the memory that is already installed.
Early DIMM-based systems use FPM or EDO DIMMs, which you should match
as closely as possible. Later DIMM-based systems may use JEDEC (PC66)
SDRAM. You can install PC66 SDRAM, but it usually makes more sense to
buy PC133 SDRAM for these systems because it can be recycled later if
you upgrade to a faster system.
