Hardware Reference
In-Depth Information
• Do you need standard (unbuffered) or registered versions?
• Do you need a specific voltage?
• What speed grade do you need?
• Do you need a specific CAS latency?
Currently, DIMMs come in SDR (SDRAM), DDR, DDR2, and DDR3 versions. They are not
interchangeable because they use completely different signaling and have different notches to prevent
a mismatch. High-reliability systems such as servers can use ECC versions, although most desktop
systems use the less-expensive non-ECC types. Most systems use standard unbuffered DIMMs, but
file server or workstation motherboards designed to support large amounts of memory might require
registered DIMMs (which also include ECC support). Registered DIMMs contain their own memory
registers, enabling the module to hold more memory than a standard DIMM. DIMMs come in a
variety of speeds, with the rule that you can always substitute a faster one for a slower one, but not
vice versa.
Some memory modules are designed to run on non-standard voltages, which may be useful when
overclocking them. Unfortunately this can also cause problems for systems where stock (non-
overclocked) memory settings are used. Standard voltages for DDR, DDR2, and DDR3 modules are
2.5V, 1.8V, and 1.5V, respectively. If you buy a DDR3 module rated at a higher voltage (1.6V or
higher) it may not perform reliably when run on the standard 1.5V setting. I've seen systems with
constant lockup and crashing problems due to improperly configured memory like this. My
recommendation is to only purchase memory rated to run on the standard voltage for that type, which
is 1.5V in the case of DDR3.
Another speed-related issue is the CAS latency. Sometimes this specification is abbreviated CAS or
CL and is expressed in a number of cycles, with lower numbers indicating higher speeds (fewer
cycles). The lower CAS latency shaves a cycle off a burst mode read, which marginally improves
memory performance. Single data rate DIMMs are available in CL3 or CL2 versions. DDR DIMMs
are available in CL2.5 or CL2 versions. DDR2 DIMMs are available in CL 3, 4, or 5. DDR3 DIMMs
are available in CL 5, 6, 7, 8, and 9. With all memory types, the lowest CL number is the fastest (and
usually the most expensive) memory type. You can mix DIMMs with different CAS latency ratings,
but the system usually defaults to cycling at the slower speeds of the lowest common denominator.
Considerations in Purchasing Obsolete Memory
Many people are surprised to find that obsolete memory types cost much more than those that current
systems use. This is because of simple supply and demand; what is least popular generally costs the
most. This can make adding memory to older systems cost prohibitive.
Most Pentium systems after 1995 used EDO SIMMs that were non-ECC and rated for 60ns access
time. If your system is older than that, you might need FPM memory instead of EDO. The FPM and
EDO types are interchangeable in many systems, but some older systems do not accept the EDO type.
Some Pentium 4 systems use RIMMs, which are available in 184-pin and 232-pin versions. Although
they appear to be the same size, they are not interchangeable. If the system supports ECC, you might
need (or want) ECC versions. You can mix ECC and non-ECC modules, but in that case the system
defaults to non-ECC mode.
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