Hardware Reference
In-Depth Information
Another speed-related issue is the CAS latency. Sometimes this specification is abbrevi-
ated CAS or CL and is expressed in a number of cycles, with lower numbers indicating
higherspeeds(fewercycles).ThelowerCASlatencyshavesacycleoffaburstmoderead,
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 ex-
pensive) 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 denominat-
or.
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 pop-
ulargenerallycoststhemost.Thiscanmakeaddingmemorytooldersystemscostprohib-
itive.
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 ver-
sions. You can mix ECC and non-ECC modules, but in that case the system defaults to
non-ECC mode.
Tip
Instead of buying “new” obsolete memory for older systems, check with computer repair
shops, Craigslist, websites specializing in surplus memory, or other users who might have a
collection of old parts.
High-reliability systems might want or need ECC versions, which have extra ECC bits.
Aswithothermemorytypes,youcanmixECCandnon-ECCtypes,butsystemscan'tuse
the ECC capability.
Replacing Modules with Higher-Capacity Versions
If all the memory module slots on your motherboard are occupied, your best option is to
remove an existing bank of memory and replace it with higher-capacity modules.
