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ECC Memory 
ECC (error correction code) technology has been around for 
some time in datacommunications and storage tecnology. An 
ECC code is a bit pattern which hasbeen extended by extra 
bits so a hamming code is formed. Such a code can bedecoded 
to the original bit pattern even if one or more bits have 
been changed.Hamming codes can be generated for one-bit 
correction capability, two-bitcorrection, etc... 
Generally the more bits that need to be corrected the longer 
the code of theextra bits which have to be added. Since the 
effeciency of the code is better onbigger data wordt, ECC 
has been used for such applications where bit errors 
areunaviodable , such as datacommunications on noisy lines 
and data storage onmedia which is not perfect. 
With 64 bit computer now being common in the marketplace, 
the memory datawidth has become wide enought for ECC memory 
to become economically feasible.First being introduced on 
mainframes and workstations, ECC memory technology isnow 
bing applied to PC platforms. The nice thing about the 
technology is thatthe ECC coding and decoding is done fully 
in hardware (in the chipset) which isfully transparent to 
the operating system. This guarantees it to work 
flawlesslyon any operating system. Because the operation is 
performed in hardware, no lossof speed is suffered. With 8 
bits added to the 64bit data single bit errorcorrection can 
be obtained. 
With SDRAM memory being burst-accessed each 10ns the 
slightest timingdeviation or glitch can cause errors. In 
fact, all requirements are so tightnowadays that even poor 
contact resistance between the module and its socketwill 
immediately cause a computer to fail. Therefore ECC memory 
is not a luxury,but a clean an easy way to enhance system 
reliability. We would recommend theuse of ECC memory for all 
systems intended for a professional environment. Forservers 
which have to run day and night ECC memory will become a 
standarditem. 
If a motherboard supports both non-ECC and ECC memory, 
usually the bios willallow the user to turn on the ECC 
function if ECC memory has been detected.Generally it is not 
possible (and also unwise) to mix ECC and non-ECC 
memorymodules. Motherboards wich require ECC memory will 
only boot if such moduleshave been installed. ECC technology 
should only be used to enhance systemreliability and not to 
make bad modules or motherboards work. 
  
One may wonder where data errors may com from if the 
computer system andmemory is in perfect condition. As it is, 
there are several sources of problemsin a computer system 
which may lead to soft errors. A soft error is a bit whichis 
altered or read out erroneously only once. Some sources of 
soft errors are: 
- Ground noise : in each digital system the ground lines 
carry the returnsupply current which varies as digital 
devices switch. Even with decentlydecoupled supply lines 
there will still be noise. 
- Crosstalk : capacitive coupling of adjacant lines generate 
noise. 
- Contact resistance : there is an ohmic resistance between 
each pin on thememory module an the connector on the 
computer system. A poor contact can alterthe timing a few 
nanoseconds easily. 
- Clock jitter : every unstability in the clock signal will 
affecttiming. 
- Alpha particle setup : chips are packed with epoxy resin. 
This materialhas, like any material, a background 
radioactivity. An atomic change of statecan at short range 
generate alpha particles which can alter the charge of a 
MOScapacitor, being the storage cell in a DRAM. Therefore 
DRAM technology is mostlysusceptible to this kind of 
radiation. 
It is obvious that no electronic system can be guaranteed to 
be totalyfunctional since there are always unpredictable 
sources of errors which maycause a system to fail. In a 
computer system the DRAM memory is usually the 
mostsusceptible part to this kind of problems because of the 
charge-storagetechnology and because of the extreme 
intagration level. Because of this, eventhe very best DRAM's 
may fail once every few years on a single bit. 
It is just to catch this type of errors that memory ECC 
technology is used.Therefore only single bit correction is 
used since the target error is one thathappens very rarely 
and is unlikely to happen with two bits at a time. 
  
Lo de dual supongo que se refiere a que puede soportar dos 
Estandares deMemoria como PC-133 o memorias DIMM de 800Mhz 
>  
>  
>  
>     Buenos dias a todos, 
>     Tengo una duda, hace tiempo que no me dedico a 
> adquirir equipos, y no se que es la memoria de doble canal 
> , o memoria dual, alguien sabe que caracteristicas tiene, 
> asi como la diferencia entre memoria ECC y NO ECC 
>  
>         José Mediero 
>  
>  
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Ing. Pedro Herrera 
Sword Vision 
svision@prodigy.net.mx 
83321027