On 18 Mar 99, at 9:01, Jim Kortisses wrote:

> Will someone please explain the difference between ecc ram and non ecc
> ram?

  ECC RAM is essentially the same as older "parity" RAM, with an extra
bit for each byte.

  Parity is a simple mechanism for detecting single-bit errors.  ECC is
a more complex system for *correcting* single-bit errors, and detecting
multiple-bit errors.

  A 72-pin DIMM has 32 data bits, and may have 4 parity bits.  To do
ECC across 32 data bits requires 5 extra bits, so this will be marketed
as "parity" RAM.
  Pentiums use RAM 64 bits wide, which is why you need DIMMs, or
*pairs* of 72-pin SIMMs.  To do ECC across 64 data bits needs only 6
extra bits; if this RAM provides parity bits, there will be 8 available.
  So:  A DIMM with parity bits will be marketed as "ECC".  A Pentium
BIOS may also support ECC as an option if parity SIMMs are installed.

  Parity is a fairly weak check, but has the advantage that it is easy
to do in hardware, in parallel with the ordinary RAM access circuitry,
so it should have no performance impact.
  ECC is a little more complicated, and so memory accesses may be
delayed slightly if this is enabled.  But it is a much more robust
check, the sort of thing you absolutely want on any critical servers.
Most people cannot justify the cost or performance on a desktop machine.


David G

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