31 Jul
2013
31 Jul
'13
12:08 p.m.
On Wed, Jul 31, 2013 at 12:41 PM, Fred Cisin <cisin at xenosoft.com> wrote:
NOTE (for those not familiar): PARITY does NOT reduce
errors, in fact,
9chips instead of 8 means 12.5% more errors.
And 12.5% more cost. When the PC market moved to SIMM memory
(predecessor of DIMM), that led to companies selling fake parity
SIMMs, by putting a parity generator chip on the SIMM instead of the
RAM chip for the parity bit.
What PARITY does is LOCK UP
the machine when there IS an error. IBM considered the inconvenience of
having to reboot and rerun the software to be less important than the
possibility of printing $96.00 instead of $64.00 on a check.
Eventually PCs stopped having parity at all. Mainstream x86
processors support ECC, which is much better than parity. DIMM
modules are either 64-bit (no ECC) or 72-bit (with ECC), so the
overhead is the same as for parity, but ECC corrects the errors
instead of locking up the machine. However, most people are still
unwilling to pay extra for ECC. Intel disables the parity circuitry
on most of their x86 processors other than their server (Xeon)
processors, to use it for product differentiation, thus raising the
cost of ECC far above just the cost of the RAM. AMD x86 processors
all have ECC enabled. That's one reason I prefer AMD in systems I
purchase. Unfortunately ECC is generally not available in laptops.
Eric