8 Apr
1999
8 Apr
'99
11:03 p.m.
On Thu, 8 Apr 1999, Richard Erlacher wrote:
> My contempt for Apple begins and ends with their total disregard for the
> value of your data.
> They designed the MAC with no memory parity assuming that you'd not mind
> if your data was corrupted without your knowledge...
Multiple studies of memory reliability (DRAM) show that parity memory is
more prone to failure than non-parity memory. If you want reliability, you
have to go to something like Error Correcting Codes (ECC) like the big boys
use. We had 39-bit memory on a 32-bit VAX (11/750) because the extra seven
bits let you *detect* two faulty bits and *correct* a single bit failure.
The Sun Enterprise servers I babysit have ECC memory - we used to get one or
two failures in the machine room per year, but they were logged and corrected
without any loss of data. My Alpha board (AXP-133 "no-name" board) uses 72-pin
*parity* SIMMs in pairs to implement ECC on a 64-bit memory bit.
The problem with parity is that yes, you do know that you had a failure, but
now you have 9 bits that might fail, not 8, raising your risk by 12%. DRAM
failures are more often total rather than intermittent. A memory test at
power-up is a better insurance policy than relying on parity to save your butt.
I did have the parity circuit on a PeeCee cough up a lung once... it was even
a five-slot original PC (256K on M.B.). We were using it into the 90's because
it was merely the terminal for a Northwest Instruments logic/CPU analyzer that
we used to check for problems in our MC68000-based serial boards. One day, the
PC would not come up. Because everything was socketed and because I owned an
IC tester, we got a bottom-of-the-totem-pole tech grunt to pull each chip and
test it. It was a faulty 4164. Labor costs: $25. Parts cost: $0.60 for a
part
we stocked thousands of for one of our older products. I still have the
machine. It still works. I wish I had the invoice for that CPU; the company
bought it new in 1981, around $5K, I know, but I'd like to know the exact
figure.
Bottom line: Apple not using parity is not a reason to trash the Mac. How
many PCs have parity since we moved to EDO and SDRAM? It's extra cost and
extra complexity and extra possibilities for failure. Unless you can correct
the failure, it's not mathematically worth the extra expense and reduced
reliability.
-ethan
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