At 11:15 02-05-98 -0400, you wrote:
>I am interested in upgrading my Pentium 166 from 32 to 64 Megs of Ram (60
>ns, EDO; 2 x 16 M SIMMS, 72 pin).
>Can someone please explain the difference between parity and non-parity
>RAM? I presume I cannot mix and match the two? If I can't mix, how do I
>tell which version (parity or non-parity) that I already have installed?
>Obviously, I want to install these 2 new SIMMS myself, but do not want to
>be caught with useless parts due to my ignorance.


Hi Michael

I hope the following will provide helpful. At least it might provide
some background knowledge.

If you know that the 32 MB of RAM that you already have is EDO, it is
almost certainly *not* parity RAM. (Parity RAM allows the computer to
detect simple memory errors.) As far as I know EDO RAM does not come
in parity form. (Certainly it would be quite rare.) Parity RAM used to
be common because RAM was not as reliable 5 or 10 years ago. When EDO
RAM came along, manufacturers figured that it does it's job well enough
that few people would require parity EDO. (If you have older Fast Page
RAM, you may have Parity FP SIMMs. But you mentioned EDO.) If you enter
your BIOS Setup and examine all the screens, you should be able to find
out whether your computer supports parity RAM and whether you do indeed
have EDO memory installed. If you have (or can download) a manual for
your motherboard, you may be able to find out what options are available
as to mixing RAM types and what performance hits this would entail.

There *is* a more expensive form of EDO RAM called ECC EDO RAM. (Error
Checking and Correcting RAM). To make use of the advanced capabilities
of ECC RAM, you would need specialized support for it from your
motherboard chipset. It is highly unlikely that you would have ECC
RAM installed unless you went out of your way to get it when you
bought your computer.

Regular RAM uses 8 memory cells per byte (8 bits) of memory. SIMMs are
built so that the memory is accessed 32 bits at a time. For example,
you buy a 4X32 SIMM (4X32 is in units of megabits) and you are getting
16 MegaBytes. (Note 4X32=16X8.)

Parity RAM provides an extra bit for every byte of memory. Parity RAM
is labeled in multiples of 9 (remember that extra bit per byte) so a
16 MB SIMM would be labeled as a 4X36 SIMM. (Note that 36 is a multiple
of 9, and not a multiple of 8 as is 32.)

Pentium class computers access memory 64 bits at a time, so *identical*
SIMMs must be accessed together to provide the 2X32 or 64 bit data
paths. (Note that DIMMs, the most common format used for SDRAM, are
already 64 bits wide, so DIMMs don't have to be added in pairs. A 168
pin DIMM is like a pair of 72 pin SIMMs in one module.) If you have
four SIMMs, the two pairs do not always have to be identical as to
type or speed. (This does depend on your motherboard however.)

It is generally acknowledged that Micron makes highest quality RAM
SIMMs. Their memory division is named Crucial Technologies.
Check out <http://www.crucial.com/> before you buy your SIMMs.
Even if you don't get the absolute lowest price, buying from
Crucial is a good way to go. If you check their website, you can find
out what the best quality RAM is selling for. (Micron offers a 30 days
return policy with full refund as well as a lifetime warranty. They
also pay for shipping.) They should be able to tell you what kind of
memory came with your system and what kind to get for your upgrade.
And if it doesn't work, they will take it back.

Parity RAM can detect *single* bit errors in memory. Say a memory byte
stores 11110000 in it's eight bits. The parity bit is chosen so that
the bits taken as a *9* bit group always add up to an even binary number.
In this example the parity bit would be a 0 since 11110000 is already
even. (Add the "1"s up. There are 4 of them.) Thus the 8 bits plus one
parity bit would store as 111100000 (note the extra "0" I put at the
end). If the computer detected that the sum of the nine bits were an
odd number (say it found 111110000), the computer would know that there
had been an error. But it would not know exactly where the error was
since it would get the same result for 11110100.

If a byte of memory is 11100000, the parity bit would be a 1 and the
memory would store 111000001 (note the "1" that I put at the end)
so that there are an even number of "1"s. If the computer found a
111001001, it would know that there was an error. With the Parity memory
scheme, the computer can detect when there has been an error (say a
cosmic ray hits a memory cell and changes it's value or maybe a memory
cell leaks some electrical charge and changes value) but it cannot fix
the error.

If *two* errors occur in the same byte, the computer cannot detect this
using the parity idea since the "even-ness" or "odd-ness" does not
change. ECC RAM allows the computer to detect multiple bit errors in the
same byte *AND* fix single bit errors. This kind of memory is usually
only used on expensive "mission critical" machines. The chipset must
look at all the bits in a SIMM in total (instead of treating each byte
individually) to make use of the ECC feature. The idea is that 1 extra
bit per byte (as in parity RAM) for error detection is overkill.
You only *need* 6 extra bits for every 64 bits to detect an error
(being that 2**6=64). Since there are 72 bits of memory for each 64 in
parity or ECC RAM, there are actually 8 "extra" bits per 64 for error
checking. The 2 memory bits beyond the required 6 can be used to correct
a single error, or tell you when there are two errors per byte.

Regards,
Bill