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# Parity

Posted on 1999-07-15
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how do you know if you have even or odd parity in ram?
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Question by:bufordk
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Expert Comment

ID: 1872195
usually this is written / printed on the chip.. or documentation..
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Accepted Solution

kayton earned 100 total points
ID: 1872196
Open the box and look at the RAM.  Count the number of chips on each SIMM or DIMM.  Odd number=parity.  Even number=no parity.
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Expert Comment

ID: 1872197
The state of even/odd parity in RAM is completely handled at the hardware level.The RAM itself only supplies the parity chip. The MB generates/checks the parity state. The parity is ODD as even parity will set parity to 0 when no RAM is installed.

M

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Author Comment

ID: 1872198
kayton but can you tell even or odd?
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Expert Comment

ID: 1872199
No. His suggestion is not valid. You cannot tell by counting as if the memory is byte wide you'll have 8/9 IC's and word wide will be 16/18. It is also possible that the RAM is not organized 9x1, you can have two x4's and an x1 and some vendors are even building x9 RAM because of the widespread use of parity.

The internal parity on *ALL* pc's is *ODD*. This from the original IBM PC Technical Reference Manual. It has ever been so and will continue to be so in the foreseeable future as there is no technical reason to change the standard.

M

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Expert Comment

ID: 1872200
The original PC, XT, 286, 386 and early 486 computers almost all required parity memory or they wouldn't boot.  These computers used SIMMs and SIPPs.  For SIMMs and SIPPs you can always tell parity or nonparity by counting chips.  Macs have always been non-parity.  Newer PCs will work with or without parity memory so it doesn't matter.  It is only on the newer computers that odd/even counting doesn't work, but for them it doesn't matter so who cares?
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Author Comment

ID: 1872201
The reason i ask is because i get asked this question all the time and i looking system info doesn't tell me. nor does looking at the chip or the box. if you install new ram say it's dimm, i've seen a parity error occur with the chip installed. remove the chip it goes away. install another chip into that same computer and it still get the parity error.

now i thought parity would be set by the motherboard hardware configurations or by your bios but in this case it seems to me that parity is not being set at all. the only thing i didn't check was if the chips didn't have parity at all (the new ones) and in this case would the mixture of parity and non-parity have this effect? i'm assuming though that the new chips did in fact have parity.
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Expert Comment

ID: 1872202
Pairty is *intrinsic* to the design of "standard" PC's. If you have non-parity ram and try to use it in a parity based system it *WILL FAIL IMMEDIATELY* during POST. If you're getting *random* parity errors and they go away when you remove the DIMM then *TOSS THE DIMM*, it's *BAD*.

Unless the BIOS specifically includes an option for "NO PARITY" then assume parity is enabled. This is part of the core design of the PC.

If the chips have parity they should be useable in *all* PC configurations. Leave parity ON. It doesn't appreciable slow down the system and it *greatly* increases the reliability.

M

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Expert Comment

ID: 1872203
mark2150-
Not so.  99% of all new PCs work fine without parity, and 99% are shipped from the manufacturer with non-parity memory since it is cheaper.  Macs won't work with parity.
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Author Comment

ID: 1872204
I understand parity vs. nonparity and error messages.
all i'm looking is there a way to find odd or even parity.
if there is a way to find out which one you'd be running or is that set by the system (BIOS) or (MotherBoard setting(jumpers?,etc.))?
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Expert Comment

ID: 1872205
It is determined by the memory you install.  If you install memory with parity and the machine boots up, it is using the parity.  If you install memory without parity, the machine obviously isn't using any.  If you install memory with parity and the comptuer won't boot, obviously you're not using the parity either.
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Author Comment

ID: 1872206
It has parity.
i've forced it to give me back parity errors to see if parity is there. but when you have parity there are two types of parity one is odd parity.
ex: 10011100 parity bit in odd parity equals "1" to ensure that the data is  infact true.

if you've got even parity the same example:
10011100 parity is "0" ensuring that the data is true.

if 10011100 has even parity and the value entered into the parity bit is "1" then a parity error occurs and the same with odd parity. if a "0" is entered into the parity bit a error message apears.

if this error occurs you really cann't tell if you have even or odd as far as i know. but there could be a way i just didn't know it.

if your saying that odd parity is no-parity i don't understand. but i think your explanning a no parity environment where a memory chip has neither odd or even.

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Expert Comment

ID: 1872207
I don't know if they are using odd-parity or even-parity.

The odd and even is the number of physical black chips on the memory SIMM.  If there is an odd number of physical black chips, then it is a parity SIMM.  Even number of chips means non-parity SIMM.  I don't know if they use odd or even parity.
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