Home > Articles > CompTIA > A+

This chapter is from the book

Memory Diagnostics—Parity

Parity is the state of either oddness or evenness assigned to a given byte (not bit) of data. Parity checking is the way that the computer uses a special set of logical rules and chips to make decisions based on the parity (state) of a particular byte. When a PC runs through the POST (Power On Self-Test), commonly called a cold boot, memory integrity is one of the first things tested. On many machines you can see this taking place as a rapidly-increasing number displays on the screen before the operating system begins to load. This checking is designed to verify the accuracy (integrity) of the bits contained in memory, and it happens after every read/write operation.

CAUTION

Remember that computers think in binary bits and bytes. The binary computer language uses "words" composed of 1s and 0s.

Originally, parity checking was a major development in data protection. At the time, memory chips were nowhere near as reliable as they are today, and the process went a long way towards keeping data accurate. The parity circuit allowed the CPU to compare what it sent to main memory with what it read from main memory. Parity checking is still the most common (and least expensive) way to check whether or not a memory cell can accurately hold data. A more sophisticated (and expensive) method uses Error Correcting Code (ECC).

Even and Odd Parity

Parity checking puts a byte into memory along with its parity bit and then reads the byte from memory. If the byte plus the parity bit matches what was sent, the memory is okay. Otherwise, we have a parity error. Parity can be set to odd, even, or off. The parity circuit works by adding one bit to every byte of data, resulting in nine bits. (Remember that a byte is already eight bits.) The value of any given bit (1 or 0) is determined at the time that data is written to memory. Then, a 1 or 0 is assigned to the parity bit, depending on whether the overall byte is made up of an odd or even number of 1s. Figure 3.2 shows various bytes of data with their additional parity bit.

Figure 3.2 Odd and even parity.

In plain odd and even parity, every byte gets 1 parity bit attached, making a combined 9-bit byte. Therefore, a 16-bit byte has 2 parity bits, a 32-bit byte has 4 parity bits, and so forth. This produces extra pins on the memory module, and this is one of the reasons why various DIMMs and SIMMs have a different number of pins.

Even parity checking, is where the total of all the 1 bits in a byte must equal an even number. If five of the bits are set to 1, the parity bit will also be set to 1 to total six (an even number). If 6 bits were set to 1, the parity bit would be set to 0 to maintain the even number six. If the parity is set to "even" and a returning byte contains an odd number of "ones," the system knows the data is corrupted. This is called a parity error.

CAUTION

In even parity, the overall number of 1s must come out to an even number.

Odd parity works in the reverse of even parity, but the concept is the same. The total number of 1s in any given byte must come out to an odd number. Once again, this is done by using either a 1 or a 0 for the parity bit, as you can see in Figure 3.2.

CAUTION

You'll receive a parity error if the parity is odd and the parity circuit gets an even number, or if the parity is even and the parity circuit gets an odd number. The circuit can't correct the error, but it can detect that the data is wrong.

Fake or Disabled Parity

Some computer manufacturers install a less expensive "fake" parity chip that simply sends a 1 or a 0 to the parity circuit to supply parity on the basis of which parity state is expected. Regardless of whether the parity is valid, the computer is fooled into thinking that everything is valid. This method means no connection whatsoever exists between the parity bit being sent and the associated byte of data.

A more common way for manufacturers to reduce the cost of SIMMs is to simply disable the parity completely, or to build a computer without any parity checking capability installed. Some of today's PCs are being shipped this way, and they make no reference to the disabled or missing parity. The purchaser must ensure that the SIMMs have parity capabilities, and must configure the motherboard to turn parity on.

Error Correction Code (ECC)

Parity checking is limited in the sense that it can only detect an error—it can't repair or correct the error. This is because the circuit can't tell which one of the eight bits is invalid. Additionally, if multiple bits are wrong but the result according to the parity is correct, the circuit will pass the invalid data as okay.

Error correction code (ECC) uses a special algorithm to work with the memory controller, and it adds an error correction code bit to each data bit when it's sent to memory. When the CPU calls for data, the memory controller decodes each error correction bit and determines the validity of its attached data bit.

The system requires twice the number of bits, but the benefit is that ECC can correct a single-bit error. Because approximately 90 percent of data errors are single-bit errors, ECC does a very good job. On the other hand, ECC costs a lot more, due to the additional number of bits.

CAUTION

Remember that ECC can correct only single-bit errors, but it can also detect multi-bit errors. Parity checking understands only that the overall byte coming out of memory doesn't match what was sent into memory. Parity checking cannot correct anything.

Usually, whoever is buying the computer will decide which type of data integrity checking they want, depending mainly on cost benefits. They can choose ECC, parity checking, or nothing. High-end computers (e.g., file servers) typically use an ECC-capable memory controller. Midrange desktop business computers typically are configured with parity checking. Low-cost home computers often have nonparity memory (no parity checking or "fake" parity).

Pearson IT Certification Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Pearson IT Certification and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Pearson IT Certification products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by Adobe Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.pearsonitcertification.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020