Home > Articles

Memory

This chapter is from the book

Cache Memory

Cache memory is a type of high-speed memory designed to speed up processing. Cache (pronounced "cash") is derived from the French word cacher, meaning to hide. A cache attempts to predict which information is about to be used, using an algorithm (logical formula) based on probabilities and proximity. Proximity means how close something is to something else; in this case, instructions or data bytes.

Typically, a memory cache is a separate SRAM chip, running much faster than DRAM. Whichever instruction or data is most likely to be used next, is stored in the cache. When the CPU looks for the next instruction, the chances are good that it will find it faster in small cache memory than in large main memory.

The Memory Hierarchy and Caches

A cache is like an expectation. If you expect to see a piece of information and it's right beside you, you can access that information much faster than if you had to go look for it. When you open a book and look at page 22, logic dictates that you'll look at the top of the page, then at the middle of the page, then at the bottom of the page, and then at the top of page 23. That's how computer caching operates.

Think of the example at the beginning of this chapter—the one where you're asked to repeat the words on the last page of the previous chapter. If you were expecting to be asked this question, you could cache the page by putting your finger in the book at that location. This would create a pointer to the page you were going to be asked to read, and you would be waiting for the probable next request—the instruction to read the page.

Because memory size is always increasing, more time is needed to decode increasingly wider addresses and to find stored information. Larger numbers mean we can have more addresses, but a memory register can store only one digit of an address. The larger the numbers, the wider the registers must be, and the wider the corresponding data bus used to move a complete address.

One solution is a memory hierarchy. "Hierarchy" is a fancy way of saying "the order of things; from top to bottom, fast to slow, or most important to least important." Memory hierarchy works because of the way that memory is stored in addresses. Going from fastest to slowest, the memory hierarchy is made up of registers, caches, main memory, and disks. When a memory reference is made, the processor looks in the memory at the top of the hierarchy (the fastest). If the data is there, it wins. Otherwise, a so-called miss occurs, at which time the requested information must be brought up from a lower level of hierarchy.

A miss in the cache (that is, the desired data isn't in the cache memory) is called a cache miss. A miss in the main memory is called a page fault. When a miss occurs, the whole block of memory containing the requested missing information is brought in from a lower, slower hierarchical level. Eventually, the information is looked for on the hard disk—the slowest storage media. If the current memory hierarchy level is full when a miss occurs, some existing blocks or pages must be removed for a new one to be brought in.

A hierarchical memory structure contains many levels of memory, usually defined by access speed. A small amount of very fast SRAM is usually installed right next to the CPU, matching up with the speed and memory bus of the CPU. As the distance from the CPU increases, the performance and size requirements for the memory are reduced.

CAUTION

SMARTDRV.SYS and SMARTDRV.EXE are DOS program utilities that provide disk caching. The efficiency of a cache is reported as its hit ratio. To send an efficiency report to the screen, issue the command SMARTDRV /S from a DOS command prompt.

L-1 and L-2 Cache Memory

The Intel 486 and early Pentium chips had a small, built-in, 16KB cache on the CPU called a Level 1 (L-1), or primary cache. Another cache is the Level 2 (L-2), or secondary cache. The L-2 cache was generally (not all the time, nowadays) a separate memory chip, one step slower than the L-1 cache in the memory hierarchy. L-2 cache almost always uses a dedicated memory bus, also known as a backside bus.

CAUTION

For the purposes of the exam, you should remember that the primary (L-1) cache is internal to the processor chip itself, and the secondary (L-2) cache is almost always external. Modern systems may have the L-1 and L-2 cache combined in an integrated package, but the exam differentiates an L-2 cache as being external. Up until the 486 family of chips, the CPU had no internal cache, so any external cache was designated as the "primary" memory cache. The 80486 introduced a 16KB internal L-1 cache. The Pentium family added a 256KB or 512KB external, secondary L-2 cache.

Technology tends to move toward consolidating components, for speed and cost efficiencies. The Super I/O chips combined many of the original XT adapters (for example, keyboard, COM and LPT ports) into a single package, and central processors soon moved in the same direction. Although caches were originally placed outside the chip die, new developments paved the way to move them inside the chip. A die, sometimes called the chip package, is essentially the foundation for the multitude of circuit traces making up a microprocessor. Today, we have internal caches (inside the CPU housing) and external caches (outside the die).

Internal and External Memory

When we speak of a chip's internal bus, we mean that the bus is cast right on the manufacturing die, along with the chip. These chip packages are sort of like an extremely small motherboard, in that they're the foundation for the many transistors, diodes, buses, caches, and a host of other electrical components we call a central processor. Don't confuse a chip package with a chipset—the entire set of chips used on a motherboard to support a CPU. The CPU is a chip package.

NOTE

A manufacturing mask is the photographic blueprint for the given chip. It is used to etch the complex circuitry into a piece (chip) of silicon.

An external bus is the place where information moves out of the chip die to another destination (for example, an L-2 cache). Because bus width is typically measured by the number of bits that the bus can process at one time, we can have an 8-, 16-, 32-, and 64-bit bus. The number of data lines, each carrying a stream of bits, indicates the width of the bus. The bus width generally depends upon where the processor is directing information.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT 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 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 InformIT. 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.informit.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