Home > Articles > Hardware

  • Print
  • + Share This
Like this article? We recommend

A Different Memory Model for Every Day of the Week

The original 8086 had used 16-bit pointers. This meant that it could access 2^16 bytes of RAM, or 64KB. Obviously, this wasn't enough for many things, and so it employed a segmented architecture. Four segment registers are present in x86 chips: the code, data, stack and extra segment registers (CS, DS, SS, and ES, respectively).

Now, because each segment register is 16 bits wide, this obviously lets you access 4GB of RAM; 2^16 segments of 2^16 bytes gives 2^32. Right? Well, no, actually. The 8086 could only address 1MB of RAM.

Now, at this point, you are probably wondering how, exactly, you could design a system like this. More astute readers will be wondering "why?" The reason is simple; they wanted segments to be able to overlap. When you specify a memory address for an 8086 instruction, the relevant segment register is left-shifted by four (giving a 20-bit number) which is then added to the specified address. The first 3 bits of the final address are dependent on the value of the segment register, the last 4 on the specified address, and the remaining 13 on both.

This mechanism allows a single physical address to have multiple logical addresses (quite a lot, actually, around 8,000). The following table shows some examples:

Segment Register:	200	206	300
Logical Address :	396	300	396
Physical Address:	3600	3596	5196

MS-DOS users may remember that some programs had a .com suffix, instead of the more conventional .exe. These programs did not ever modify the segment registers, and so could be placed anywhere in memory. Device drivers were often written using this mechanism, because it meant that they could run concurrently with other programs, being invoked as the result of an interrupt.

One megabyte of RAM sounded like a lot back in 1981, when a lot of computers still only had 64KB. It soon started to sound small, however. To get around this, the Expanded Memory Specification (EMS) was created. DOS applications were allowed to use 640KB of memory, leaving 384KB for the operating system and memory-mapped I/O. EMS allowed an application to use some of this memory, typically 64KB. It would have to request it from the operating system, however, which allowed some tricks to be performed. In a machine with an EMS board containing extra RAM, some extra hardware sitting on the address bus would map a different 64KB into the top bit of the address space on request. Applications could use all of the extra physical memory, but they had to use it 64KB at a time.

The 80286 didn't improve matters greatly. Twenty-four bit physical addressing was added, although this was still a segmented mode, where 16-bit pointers were taken as relative to a 24-bit segment. The 80286 booted in a fully 8086 compatible mode, known as "real mode." Unfortunately, the designers neglected to include a method of swapping back from protected to real mode, which meant that a system in protected mode could not run legacy code. For this reason, protected mode was rarely used. There were two ways later discovered to switch back into real mode:

  1. One involved going via the keyboard controller (which was very slow).
  2. The other involved setting up an invalid interrupt vector and raising an interrupt (which was very ugly).

The 80386 improved things slightly, but began to suffer from feature creep. The segmented addressing mechanism was extended to 32-bit segments and 32-bit offsets (still only giving 32-bits of address space), but paging was added. They fixed real mode, so you could switch back to it from protected mode, but also added a virtual 8086 mode, allowing 8086 operating systems to be virtualized.

Paging is quite useful. It's a nice way of implementing virtual memory, because every page is the same size, so you can easily re-order them or put them on disk. Segments are also nice sometimes; you can do a few nice tricks with them. Using segments, you can cleanly isolate segments of memory—for example, to do hardware bounds-checking on arrays, or giving each object in a system its own segment to stop it touching others. Most of the time, however, they are ignored. One group of people in Cambridge had just started using them for a little piece of software called Xen. They used the segmented addressing to protect the hypervisor. Minix 3 also makes use of segmented memory. Of course, now that someone is actually using a feature, it is declared obsolete, and AMD removed it from x86-64.

Now we've gone from 16-bit to 64-bit, but we missed a little detour along the way. Starting with the Pentium Pro, Intel introduced the Physical Address Extension (PAE) mode. This gave a 36-bit addressing mode. Somewhat unusually, this was done via the paging system, rather than once again modifying the segmentation system.

PAE provides a few things, such as 2MB or even 4MB pages. For systems where segments are large, this can be beneficial, because more of the page table can be kept in cache. This is most apparent when moving to a 36-bit address space, where PAE requires an extra level of indirection for 4MB pages, but not for 2MB.

You may have read in recent years about the NX bit being added to x86 chips. You have always been able to set memory as non-executable in a protected mode x86 chip, but this used to only be at the segment level. If you look at the memory layout diagrams for OpenBSD, you will see quite how much effort this is to use for everyday applications (such as implementing the mprotect function specified by POSIX). The newer chips allow read-but-not-execute permission to be granted on a page granularity. This is slightly more expensive to test, but it is easier to use. x86-64 chips get rid of the segmented protections entirely, so you need to use page level protections for everything while in 64-bit mode. This extra cost is reduced on AMD chips by passing it off to the on-die memory controller.

  • + Share This
  • 🔖 Save To Your Account

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.


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.


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.


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.


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


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


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.


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.


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