Home > Articles > Hardware > Upgrading & Repairing

Like this article? We recommend

The EPIC Battle Between CISC and RISC

Early CPUs were intended to be programmed in assembly languages. Their instruction sets looked quite like a high-level programming language. The most famous example of this design was the VAX, which included things like an "evaluate polynomial" instruction.

Later versions of these chips didn’t actually include all of these instructions in hardware; they had a core instruction set that included the basic features and implemented the rest using microcode—publicly visible instructions that were then translated into micro-operations (μops) for executing. This approach had the advantage that, since many of the instructions were implemented in software, it was cheap to release bug fixes. The original floppy disk was designed for distributing microcode updates.

As higher-level languages started becoming popular, more code was being generated by compilers than by humans. Compiler writers started discovering that complex instructions didn’t map directly to language constructs, so they used simpler ones. The more complex instructions were just wasting space on the die.

The idea of just exposing a simple instruction set to compiler writers was born. Reduced Instruction Set Computing (RISC) wasn’t just about a small instruction set—it was about orthogonality. Older CISC designs often had several ways of doing the same thing, but compiler writers would just use the most efficient one.

Early RISC designs had very few instructions. Most omitted even multiply and add instructions, since these operations could be implemented using a combination of adds and shifts. This turned out not to be such a great idea. The minimum amount of time in which an instruction can complete is one cycle, and chips with divide instructions eventually were able to complete them in fewer instructions than a chip that executed the shifts and adds, especially on floating-point values, where extra normalization steps are required and the mantissa and exponent must be handled separately.

Modern RISC CPUs include quite a lot of features, and RISC is almost a misleading name. Typically, they’re still load-store architectures (that is, operations involve loading values into registers, operating on them, and then storing the results back in memory), while CISC architectures have instructions for operating directly on values in memory (which are implemented by load-operate-store μops). The instruction sets are still more or less orthogonal; there’s only one sensible way of doing something. Of course, in theory you could implement any of the other instructions with just a load, a store, an add, and a conditional jump, but more complex instructions are added only where they’re less costly than compound instructions. An example is the vector unit found on many RISC chips, which executes an instruction on two to four values.

Intel’s x86 architecture is the last surviving CISC chip, and has a particularly baroque architecture, including things like string-comparison instructions. All x86 CPUs since the Pentium have contained a more RISC-y core and have translated these instructions into sequences of μops that are executed internally. Starting with the Core microarchitecture, Intel has done this in reverse, reassembling sequences of μops into sequences that can be executed with a single instruction.

The only real difference between a RISC and a CISC chip these days is the public instruction set; the internal instruction sets are likely to be similar. RISC and CISC are not the only possible alternatives, however. RISC came from a desire to simplify the core, and a group at Yale in the early 1980s worked out that you could take this design even further. A pipelined CPU has to do a lot of work to determine which instructions can be executed concurrently. Imagine the following sequence of operations:

  1. A = B + C
  2. D = E + F
  3. G = A + D

The first two of these operations are completely independent, but the third can’t be executed until both of the others are finished. The CPU has to devote some effort to detecting this situation, and making sure that it waits. A CPU that supports out-of-order execution might try inserting some later instructions between 2 and 3.

A Very Long Instruction Word (VLIW) processor doesn’t do this at all. Instead, instructions are grouped into blocks that can be executed independently. This technique required the compiler to do a lot more work, though. It had to detect these dependencies at compile time, but it also had to know what execution units the CPU had. For example, there was no point in issuing a block of four integer operations to a CPU that had two integer units, a floating-point unit, and two load-store units. This arrangement meant that a lot of long instructions would have space in them (wasting a lot of instruction cache space), and the next-generation CPU that had more execution units couldn’t run existing code any faster.

Intel took the VLIW concept and tweaked it slightly to produce Explicitly Parallel Instruction Computing (EPIC). Instead of fixed-length blocks of parallel instructions, Intel added a flag to the instruction that allows the start and end of parallel blocks to be marked. If a long block of instructions can be executed in parallel, a future generation can execute it faster; if a block is very short, it doesn’t waste any space.

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