Printer Friendly
Dictionary, Encyclopedia and Thesaurus - The Free Dictionary
1,508,979,214 visitors served.
forum mailing list For webmasters
?
New: Language forums
Dictionary/
thesaurus
Medical
dictionary
Legal
dictionary
Financial
dictionary
Acronyms
 
Idioms
Encyclopedia
Wikipedia
encyclopedia
?

A20 line
(redirected from Gate A20)

    Also found in: Wikipedia 0.04 sec.

A20 line

Refers to the address line in an x86-based PC that points beyond one megabyte. When IBM introduced the 286-based PC AT in 1984, its 24 address lines could reach 16MB of RAM, whereas the previous 8088 CPU could only reach 1MB and had the oddity of wrapping addresses beyond 1MB back to zero. In order to provide compatibility with the 8088, IBM made the A20 line switchable so that it could be disabled when running in Real Mode (within 1MB), and only lines A0 through A19 would be operable. Thus, earlier applications that depended on the wrap-around quirk would continue to run.

A Legacy Example
The A20/memory wrap-around is an example of an early function that continues to be supported for years to come. PC operating systems simulate the wrap-around when running in Virtual 8086 Mode, and the PC BIOS's still have an option to turn the A20 line on and off as required. See Virtual 8086 Mode, Real Mode, address bus and binary values.



How to thank TFD for its existence? Tell a friend about us, add a link to this page, add the site to iGoogle, or visit webmaster's page for free fun content.
?Page tools
Printer friendly
Cite / link
 Email
Feedback
? Mentioned in
No references found
 
Encyclopedia browser? ? Full browser
 
 
Encyclopedia
?

Disclaimer | Privacy policy | Feedback | Copyright © 2009 Farlex, Inc.
All content on this website, including dictionary, thesaurus, literature, geography, and other reference data is for informational purposes only. This information should not be considered complete, up to date, and is not intended to be used in place of a visit, consultation, or advice of a legal, medical, or any other professional. Terms of Use.