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.
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.
 | Reproduced with permission from Computer Desktop Encyclopedia. Copyright (c) 1981-2009 The Computer Language Company Inc. All rights reserved. |
Additional Resources
- Toshiba HD-A30
- For home theater buffs, Toshiba has been the face of HD DVD in the format war. Toshiba delivered the first ever standalone HD DVD player, the HD-A1, and followed it up with a full second-generation line including the HD-A2, HD-A20, and HD-XA2. Coming into the 2007 holiday season, Toshiba is...
- Product reviews 2007-10-12
- Toshiba HD-A20 - HD DVD player
- Toshiba has already announced its third-generation HD DVD players, but so far we're a little skeptical about the real-world benefits of the promised upgrades. This means Toshiba's current line of HD DVD players could become a smart value buy over the next few months, as retailers look to sell old...
- Product reviews 2007-08-08
- Toshiba HD-D2
- Editors' note: The Toshiba HD-D2 is virtually identical to the Toshiba HD-A2. The review and images below are taken from our HD-A2 review, but they accurately reflect the capabilities of the HD-D2, as the two products are essentially the same.So far our biggest knock on dedicated players of HD DVD...
- Product reviews 2007-05-30
- Toshiba HD-A2
- So far our biggest knock on dedicated players of HD DVD and Blu-ray discs has been that price puts them out of reach of the average consumer. HD DVD players have always been less expensive than Blu-ray players, however. The first-generation Toshiba HD-A1, for example, debuted at half the price...
- Product reviews 2007-05-22
- PowerShot A20 3.9.0 (Windows)
- PowerShot A20. Shoot high quality photos with Canon Powershot Digital cameras. Full line of powerful, stylish and easy to use cameras will help you capture those precious moments and share with your friends and family.
- Software downloads 2001-08-06
Neighboring Terms
Premier Vendor Content Whitepapers, webcasts & resources from our Power Center Sponsors
SmartPlanet
- Thought-provoking progressive ideas on diverse topics that intersect with technology, business, and life, and matter to the world at large. Visit SmartPlanet
- More from IBM
-
- Innovate your business' process model, play against the market, compete against others on our scoreboards and WIN! Try INNOV8 2.0: A BPM Simulator
- Enabling Real-World Business Transformation through IBM Service Management Read the EMA Analyst Report
