DOCUMENT:Q71704 17-DEC-2000 [msdos] TITLE :MS-DOS: Reasons for A20 Line Switching PRODUCT :Microsoft Disk Operating System PROD/VER:MS-DOS:5.x,6.0,6.2,6.21,6.22 OPER/SYS: KEYWORDS: ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft MS-DOS operating system versions 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22 ------------------------------------------------------------------------------- SUMMARY ======= This article explains why programs that enable the A20 line, such as HIMEM.SYS, always disable the line before returning control to the system. MORE INFORMATION ================ When the A20 address line is enabled, a program can access the extended memory in the HMA (high memory area). If the A20 line is not enabled, calls to addresses above 1024K wrap back around to the bottom of memory rather than accessing the extended memory within the HMA. At one time, programmers used the method of addressing memory above 1024K, knowing that the address would actually wrap back around to the beginning of memory. Programmers found the wraparound effect to be useful for some algorithms. If the A20 line is not disabled when a program that was designed to wrap around is executed, the HMA will be overwritten. For this reason, any program that enables the A20 line must disable it before returning control to the system. Additional query words: 6.22 5.00 5.00a 6.00 6.20 ====================================================================== Keywords : Technology : kbMSDOSSearch kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS500 kbMSDOS500a Version : MS-DOS:5.x,6.0,6.2,6.21,6.22 ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 2000.