Microsoft KB Archive/71704

= MS-DOS: Reasons for A20 Line Switching =

Article ID: 71704

Article Last Modified on 5/12/2003

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APPLIES TO


 * Microsoft MS-DOS 5.0 Standard Edition
 * Microsoft MS-DOS 5.0a
 * Microsoft MS-DOS 6.0 Standard Edition
 * Microsoft MS-DOS 6.2 Standard Edition
 * Microsoft MS-DOS 6.21 Standard Edition
 * Microsoft MS-DOS 6.22 Standard Edition

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This article was previously published under Q71704



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: KB71704

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