Q71704: MS-DOS: Reasons for A20 Line Switching

Article: Q71704
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:5.x,6.0,6.2,6.21,6.22
Operating System(s): 
Keyword(s): 
Last Modified: 17-DEC-2000

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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 
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SUMMARY
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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
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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

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Keywords          :  
Technology        : kbMSDOSSearch kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS500 kbMSDOS500a
Version           : MS-DOS:5.x,6.0,6.2,6.21,6.22

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