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What is the meaning of Local vs Global Enable/Disable A20 (How-tos)

posted by tom , Germany (West), 01.02.2025, 01:18
(edited by tom on 01.02.2025, 01:37)

From memory, so be careful:

Global enable/disable unconditionally sets the A20 line

Local enable/disable increments/decrements a counter. only if the counter
switches between 0 and !=0 the A20 line is changed.

So if LocalEnable() is called 10 times, LocalDisable() must also be called
10 times before the actual A20 status changes back to disabled.

However, I may be totally wrong

Fortunately, the source to MS HIMEM is (and always was) publicly available,
and can be regarded as the definition of these functions.
FD HIMEM is supposed to mimic it precisely (which is not certain).

So you can look it up yourself without relying on unprecise descriptions.

[Edited to add]However I am not aware of a guideline how these functions are to
be used, for what purpose. If someone knows, please report.

Complete thread:

• What is the meaning of Local vs Global Enable/Disable A20 - ecm, 31.01.2025, 23:21 (How-tos)
  • What is the meaning of Local vs Global Enable/Disable A20 - ecm, 31.01.2025, 23:40
    • What is the meaning of Local vs Global Enable/Disable A20 - tom, 01.02.2025, 01:18