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Using Multiple CPU Cores in DOS? (Miscellaneous)

posted by RayeR(R) Homepage, CZ, 20.09.2011, 03:14

> Since I hadn't messed directly with PM before (all I had ever done was set
> up some Descriptor Table entries), I thought that as well. But now that
> I've done some testing, I know it's not true. A20 has to be enabled to
> access memory above 1 MB, even if the CPU is in PM. I tried it with A20
> disabled -- it didn't crash, but the data was garbage.

Aha, thanks for explanation. So when going to PM it's needed to enable A20 logic and it's leaved enabled for whole time it runs in PM so it doesn't cause performance loss. But when need to switch to RM A20 logic is disabled (but it seems it's not necessary for newer programs) and it takes extra time (enabling too). If I remember well A20 was controlled via KBC chip through some IO port on old PC so it was really slow (IO cycle went to KBC through slow ISA bus). On modern PC it should be faster but it's legacy stuff so maybe there's little attention on good performance now...

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DOS gives me freedom to unlimited HW access.

 

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