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Using Multiple CPU Cores in DOS? (Miscellaneous)

posted by bretjohn(R) Homepage E-mail, Rio Rancho, NM, 17.09.2011, 02:03

> I think that A20 is fiddled only when acessing HMA so it's useless for
> reaching MMIO at higher address. And when himem access XMS it shouldn't do
> extra settings of A20. It seems to mile like 2 different things without any
> relations.

That seems logical to me also, but there must be something going on in the BIOS that makes it so slow. It seems like all it would need to do is a variation on what HIMEMX does, which should be very fast (haven't done my testing yet, though).

I did notice in the code for the latest HIMEMX that basically all the INT 15.87 trap does is keep track of the A20 state, so the BIOS must be doing something with A20. And, in the comments at the top of the source it talks about how slow this is. I'm just guessing that maybe the BIOS messes with A20 every time, even if it doesn't actually need to. Maybe Tom/Japheth/whoever (somebody who's worked on HIMEMX or similar so has had to do some research already) can shed some light on it.


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