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Using Multiple CPU Cores in DOS? (Miscellaneous)

posted by bretjohn(R) Homepage E-mail, Rio Rancho, NM, 18.09.2011, 04:27

> > ... but there must be something going on in the BIOS that makes it so
> > slow.
> measurements show that something is making it slow - whatever it is.

I did some experimenting today on one of my computers, and it looks like toggling the A20 line is the sole culprit. I can detail the exact procedures I took if someone wants. Anyway, I can get the timing down to approximately the same as V86 mode by taking A20 out of the equation.

> the BIOS doesn't care about A20. unexpected things would happen if int15.87
> is called with A20 disabled.

That may be true for some BIOS's, but not all. At least on the system I was testing today, the BIOS messes with the A20 line even if it doesn't need to (that is, it takes the time to enable A20 even if it's already enabled).

Anyway, I think I have a way around this problem figured out. Unfortunately, in order to work, it's going to need the A20 line on all the time. I need to get some opinions on how "appropriate" this is, though.

I've seen statements that vary from (paraphrasing), "There are a lot of programs that depend on A20 being disabled so you shouldn't EVER leave it enabled," to "There are only a few poorly written 8086-era programs that depend on A20 being disabled, and nobody uses them any more." I think the latter statement is more accurate, but just wondered what others feel/know about the real situation.

Thanks for all your help Tom, BTW.


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