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From: ralf+@cs.cmu.edu (Ralf Brown)
Newsgroups: comp.os.msdos.programmer
Subject: Re: memory
Message-ID: <1991Apr6.045151.9855@cs.cmu.edu>
Date: 6 Apr 91 04:51:51 GMT
References: <1364@bragi.ctron.com> <252u3715.670712175@fergvax> <1396@bragi.ctron.com>
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Organization: School of Computer Science, Carnegie Mellon
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In article <1396@bragi.ctron.com> dj@bragi.ctron.com (DJ Delorie) writes:
}Gate-A20 is a kludge put into the original AT to make it more backward
}compatible with the XTs.  It turns out that some applications
}*required* that the memory space wrap at the 1M boundary (examples:
}CPM-86 and MS Flight Simulator 2.x).  When you enable gate-A20, the

Add *all* versions of MSDOS, at least when invoked via the CP/M compatibility
entry point at offset 5 in the PSP.  That entry does a FAR jump to absolute
1000C0h, which is expected to wrap to 000C0h.  That location contains
another FAR jump into the DOS kernal; DESQview is smart enough to make a copy
of that jump instruction beyond the 1M mark when it loads itself into the
HMA.

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