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From:	"Rod Pemberton" <dont_use_email AT xnohavenotit DOT cnm>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Is int86() designed and implemented correctly?
Date:	Sat, 15 Feb 2014 05:16:29 -0500
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For interrupts other than Int 21h, int86() calls _int86
in assembly which generates an interrupt in PM.  For
many, but not all, Int 21h sub-functions, int86() does
the same.  Supposedly, this is to allow installed PM
interrupt routines to execute.

However, there are thirteen Int 21h sub-functions
which have been extended for setting up pointers,
transfer buffer, return values etc within int86().
These thirteen sub-functions *do not* call the _int86
assembly routine which generates a PM interrupt.
Instead, int86() calls __dpmi_int() for them.
__dpmi_int() calls the DPMI host's simulate real mode
interrupt DPMI API call, thereby generating a RM
interrupt, instead of a PM interrupt.  Shouldn't these
extended Int 21h sub-functions have been designed to
call int86() to generate a PM interrupt?

Using __dpmi_int() would seem to *bypass* installed PM
ISR routines for those Int 21h sub-functions, calling
RM instead.  Is this correct?  Since it seems these
sub-functions can't be called in PM via DJGPP, how does
someone call installed custom ISRs for the thirteen Int 21h
sub-functions?  Does assembly need to be used to generate
a PM Int 21h interrupt directly?  Or, am I missing something,
like these get reflected back to PM somewhere, somehow?


Rod Pemberton

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