Xref: news2.mv.net comp.os.msdos.djgpp:8508
From: "david l." <danger37 AT znet DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: FP_SEG & FP_OFF equivelent for DJGPP?
Date: Tue, 10 Sep 1996 19:25:04 -0700
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Samuel Vincent wrote:
> 
> On Fri, 6 Sep 1996, Matt wrote:
> 
> > I am converting some real mode cd player code to djgpp and nowhere can
> > I find the equivelent.  I need the seg and offset for a variable that
> > is required for the real mode int.  Any Ideas?
> >
> > {this is probably a very dumb question}
> >
> > Matt Kittleson
> 
> This is probably in the FAQ somewhere...
> FP_SEG in 16 bit compilers would deal with a real mode segment
> FP_OFF in 16 bit compilers would deal with a real mode offset.
> Both of these would be 16 bit values.
> 
> In DJGPP as in most 32-bit compilers, you deal mainly with one
> large selector (the protected mode equivolent of a real mode segment, but
> more complicated than just that.)  Every pointer is a 32-bit value and is
> only the offset in the program space. (A 32-bit offset.)  This is much
> like a near pointer in real mode, only, instead of being limited to 64k
> near pointers due to the 16-bit value, you are 'limited' to 4 gigabytes of
> memory with the new 32-bit value.
> 
> Now how, might you ask, do you get a "far pointer?"  I believe the answer
> the your question is the functions in the header file <sys/farptr.h>.
> There are macros in there which will retrieve or store values across
> different selectors, and they are very efficient.  In most cases you will
> want to use these.  Some may tell you about a method of disabling memory
> protection and accessing the "far pointers" directly, but this will only
> work in a ring 0 DPMI provider, such as CWSDPMI.  It will not work under
> any Windows or OS/2 dos shell, or any linux dos emulator.  If you wish for
> information on this technique, ask the others in/on this newsgroup/mailing
> list.  I never use that technique.  The farptr.h macros and functions are
> just as fast, when used correctly.
> 
> -Sam