Xref: news2.mv.net comp.os.msdos.djgpp:7946
From: Elliott Oti <e DOT oti AT stud DOT warande DOT ruu DOT nl>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: what is dpmi....
Date: Tue, 27 Aug 1996 13:15:11 -0700
Organization: Academic Computer Centre Utrecht, (ACCU)
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Message-ID: <3223574F.43AE@stud.warande.ruu.nl>
References: <9608271823 DOT AA18073 AT pdv DOT com>
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Franklin asked what DPMI was 
>  and what can i use this for...(please correct my grammar if bad)
> 
>  regards, Franklin Castillo

DPMI = Dos Protected Mode Interface
DPMI servers are programs that control a 386/486/586 in protected mode
and are responsible for:
1. DOS (low memory) and expanded memory management
2. Management of interrupts (hard and software)
3. Page and LDT management
4. Context switching
5. Debugging.

In the good old days,right after 640k was enough for everyone,people started 
pluggin' extra memory into their PC's, and Intel developed this newfangled
workstation chip called the 80286 which had a thing called protected mode.
In order to access that extra memory, MS, Intel and Lotus developed the
Expanded Memory specification (EMS) to handle expanded memory, and later the
Extended Memory Specification (XMS) for extended memory above 1Mb.
Out of the two standards,and to cope with this new 286 protected mode thingy ,
the VCPI and later DPMI standards developed.

DPMI and VCPI servers are programs that handle the messy stuff of finding
and allocating memory, switching to & from protected mode etc, like
Windows (DPMI), CWSDPMI( DPMI), go32 ( VCPI ), Dos4GW ( VCPI) etc.
DPMI is a successor to VCPI ( Virtual Control Program Interface ); VCPI
servers have control of the system in V86 mode -- this means programs have
to beg 'em for memory, and say aye and amen -- but in protected mode
the client ( ie the application program) has control & can do essentially
what it likes.
With DPMI this changes; DPMI servers assign priviledge levels to programs
running under them.The DPMI host itself has the highest level and the client
runs at a lower priviledge level. The lower the level, the more permission has to
be asked of the DPMI host before a program can do something. Clients
running at the lowest level not only have to ask politely for memory,
they can't issue interrupts, outports, inports, or farts without permission.
Or at least they can try, but they won't neccessarily get away with it.

Using DPMI directly is a real pain in the arse and should be avoided as much
as posible. There are 80+ functions you can call under a DPMI host and so
you want to use a DOS extender to ease the pain, or a compiler that shields
you from the nasty complexities of DPMI function calls. Which is why I, after
hours of numbly & fruitlessly poring thru the DPMI specs in yesteryear, have
nothing but profound admiration for C.W Sandman and DJ Delorie who not only
understand the stuff, but actually MAKE extenders/DPMI servers that *WORK*.

Elliott