To:	babcock AT cfa DOT harvard DOT edu
Cc:	djgpp AT sun DOT soe DOT clarkson DOT edu
Subject:	Re: Some help about djgpp C/C++
Date:	Wed, 11 May 1994 12:49:26 -0700
From:	Darryl Okahata <darrylo AT sr DOT hp DOT com>

> As I enter this, I realize that I don't know how to obtain a selector which
> points at a physical address outside the first MB.  I suspect that there is a
> standard DPMI function call to do this, but is there a C routine which
> implements this call, and does go32 support this function?  This question has

     I believe that the DPMI function you want is 0x0800.  I don't think
GO32 implements it, though, if a DPMI server does not already exist.
From the DPMI 0.9 specs (available as
ftp.cica.indiana.edu:/pub/pc/win3/misc/dpmi_txt.zip):

-------------------------------------------------------------------------------
                            16. PHYSICAL ADDRESS MAPPING


            Memory mapped  devices such as network adapters and displays
            sometimes have  memory mapped at physical addresses that lie
            outside of  the normal  1Mb of memory that is addressable in
            real  mode.     Under  many  implementations  of  DPMI,  all
            addresses are  linear addresses  since they  use the  paging
            mechanism of  the 80386.  This service can be used by device
            drivers to convert a physical address into a linear address.
            The linear  address can  then be  used to  access the device
            memory.

            Some implementations  of DPMI  may  not  support  this  call
            because it  could be  used to  circumvent system protection.
            This call  should only  be used  by programs that absolutely
            require direct access to a memory mapped device.

            To Call

                 AX = 0800h
                 BX:CX = Physical address of memory
                 SI:DI = Size of region to map in bytes

            Returns

                 If function was successful:
                 Carry flag is clear.
                 BX:CX =  Linear address  that can be used to access the
                      physical memory

                 If function was not successful:
                 Carry flag is set.

            Programmer's Notes

                 o    Under DPMI  implementations that  do not  use  the
                      80386 paging  mechanism, the  function will always
                      succeed and  the address returned will be equal to
                      the physical  address parameter  passed into  this
                      function.
                 o    It is  up to  the caller  to build  an appropriate
                      selector to access the memory.
                 o    Do not  use this  service to access memory that is
                      mapped in the first megabyte of address space (the
                      real mode addressable region).
-------------------------------------------------------------------------------

     The above is confusing, and I have no idea how it is used (there's
got to be a way of associating the resulting linear address with some
selector, but how?).  Don't ask me any questions.

     -- Darryl Okahata
	Internet: darrylo AT sr DOT hp DOT com

DISCLAIMER: this message is the author's personal opinion and does not
constitute the support, opinion or policy of Hewlett-Packard or of the
little green men that have been following him all day.

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