Physical Address Mapping

Converts a physical address into a linear address.
  This function allows device drivers running under DPMI
hosts which use paging to reach physical memory that is
associated with their devices above the 1 MB boundary
Examples of such devices are the Weitek numeric coprocessor
(usually mapped at 3 GB), buffers that hold scanner bit
maps, and high-end displays that can be configured to make
display memory appear in extended memory.

Call With:

   AX     = 0800H
   BX:CX  = physical address of memory
   SI:DI  = size of region to map (bytes)

Returns:

   if function successful
   Carry flag= clear
   BX:CX  = linear address that can be used to access the
           physical memory

   if function unsuccessful
   Carry flag= set
   AX     = error code
           8003H   system integrity (DPMI host memory
                   region)
           8021H   invalid value (address is below 1 MB
                   boundary)

Notes:

o  It is the caller's responsibility to allocate and
   initialize a descriptor for access to the memory.
o  This function should only be used by clients that
   absolutely require direct access to a memory mapped
   device at physical addresses above 1 MB.  Clients should
   not use this function to access memory below the 1 MB
   boundary (the real mode addressable region).  See also
   Int 31H Functions 0002H, 0508H, and 0509H.
o  When this function is called, the DPMI host either
   creates page table entries that directly map the physical
   addresses requested and returns the linear address of the
   created page table entries, or else just returns the
   linear address of the memory region that is already used
   to map the requested device.  For example, if the client
   attempts to map a Weitek coprocessor and the host already
   has a linear region set up to map the Weitek chip and
   virtualize it, it would simply return the linear address
   of the existing region.  If the host does not virtualize
   the Weitek chip, it would create 16 page table entries
   that map the 64 KB Weitek address space and return a
   linear address corresponding to the new page table
   entries.
o  If the host is not virtualizing the device, it must
   disable any memory caching on the mapped pages;  in
   particular, on the 80486 the host must set the PCD (page
   cache disable) bit in the page table entries.
o  The host is permitted to fail any memory mapping call.
   However, the host should support this function whenever
   possible, to achieve compatibility with application
   programs that use memory-mapped devices of which the host
   is not aware.  Useful guidelines are that the host should
   fail any attempt to map addresses below 1 MB, or
   addresses which the host considers to be general-purpose
   RAM memory.  Attempts to map any other physical address
   should succeed, since the host should either (a) already
   know about the device and be able to return a linear
   address used to access the device, or (b) assume the
   program is attempting to map a legitimate device of which
   the host has no knowledge.

o  Programs and device drivers which need to perform DMA I/O
   to physical addresses in a virtualized hardware
   environment should use the Virtual DMA Services (see the
   Glossary entry for the Virtual DMA Services
   Specification).   Also see page 10 of the DPMI execution
   environment section.