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====== Int 31H, AH=08H, AL=00H ======

===== Version =====

0.9

===== Brief =====

Physical Address Mapping

===== Input =====

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

  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 =====

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.

It is the caller's responsibility to allocate and initialize a descriptor for access to the memory.

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.

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.

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.

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.

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.

===== See also =====

   

===== Note =====

Text based on [[http://www.delorie.com/djgpp/doc/dpmi/]]

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