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study/sabre/os/files/ProtectedMode/DPMI1.0Spec/api/310800.html

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+<h2>Physical Address Mapping
+<img src="../0.9.gif" alt="[0.9]" width=22 height=17></h2>
+
+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.<p>
+
+<b>Call With</b><br>
+<img src="r/ax+bx+cx+si+di.gif" alt="" width=245 height=59><br>
+
+AX     = 0800H<br>
+BX:CX  = physical address of memory<br>
+SI:DI  = size of region to map (bytes)<p>
+
+<b>Returns</b><br>
+<img src="r/ax+bx+cx+c.gif" alt="" width=245 height=59><br>
+
+<i>if function successful</i><br>
+Carry flag = clear<br>
+BX:CX  = linear address that can be used to access the
+physical memory<br>
+
+<i>if function unsuccessful</i><br>
+Carry flag = set<br>
+AX     = <a href="errors.html">error code</a>
+
+<table border=1 cellspacing=0 cellpadding=4>
+
+<tr><td>8003H</td><td>system integrity (DPMI host memory
+region)</td></tr>
+<tr><td>8021H</td><td>invalid value (address is below 1 MB
+boundary)</td></tr>
+</table>
+
+<h2>Notes</h2><ul>
+
+
+<li>It is the caller's responsibility to allocate and initialize a
+descriptor for access to the memory.<p>
+
+<li>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
+<a href="310200.html">Int 31H Functions 0002H</a>, <a
+href="310508.html">0508H</a>, and <a href="310509.html">0509H</a>.<p>
+
+<li>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.<p>
+
+<li>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.<p>
+
+<li>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.<p>
+
+<li>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.<p>
+
+</ul>
+
+