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

• 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.
• It is up to the caller to build an appropriate selector to access the memory.
• Do not use this service to access memory that is mapped in the first megabyte of address space (the real mode addressable region).