Int 4B Fn 8104  - Virtual Dma Specification - Unlock Dma Region            [d]

   AX = 8104h
   DX = flags
       bit 0: reserved (zero)
       bit 1: data should be copied out of buffer
       bits 2-15 reserved (zero)
   ES:DI -> DMA descriptor structure (see AX=8102h) with region size,
       physical address, and buffer ID fields set

Return: CF clear if successful
       DDS physical address field set
       DDS buffer ID field set (0000h if no buffer allocated)
   CF set on error
       AL = error code (see AX=8102h)
       DDS region size field filled wth maximum contiguous length in bytes

Note:  Windows 3.0 does not check whether the region extends beyond the end of
     a segment
BUG:   Windows 3.0 in enhanced mode does not return a correct code on error

See Also: AX=8103h,AX=8106h