INT 4B - Virtual DMA Specification - LOCK DMA REGION
        AX = 8103h
        DX = flags (see below)
        ES:DI -> DMA descriptor structure (see AX=8102h)
Return: CF clear if successful
            DDS physical address field filled in
            DDS buffer ID field filled (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
BUGS:   Windows 3.0 does not correctly support automatic remapping or copying
          in enhanced mode
        Windows 3.0 in enhanced mode does not return a correct code on error
SeeAlso: AX=8104h,AX=8105h

Bitfields for flags:
 bit 0  reserved (zero)
 bit 1  data should be copied into buffer (ignored if bit 2 set)
 bit 2  buffer should not be allocated if region noncontiguous or crosses
        physical alignment boundary specified by bits 4-5
 bit 3  don't attempt automatic remap
 bit 4  region must not cross 64K physical alignment boundary
 bit 5  region must not cross 128K physical alignment boundary
 bits 6-15 reserved (zero)

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