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INT 4B - Virtual DMA Specification - UNLOCK DMA REGION
        AX = 8104h
        DX = flags
            bit 0: reserved (zero)
                1: data should be copied out of buffer
             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
SeeAlso: AX=8103h,AX=8106h