INT 4B - Virtual DMA Specification (VDS) - GET VERSION
        AX = 8102h
        DX = 0000h
Return: CF clear if successful
            AH = major version number
            AL = minor version number
            BX = product number (see below)
            CX = product revision number
                always 0000h for QMAPS and HPMM.SYS
                always 0001h for Microsoft's EMM386.EXE v4.20-4.41
            SI:DI = maximum DMA buffer size
            DX = flags
                bit 0: PC/XT bus (DMA in first megabyte only)
                bit 1: physical buffer/remap region in first megabyte
                bit 2: automatic remap enabled
                bit 3: all memory is physically contiguous
                bits 4-15 reserved (zero)
        CF set on error
            AL = error code (see below)
Note:   bit 5 of 0040h:007Bh is supposed to be set if VDS is supported; this is
          apparently not always the case
SeeAlso: INT 2C/AX=002Bh,INT 31
Index:  installation check;Virtual DMA Spec

Values for product number:
 0000h  for Quadtel's QMAPS and Hewlett-Packard's HPMM.SYS
 0001h  for Microsoft's EMM386.EXE
 0003h  for Windows 3.x WIN386.EXE
 0EDCh  for DR-DOS 6.0 EMM386.SYS
 4560h  ("E`") for Qualitas' 386MAX
 4D43h  ("MC") for V Communication's Memory Commander
 5145h  ("QE") for Quarterdeck's QEMM-386
 524Dh  ("RM") for Helix's Netroom RM386

Values for error code:
 01h    region not in contiguous memory
 02h    region crossed a physical alignment boundary
 03h    unable to lock pages
 04h    no buffer available
 05h    region too large for buffer
 06h    buffer currently in use
 07h    invalid memory region
 08h    region was not locked
 09h    number of physical pages greater than table length
 0Ah    invalid buffer ID
 0Bh    copy out of buffer range
 0Ch    invalid DMA channel number
 0Dh    disable count overflow
 0Eh    disable count underflow
 0Fh    function not supported
 10h    reserved flag bits set in DX

Format of DMA descriptor structure (DDS):
Offset  Size    Description
 00h    DWORD   region size
 04h    DWORD   offset
 08h    WORD    segment/selector
 0Ah    WORD    buffer ID
 0Ch    DWORD   physical address

Format of Extended DMA descriptor structure (EDDS):
Offset  Size    Description
 00h    DWORD   region size
 04h    DWORD   offset
 08h    WORD    segment/selector
 0Ah    WORD    reserved
 0Ch    WORD    number available
 0Eh    WORD    number used
 10h    DWORD   region 0 physical address
 14h    DWORD   region 0 size in bytes
 18h    DWORD   region 1 physical address
 1Ch    DWORD   region 1 size in bytes
        ...

Format of Extended DMA descriptor structure (EDDS) with page table entries:
Offset  Size    Description
 00h    DWORD   region size
 04h    DWORD   offset
 08h    WORD    segment/selector
 0Ah    WORD    reserved
 0Ch    WORD    number available
 0Eh    WORD    number used
 10h    DWORD   page table entry 0 (same as 80386 page table entry)
 14h    DWORD   page table entry 1
        ...
Note:   bits 1-11 of the page table entries should be zero; bit 0 set if page
          is present and locked

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