Compatibility: XMS 2.0+
Expects: AH    10H
DX    desired size of UMB , in paragraphs (16-byte units)
Returns: AX    status: 0000H = successful
0001H = error and BL=XMS error code (if BL >= 80H)
BX    segment of the UMB
DX    (success) Actual size of the UMB
(failure) Size of largest UMB available
Info: This is a low-level way to gain ownership of a UMB  or to learn
the size of the largest available UMB.

See Accessing Upper Memory for the correct (DOS-blessed) way to
get a UMB when running DOS 3.0+.

Notes:   To learn the size of the largest available UMB, set DX=ffffH on

  UMBs are guaranteed not be affect the EMS frame.

  UMBs are paragraph-aligned.

See Also: XMS Functions
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XMS 10H: Request Upper Memory Block