XMS 10H: Request Upper Memory Block

                                                     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
          Extended Memory Specification (XMS)
          DOS Functions