Compatibility: XMS 2.0+
Expects: AH    11H
DX    segment address of he UMB (as obtained via XMS 10H)
                                                                  
Returns: AX    status: 0000H = successful
0001H = error and BL=XMS error code (if BL >= 80H)
                                                                  
Info: This frees ownership of a UMB obtained via XMS 10H (request UMB).

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

Notes: When a UMB has been released, any code and data in it becomes
invalid and should not be accessed.

See Also: XMS Functions
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XMS 11H: Release Upper Memory Block