Compatibility: XMS 2.0+
Expects: AH    09H
DX    desired size of block, in K-bytes
                                                                  
Returns: AX    status: 0000H = successful
0001H = error and BL=XMS error code (if BL >= 80H)
DX    XMS handle
                                                                  
Info: This allocates a block of extended memory and returns a handle to
use in future reference to that block.

If no memory is allocated, DX returns with 0000H (null handle).

By default, XMS supports 32 handles.  Use HIMEM.SYS /NUMHANDLES=
in the DEVICE= CONFIG.SYS command to increase this max.

See Also: XMS Functions
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XMS 09H: Allocate Extended Memory Block