INT 2F - DOS 5.0 - ALLOCATE HMA SPACE
        AX = 4A02h
        BX = number of bytes
Return: ES:DI -> start of allocated HMA block or FFFFh:FFFFh
        BX destroyed
Notes:  this call is not valid unless DOS is loaded in the HMA (DOS=HIGH)
        called by Windows 3.1 DOSX.EXE