Compatibility: 5.0+
Expects: AX    5803H
BX    01H = link upper  memory for allocations
00H = unlink (all allocations go to conventional  mem)
                                                                  
Returns: AX    error code if CF is set to CY
                                                                  
Info: Enables or disables access to upper memory.

See Accessing Upper Memory for info on allocating from UMBs.

The command dos=umb must be executed in CONFIG.SYS before is it
possible to link and allocate upper memory.

This call fails with error code 0001h (invalid function) on 8088
and 286 CPUs and on 386+ CPUs in which a 386 memory manager such
as Emm386.exe was not installed and/or dos=umb was not executed.

Programs which set the link state should use fn 5802H and save
the current setting and be sure to change it back before
terminating.

See Also: Fn 48H (allocate memory)
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DOS Fn 5803H: Set Upper-Memory Link State