DOS Fn 5803H: Set Upper-Memory Link State

                                                         Compatibility: 5.0+ 
 Expects: AX    5803H
          BX    01H = link upper▲ memory for allocations
                00H = unlink (all allocations go to conventional▲ mem)
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 Returns: AX    error code if CF is set to CY
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    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)
          Memory Control Functions
          DOS Functions
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