Compatibility: 2.0+
Expects: AH    49H
ES    segment address (paragraph) of allocated memory to release
                                                                  
Returns: AX    error code if CF is set to CY
                                                                  
Info: Deallocates (frees) the block of memory starting at ES:0000.
This block will become available for other system resources.

DOS prevents a process from freeing blocks of memory that were
allocated by another process (the PID  of the owner is saved
in the MCB).  The undocumented DOS Fn 50H lets you become the
"current process" and thus free blocks you don't really own.

It is recommended that a TSR program free its environment block
during installation (that is, if it won't need it again!).  This
allows the next program to reuse that block.

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DOS Fn 49H: Free Allocated Memory Block