Function 4Ah (74)        Modify Memory Allocation (SETBLOCK)

    Changes the size of a memory block that was allocated via Function
    48h.

       On entry:      AH         4Ah
                      BX         New block size, in paragraphs
                      ES         Segment address of the memory block to be
                                 modified

       Returns:       AX         Error code, if CF is set
                      BX         Maximum memory available, if CF is set and
                                 request was to expand memory block

       Error codes:   7          Memory control blocks destroyed
                      8          Insufficient memory
                      9          Invalid block address

                      Call Function 59h for extended error code information
                      (DOS 3.0 and above).

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    This function is used to increase or decrease the size of a memory
    block allocated by Function 48h. ES contains the segment address of
    the block whose size will be changed. BX contains the new size of the
    block, in paragraphs (16-bytes units).