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  Function 49H (decimal 73) frees a block of memory for subsequent reuse by
  DOS or by other programs. Call function 49H with ES containing the
  paragraph address (segment) of the start of the memory block. If the
  memory is successfully freed, the function clears the carry flag.
  Otherwise, the carry flag is set, and AX contains an error code: 07H
  (memory control blocks destroyed) or 09H (invalid memory-block address).

  Although function 49H is usually used to free memory previously allocated
  through function 48H, it will free any memory block. For example, a
  Terminate-and-Stay-Resident program can free its environment block by
  calling function 49H with ES containing the paragraph address of the
  environment block. (See the discussion of function 31H in this chapter.)