FREE MEMORY

 Command:       $AF $0B
 Data:          $ww $xx $yy $zz

 Free the memory allocated with the ALLOCATE MEMORY command.  This command
 has the same restrictions as ALLOCATE MEMORY.  Memory is released by
 calling freevec() on the address <$wwxxyyzz>

 Command ACK:   $00 $0B
 Command DONE:  $1F $0B