I copied the hello.mixal example from:
https://www.gnu.org/software/mdk/manual/html_node/Writing-a-source-file.html#Writing-a-source-file

But when I compile it with mixasm it gives a warning:

$ mixasm --version
mixasm, MIX assembler (GNU MDK 1.2.10)

Copyright (C) 2000-2014 Free Software Foundation, Inc.
There is NO warranty.  You may redistribute this software
under the terms of the GNU General Public License.
For more information about these matters, see the files named COPYING.
$ cat hello.mixal
*

• hello.mixal: say 'hello world' in MIXAL

*

• label ins    operand     comment

TERM    EQU    19          the MIX console device number
        ORIG   3000        start address
START   OUT    MSG(TERM)   output data at address MSG
        HLT                halt execution
MSG     ALF    "MIXAL"
        ALF    " HELL"
        ALF    "O WOR"
        ALF    "LD   "
        END    START       end of the program
$ mixasm hello.mixal
hello.mixal:0: warning: undefined symbol: halt
(1 warning(s))