CPS 104			 Problem PA


	    MYMIPS Self-Moving Program (CRAWLER)

     Write  a  MYMIPS  program	which  moves  itself  around
memory,	 following "directions"	which are read from standard
input.	The input consists of a	sequence of  integers.	 The
program	 processes these integers one at a time.  To process
one input integer, assume the program and all its  data	 are
currently  located  at	locations  Q  through  Q+N of MYMIPS
MEMORY.	 (Initially, Q==0).  Upon reading  an  integer	with
value  K,  the program moves itself to locations Q+K through
Q+N+K, and executes a "syscall break" instruction  at  loca-
tion Q+K+16.  It then repeats these steps until	some integer
is read	that specifies a new value of Q	less than 800  bytes
away  from  the	current	value of Q.  When such an integer is
read, the program should exit silently.

     Your interpreter should  be  modified  to	execute	 the
"syscall break"	by executing

	printf(	"Break at PC=%08X\n",PC);

where PC holds the location of the "syscall break", and	CON-
TINUING	 execution  with the next instruction.	Your program
must not contain any "sycall print-string" operations.

     Be	certain	that, after the	 program  moves	 itself	 and
executes the "syscall break", its further execution does not
depend on any values stored in its previous  locations.	  We
will test that this is the case, by having our interpreter's
"syscall break"	operation store	something into all locations
other  than  an	800 byte region	starting 16 bytes before the
location of the	"syscall break".

     Your program should test K	for validity, and exit on  K
values that cannot produce a valid new location	for the	pro-
gram;  however,	the program  should  provide  for  K  values
which are positive, or negative, as long as their magnitudes
are acceptable.

     Part of your task is to get your  interpreter  to	work
correctly,  also.   You	 may  test  your program against our
reference interpreter, but that	interpreter  will  NOT	have
the  required  modification built in, so correctness of	your
program	may be hard to determine while using it.  You should
submit the corrected interpreter along with your MYMIPS	pro-
gram.  The members of the original team	that  developed	 the
team  interpreter  should each submit independently modified
versions of that interpreter.