# fact.eas -- factorial, via recursion
# SCCSID("%Z%%P%	%I%")

# Caller
A N1 N<READNUM T	# obtain argument
A N1 N<FACT T		# call factorial function
A N1 N<WRITENUM T	# print result
N10 O N1 N0 T		# print a newline and exit

# One input arg: function prologue
>FACT: N1 H		# roll argument above return address	# ... addr arg

# Function body
# Factorial of 1 is just 1
N0 H			# duplicate top for test		# ... addr arg arg
N1 S A N0 N1 S S T	# skip if non-zero (ie. top wasn't 1)	# ... addr arg arg-1 here+1 T
N1 N<FACTdone T		# argument _was_ 1: we're finished	# ... addr arg
N0 H			# dup					# ... addr arg arg
N1 S			# stack is now: ... addr n n-1		# ... addr arg arg-1
A N1 N<FACT T N1 H	# recursive call (tail recursion!)	# ... addr fact(arg-1) arg 
A N1 N<MULTIPLY T	# multiply Nby fact(n-1)		# ... addr fact(arg-1)*arg
# result is on the stack: fall through to the epilogue

# Finished: function epilogue
>FACTdone: N1 N2 H T

*readnum.eas
*writenum.eas
*multiply.eas
*writestr.eas