# A recursive factorial function (or so I hope).
# The stack frame for each invocation has:
#    Return-Address (for recursive calls)
#    Parameter (for recursive calls)
#    Base of previous frame (for recrusive calls)
# In addition
#    R(100) holds the return address
#    R(101) holds the parameter
#    R(102) holds the base of the previous frame (we don't know its
#       size, so we need to remember it
#    R(102) holds the return value from the recursive call
BUILTIN L(print) L(WELCOME)
CALL L(fact) 4
BUILTIN printlnInt RV
JUMP 1 L(DONE)
LABEL fact
# Update the frame pointer
MOVE R(102) FP
MOVE FP SP
# Update the stack pointer
BINOP MINUS SP 4
MOVE SP ACC
# Fill in the return address
MOVE R(100) RA
# Fill in the parameter
MOVE R(101) A0
# If the parameter is less than 1, we're done
CJUMP LT R(101) 1 L(BASECASE)
# The recursive case
LABEL RECCASE
# Put stuff on the stack frame
MOVE M(FP) R(100)
BINOP MINUS FP 1
MOVE M(ACC) R(101)
BINOP MINUS FP 2
MOVE M(ACC) R(102)
# Compute the new parameter: R(101) - 1
BINOP MINUS R(101) 1
# And the recusive call
CALL L(fact) ACC
# Grab the result
MOVE R(103) RV
# Restore information
MOVE R(100) M(FP)
BINOP MINUS FP 1
MOVE R(101) M(ACC)
BINOP MINUS FP 2
MOVE R(102) M(ACC)
# Compute the result
BINOP MUL R(103) R(101)
MOVE RV ACC
JUMP 1 L(ENDfact)
# The base case
LABEL BASECASE
# Set the return value
MOVE RV 1
JUMP 1 L(ENDfact)
# Clean up after ourselves
LABEL ENDfact
# Restore the stack pointer
MOVE SP FP
# Restore the frame pointer
MOVE FP R(102)
# Return to where we came from
JUMP 1 R(100)
# Here's a carriage return
LABEL CR
STRING \n
# Were's a welcome message
LABEL WELCOME
STRING Welcome to factorial\n
LABEL DONE
END