# File:        hmwk3-1.asm
# Written by:  Larry Merkle, Dec. 10, 2002
#
# Modified by: Archana Chidanandan, Dec. 11, 2003
#	       a. Name changed from "Move" to "MoveObject"
#
# Modified by: Archana Chidanandan, Sept. 9, 2004
#		 a. Upadated descriptions of the procedures.
#
# Given a current position component and a velocity component, the MIPS procedure
# MoveObject calculates a new position component.  It assumes periodic boundaries 
# at positions zero and eleven in both directions, so a ball that goes off of one 
# edge comes back at the opposite edge.
#
###############################################################################
#
# NOTE:  The procedure adheres STRICTLY to the MIPS register usage conventions.
#
###############################################################################
#
#
#
# Register usage in MoveObject:
#
# $a0 - Position component
# $a1 - Velocity component
# $v0 - Updated position component
#


	.text			# Text section of the program (as opposed to data).

main:
#------------------------------------------------------------------------------
#          Initialize the x and y positions and velocities of the "ball".
#          Call MoveBall repeatedly (infinite loop) to update the x and y 
#          positions of the "ball".
#------------------------------------------------------------------------------

#
# Insert your code here.
#

MoveBall:
#------------------------------------------------------------------------------
#          Call MoveObject twice. First, to update the x position of the "ball".
#          Second, to update the y position of the "ball".
#------------------------------------------------------------------------------

#
# Insert your code here.
#


MoveObject:

# Note: You may not modify this procedure.

#---------------------------------------------------------------------------------
#
# Given a current position component and a velocity component, this MIPS procedure
# calculates a new position component.  It assumes periodic boundaries at positions 
# zero and eleven in both directions, so a ball that goes off of one edge comes back
# at the opposite edge.
#
#----------------------------------------------------------------------------------
	add	$t0, $a0, $0	# position component
	add	$t1, $a1, $0	# velocity component
	
	add	$t0, $t0, $t1	# update component ignoring walls
					# i.e. add the velocity to the position

cklb:
	slt	$t2, $t0, $0	# set flag if component is less than zero
	beq	$t2, $0, ckub	# if flag is clear, ball didn't "cross" wall at zero
	addi	$t0, $t0, 11	# bring ball back in the other side
	j	cklb		      # a fast ball could "cross" more than once
	
ckub:
	slti	$t2, $t0, 11	# set flag if component is not at least eleven
	bne	$t2, $0, exit	# if flag is set, ball didn't "cross" wall at eleven
	addi	$t0, $t0, -11	# bring ball back in the other side
	j	ckub		      # a fast ball could "cross" more than once
	
exit:

	move	$v0, $t0	# set return value 

	# Wipe out other non-preserved registers for the fun of it
	li	$t0, -1		
	li	$t1, -1
	li	$t2, -1
	li	$t3, -1
	li	$t4, -1
	li	$t5, -1
	li	$t6, -1
	li	$t7, -1
	li	$t8, -1
	li	$t9, -1
	li	$a0, -1
	li	$a1, -1
	li	$a2, -1
	li	$a3, -1
	li	$v1, -1
	
	jr	$ra		# return to the calling procedure