# INEL 4206 - Microprocessors
# University of Puerto Rico Mayaguez
#
# Program that performs division by repeated subtraction
# The operands reside in global variables x and y
# The result of x/y ends up in global variable res
# The remainder of the division ends up in x
#
# Instructional Objectives
# + teach how to program control structures (while)
# + teach load and store pseudo instructions
# + teach how to use I/O system calls
# + teach how to implement simple procedures
#   (no params, no recursion, no return values)
# + teach how to implement procedures using stacks
#   (support recursion and procedures calling other procedures)
	.data				# Use HLL program as a comment
x:	.word	0			#   int x = 0;
y:	.word	0			#   int y = 0;
res:	.word	0			#   int res = 0;
pf1:	.asciiz "Result = "
pf2:	.asciiz "\nRemainder = "
	.globl	main
	.text
# main function 
# PENDING ISSUES: main must allocate a stack frame as well
main:					# main assumes registers sx unused
	li	$s1, 12			# x = 12;
	usw	$s1, x
	li	$s2, 5			# y = 5;
	usw	$s2, y
	li	$s3, 0			# res = 0;
	usw	$s3, res
	move	$s4, $ra		# Save $ra in $s4, callee saved
	jal	d			# div();
	move	$ra, $s4		# Restore $ra
	ulw	$s1, x			# Must reload variables from memory
	ulw	$s2, y
	ulw	$s3, res
	la	$a0, pf1		# printf("Result = %d \n");
	li	$v0, 4			#   //system call to print_str
	syscall
	move	$a0, $s3		
	li	$v0, 1			#   //system call to print_int
	syscall
	la	$a0, pf2		# printf("Remainder = %d \n");
	li	$v0, 4			#   //system call to print_str
	syscall
	move	$a0, $s1		
	li	$v0, 1			#   //system call to print_int
	syscall
	jr	$ra			# return; 

# div function
# 
d:	sub	$sp, $sp, 36		# Alloc space for 36 byte stack frame
	sw	$a0, 32($sp)		# Save argument registers
	sw	$a1, 28($sp)		
	sw	$a2, 24($sp)
	sw	$a3, 20($sp)		
	sw	$ra, 16($sp)		# Save other registers as needed
	sw	$fp, 12($sp)		
	sw	$s1, 8($sp)		# Save callee saved registers ($sx)
	sw	$s2, 4($sp)		
	sw	$s3, 0($sp)
					# No need to save $s4, since not used
	addu	$fp, $sp, 32
					# Allocate registers for globals
	ulw	$s1, x			#   x in $s1
	ulw	$s2, y			#   y in $s2
	ulw	$s3, res		#   result in $s3
while:	bgt	$s2, $s1, endwhile	# while (x <= y) {
	sub	$s1, $s1, $s2		#   x = x - y
	addi	$s3, $s3, 1		#   res ++
	j	while			# }
endwhile:				
	usw	$s1, x			# Update variables in memory
	usw	$s2, y
	usw	$s3, res
	lw	$ra, 16($sp)		# Restore saved registers
	lw	$fp, 12($sp)
	lw	$s1, 8($sp)
	lw	$s2, 4($sp)
	lw	$s3, 0($sp)
	addu	$sp, $sp, 32		# pop stack frame
enddiv:	jr	$ra			# return; 
#