parity.s

.globl main

.data
L: .word 3
A: .half 0x002A, 0xA5FF, 0x5BE1

.text
main:
	# prepare arguments for the parity function
	lw $a1, L
	la $a0, A
	
	jal parity
	
	move $a0, $v0 # a0 = v0
	li $v0, 1 # print the parity word
	syscall
	
	move $a2, $a0 # a2 = a0
	
	jal parity_bit
	
	move $a0, $v1 # a0 = v1
	li $v0, 1 # print the parity bit of the parity word
	syscall
	
	li $v0, 10 # exit
	syscall
parity:
	# push stack
	subi $sp, $sp, 8
	sw $ra, 4($sp)
	sw $fp, 0($sp)
	
	# result
	move $v0, $zero
	
	# index
	move $v0, $zero
	
	# exit condition
	sll $t1, $a1, 1 # t1 = a1 << 1 => a1 * 2 (L * half_size)
	
	loopA:
		beq $t0, $t1, loopAExit # exit if index == L * half_size
		add $t4, $a0, $t0 # t4 = a0 + t0, array_address + index_address => the current half address
		
		lh $a2, 0($t4) # a2 = L[t4]
		jal parity_bit # v1 = parity_bit(a2: half_word)
		
		sll $v0, $v0, 1 # v0 <<= 1 making space for the next parity bit
		add $v0, $v0, $v1 # v0 += v1, adding the parity bit to the result
		addi $t0, $t0, 2 # t0 += 2
		
		j loopA
	loopAExit:
	
	# pop stack
	lw $fp, 0($sp)
	lw $ra, 4($sp)
	addi $sp, $sp, 8
	
	jr $ra
	
parity_bit:
	# push stack
	subi $sp, $sp, 8
	sw $ra, 4($sp)
	sw $fp, 0($sp)
	
	# result
	move $v1, $zero
	
	# bitmask
	move $t5, $zero
	addi $t5, $t5, 1 # t5 = 1
	
	# bit counter
	move $t7, $zero # t7 = 0
	
	# exit condition
	move $t6, $zero
	addi $t6, $t6, 16
	
	loopHalf:
		beq $t7, $t6, loopHalfExit # exit if bit counter == 16 (half_size = 16)
		and $s0, $t5, $a2 # s0 = t5 & a2, s0 stores the current bit of the word
		
		srlv $s0, $s0, $t7 # s0 >>= t7, transfer the MSB to the LSB
		
		xor $v1, $v1, $s0 # v1 ^= s0
		sll $t5, $t5, 1 # t5 <<= 1, update the bitmask
		addi $t7, $t7, 1 # t7 += 1
		
		j loopHalf
	loopHalfExit:
	
	# pop stack
	lw $fp, 0($sp)
	lw $ra, 4($sp)
	addi $sp, $sp, 8
	
	jr $ra