MFC0 $k0, $13
	nop
	sll $k0, 26
	srl $k0, 28    #Setting bits 2-5 to the least significant bits
	beq $k0, $zero, ext_int #If bits are 0 it is an external interupt.
	nop

	addiu $k1, $zero, 8
	bne $k0, $k1, kernel_loop   #Making sure the exception was caused by a syscall
	nop

	addiu $k0, $zero, 0x102
	bne $v0, $k0, return	#Making sure we want to output
	nop

	andi $k0, $k0, 0
	lui $k0, 0xffff
	sb $a0, 8($k0)		#Outputting the character in a0
	b return
	nop



ext_int:MFC0 $k0, $13
	nop
	addiu $k1, $zero, 0x400   #Mask to check bit 10
	and $k1, $k1, $k0
	bne $k1, $zero, timer_int  #If the bit is set it is a timer interupt
	nop
	b kernel_loop		#Else kernel loop
	nop




timer_int:

	li      $k0, 0xFFFF0010		#Clearing the timer interupt.
	li      $k1, 0b101001
	sw      $k1, 0($k0)

	addiu	$sp, $sp, -4
	sw	$ra, 0($sp)		#store return address on stack
	addiu	$sp, $sp, -4
	sw	$sp, 0($sp)		#store frame pointer on stack
	move	$fp, $sp		#establish new frame pointer
	addiu	$sp, $sp, -4
	sw	$t0, 0($sp)		#Storing the registers used by the processes on the stack.
	addiu	$sp, $sp, -4
	sw	$t5, 0($sp)
	addiu	$sp, $sp, -4
	sw	$a0, 0($sp)

	li $k0, 0x80000000
	addiu	$k0, $k0, curpcb
	lw	$k1, 0($k0)		#load address for current PCB
	mfc0	$k0, $14		#load program counter
	sw	$gp, 4($k1)		#store global pointer
	sw	$sp, 8($k1)		#store stack pointer
	sw	$k0, 0($k1)		#store program counter
	li $k0, 0x80000000
	addiu	$k0, $k0, pcb3		#load address for pcb3
	bne	$k1, $k0, increase		#if current pcb is not pcb3, start popping
	nop
	lui	$k1, 0x1
	sll	$k1, 15
	addiu	$k1, $k1, pcb1		#load address for pcb1
	b 	pop
	nop

increase:
	addiu	$k1, $k1, 12		#go to next pcb

pop: 	li $k0, 0x80000000
	addiu	$k0, $k0, curpcb	#load curpcb address
	sw	$k1, 0($k0) 		#store new current process in current process pcb
	lw	$ra, 0($k1)		#load program counter of next process
	lw	$gp, 4($k1)		#load global pointer of next process
	lw	$sp, 8($k1)		#load stack pointer of next process
	lw	$a0, 0($sp)		#load global pointer for reference
	addiu 	$sp, $sp, 4
	lw	$t5, 0($sp)		#Loading registers used by the proccesses
	addiu	$sp, $sp, 4
	lw	$t0, 0($sp)
	addiu   $sp, $sp, 4
	lw	$fp, 0($sp)
	addiu   $sp, $sp, 4

	jr $ra
	RFE


return:	MFC0 $k0, $14		#Loading the exception address
	nop
	addiu $k0, $k0, 4 	#The next instruction
	jr $k0
	RFE

kernel_loop:    b kernel_loop
        	nop