# First section:  user data,
# each process has its own "private" area.

        .data
glob1:  .byte   'A'
        .space  999
glob2:  .byte   '0'
        .space  999
glob3:  .byte   'a'
        .space  999

# Second section: user code.
#
# The first instructions initializes IO and Timer.
# The last instruction in main are only used for start
# up, they initialize the first process.

        .text
        .set noreorder
main:
        li    $t0, 0xFFFF0010   # address to Timer registers:
                                # +0: Timer control register
                                # +4: Timer count register
                                # +8: Timer compare register

        li	$t1, 0
        sw	$t1, 4($t0)       # reset counter register

        li	$t1, 63           # count from 0 to 63
        sw	$t1, 8($t0)       # compare register := 63

        li	$t1, 0b101001     # "101001": compare interrupt enable,
                                #           compare reset enable,
                                #           timer start
        sw	$t1, 0($t0)       # control register := "101001"

        li	$t0, 0xFFFF0000   # address to I/O registers:
                                # +0: Input control register

        li	$t1, 0b10         # "10": interrupt enable
        sw	$t1, 0($t0)       # control register := "10"

        li	$t0, 0x0C03       # enable HW Interrupt 1 (input): bit 11
                                # enable HW Interrupt 0 (timer): bit 10
                                # set user mode:     bit 1
                                # enable interrupts: bit 0

        mtc0  $t0, $12          # CP0 status := 0x0Cma03

        la    	$gp, glob1        # dirty setup for process 1



proc1:  # ++++++++++ first process +++++++++

        # Description: proc1 reads the byte stored at 0($gp),
        # prints it, increments to the next character,
        # saving that back into 0($gp).  After printing 'Z',
        # this process should then start over again with 'A', in
        # an endless loop.  The symbol "glob1" may not be used.


	lb	$t0, 0($gp)	#Loads a byte
	nop
	ori	$t1, $zero, 90	#Loads 90 into t0
	move	$a0, $t0	#

	lui 	$v0, 0x102
	nop
	syscall
	nop
	beq	$t0, $t1, reset
	nop

add:
	addiu 	$t0, $t0, 1
	sb	$t0, 0($gp)

        j proc1
        nop

reset:
	addiu	$t0, $t0, -25
	sb	$t0, 0($gp)
	j	proc1
	nop


proc2: # ++++++++++ second process +++++++++

        # Description: almost identical to the code of proc1 above,
        # only 2-3 lines should differ.  Prints '0' through '9' in
        # an endless loop. The symbol "glob2" may not be used.


	lb	$t0, 0($gp)	#Loads a byte
	nop
	ori	$t1, $zero, 57	#Loads 90 into t0
	move	$a0, $t0	#

	lui 	$v0, 0x102
	nop
	syscall
	nop
	beq	$t0, $t1, reset2
	nop

add2:
	addiu 	$t0, $t0, 1
	sb	$t0, 0($gp)

        j proc2
        nop

reset2:
	addiu	$t0, $t0, -9
	sb	$t0, 0($gp)
	j	proc2
	nop



proc3:  # +++++++++ third process ++++++++++

        # Description: almost identical to the code of proc1 above,
        # only 2-3 lines should differ.  Prints 'a' through 'z' in
        # an endless loop. The symbol "glob3" may not be used.


	lb	$t0, 0($gp)	#Loads a byte
	nop
	ori	$t1, $zero, 122	#Loads 90 into t0
	move	$a0, $t0	#

	lui 	$v0, 0x102
	nop
	syscall
	nop
	beq	$t0, $t1, reset3
	nop

add3:
	addiu 	$t0, $t0, 1
	sb	$t0, 0($gp)

        j proc3
        nop

reset3:
	addiu	$t0, $t0, -25
	sb	$t0, 0($gp)
	j	proc3
	nop


# Third section: data structures for the kernel:
# Process Control Block (PCB) consists of three words:
# pcb: .word  (next Program Counter for this process)
#      .word  (contents of $gp for this process)
#      .word  (contents of $sp for this process)
# All other context is saved on the process' own stack
# during exception handling and SYSCALL.

        .section .kdata
curpcb: .word  pcb1
pcb1:   .word  0, 0, 0
pcb2:   .word  proc2, glob2, 0x7fffbf94
pcb3:   .word  proc3, glob3, 0x7fff7f94

# Fourth section: kernel code.

        .section .ktext , "xa"
        .set noreorder

kernel_loop:

	mfc0	$t2, $13
	nop
	ori 	$t5, $zero, 8
	sll	$t2, $t2, 26
	srl	$t2, $t2, 28


	beq	$t2, $t5, check
	nop
	beq	$zero, $t2, exc
	nop
	b kernel_loop
	nop
check:
	lui	$t3, 0x102
	nop
	beq	$v0, $t3, back
	nop
	rfe
	nop

back:
	lui	$t4, 0xFFFF
	sb 	$a0, 8($t4)
	nop
	nop
	mfc0	$k0, $14
	nop

	addiu 	$k0, $k0, 4
	jr	$k0
	rfe

exc:
	ori	$t3, $zero, 1
	mfc0	$t2, $13
	sll	$t2, $t2, 20
	srl	$t2, $t2, 31

	beq	$t2, $t3, timerint

	nop
	b kernel_loop
	nop


timerint:
	#loads in the current stack pointer
	la	$s0, curpcb
	nop

ignore:
	mfc0	$k1, $14
	nop
	move	$ra, $k1
	sw	$k1, 0($s0)
	sw	$gp, 4($s0)

    addiu $sp, $sp, -4
	sw	$ra, 0($sp)
    addiu $sp, $sp, -4
	sw	$fp, 0($sp)
    move $fp, $sp
    addiu	$sp, $sp, -100
	sw	$gp, -4($fp)
	sw	$s0, -8($fp)

	sw	$s1, -12($fp)
	sw	$s2, -16($fp)
	sw	$s3, -20($fp)
	sw	$s4, -24($fp)
	sw	$s5, -28($fp)
	sw	$s6, -32($fp)
	sw	$s7, -36($fp)

	sw	$t0, -40($fp)
	sw	$t1, -44($fp)
	sw	$t2, -48($fp)
	sw	$t3, -52($fp)
	sw	$t4, -56($fp)
	sw	$t5, -60($fp)
	sw	$t6, -64($fp)
	sw	$t7, -68($fp)
	sw	$t8, -72($fp)
	sw	$t9, -76($fp)

	sw	$a0, -80($fp)
	sw	$a1, -84($fp)
	sw	$a2, -88($fp)
	sw	$a3, -92($fp)

	sw	$v0, -96($fp)
	sw	$v1, -100($fp)

    .set noat
	    sw	$at, -104($fp) # FIX
	.set at

	la $s2, pcb2
	la $s4, pcb3

	beq $s0, $s2, pcb2_3
	nop
	beq $s0, $s4, pcb3_1
	nop
	b pcb1_2
	nop

pcb3_1:
	la	$s3, pcb1
	move $s0, $s3
	lw  $gp, 4($s0)
	nop
	b	load
	nop

pcb1_2:
	la	$s3, pcb2
	move $s0, $s3
	lw $gp, 4($s0)
	nop
	b load
	nop

pcb2_3:
	la	$s3, pcb3
	move $s0, $s3
	lw $gp, 4($s0)
	nop
load:

# Clear Timer Interrupt
	li      $k0, 0xFFFF0010
	li      $k1, 0b101001
	sw      $k1, 0($k0)

	#---------------------
	lw	$sp, 8($s0)
	lw  $ra, 0($s0)

	move $fp, $sp
	addiu $fp, $fp, -8

	sw $gp, 4($fp)
	sw $s0, -4($fp)

	lw  $gp,  4($fp)
	nop
	lw	$s0, -4($fp)
	nop

	lw	$s1, -8($fp)
	nop
	lw	$s2, -12($fp)
	nop
	lw	$s3, -16($fp)
	nop
	lw	$s4, -20($fp)
	nop
	lw	$s5, -24($fp)
	nop
	lw	$s6, -28($fp)
	nop
	lw	$s7, -32($fp)
	nop

	lw	$t0, -36($fp)
	nop
	lw	$t1, -40($fp)
	nop
	lw	$t2, -44($fp)
	nop
	lw	$t3, -48($fp)
	nop
	lw	$t4, -52($fp)
	nop
	lw	$t5, -56($fp)
	nop
	lw	$t6, -60($fp)
	nop
	lw	$t7, -64($fp)
	nop
	lw	$t8, -68($fp)
	nop
	lw	$t9, -72($fp)
	nop

	lw	$a0, -76($fp)
	nop
	lw	$a1, -80($fp)
	nop
	lw	$a2, -84($fp)
	nop
	lw	$a3, -88($fp)
	nop

	lw	$v0, -92($fp)
	nop
	lw	$v1, -96($fp)
	nop

    .set noat
	    lw	$at, -104($fp) # FIX
		nop
	.set at

	jr $ra
	rfe