MIPS interupt I/O, confused on what this source does

[voidflux]

Hello everyone i'm trying to understand this source but i'm having troubles. Its dealing with interupt input and output with mips 32 asm. I'm suppose to figure out how many times the interupt (trap hander is invoked) when the user inputs:
computer
organization
design
cse
331
SPIM
The source is posted here:
http://cpp.sourceforge.net/?show=26042

If i can figure out what the code does I think i'll be able to figure out where I write the output so it can count how many times the interupt or trap is called.

Thanks.

# SPIM S20 MIPS simulator.

# Test of memory mapped IO.

#

# Copyright (C) 1992-2004 James Larus, [email protected].

# ALL RIGHTS RESERVED.

#

# SPIM is distributed under the following conditions:

#

# You may make copies of SPIM for your own use and modify those copies.

#

# All copies of SPIM must retain my name and copyright notice.

#

# You may not sell SPIM or distributed SPIM in conjunction with a commerical

# product or service without the expressed written consent of James Larus.

#

# THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR

# IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED

# WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR

# PURPOSE.

#



# $Header: /u/faculty/larus/Software/larus/SPIM/Tests/RCS/tt.io.s,v 1.3 1994/01/18 03:23:44 larus Exp $



# Standard startup code.  Invoke the routine "main" with arguments:

#    main(argc, argv, envp)

#


###I'm not sure what this code does exactly, can anyone let me know what its doing
##when you enter 6 words?


    .text

    .globl __start

__start:

    lw $a0 0($sp)            # argc

    addiu $a1 $sp 4        # argv

    addiu $a2 $a1 4        # envp

    sll $v0 $a0 2

    addu $a2 $a2 $v0

    jal main

    nop



    li $v0 10

    syscall            # syscall 10 (exit)



    .globl __eoth

__eoth:



    .data

prompt: .asciiz "SPIM IO Test.\nOnly runs with -mapped_io flag.\nPlease type 6 input lines:\n"

nl:    .asciiz "\n"



    .text

    .globl main

main:

    li $v0 4

    la $a0 prompt

    syscall





# Register usage:

#        s0        loop counter

#        t0        address of recv_ctrl

#        t1        address of recv_buffer

#        t2        address of trans_ctrl

#        t3        address of trans_buffer

#        t4, t5        temporaries

#        t6        char read from input

#        t7        1 => char in t6



    li $s0 3        # loop counter





    li $t0 0xffff0000    # recv ctrl

    li $t1 0xffff0004    # recv buf

    li $t2 0xffff0008    # trans ctrl

    li $t3 0xffff000c    # trans buf



# First, read and echo 3 lines of input by polling the IO registers, not through

# interrupts:



    mtc0 $0 $12        # Clear IE bit in Status reg to disable interrupts



l1:

    lw $t4 0($t0)        # Wait for receiver ready

    and $t4 $t4 1

    beq $t4 0 l1



    lw $t6 0($t1)        # Read character



l2:

    lw $t4 0($t2)        # Wait for transmitter ready

    and $t4 $t4 1

    beq $t4 0 l2



    sw $t6 0($t3)        # Write character



    beq $t6 0xa decr    # New line (nl)

    bne $t6 0xd l1        # Carriage return (cr)



decr:

    add $s0 $s0 -1        # Decrement line counter

    bne $s0 0 l1        # If not zero, get another line





# Second, read and echo 3 lines of input by through interrupts:



    mfc0 $t4 $13

    and $t4 0xffff00ff    # Clear IP bits in Cause register

    mtc0 $t4 $13



    li $s0 3        # loop counter



    li $t4 0x2        # Enable device interrupts

    sw $t4 0($t0)

    sw $t4 0($t2)



    mfc0 $t4 $12        # Enable interrupts and mask in Status reg

    ori $t4 $t4 0xff01

    mtc0 $t4 $12





l3:    b l3            # Loop waiting for interrupts





# Trap handler. Replaces the standard SPIM handler.



    .ktext 0x80000180

    mfc0 $t4 $13        # Get ExcCode field from Cause reg

    srl $t5 $t4 2

    and $t5 $t5 0x1f    # ExcCode field

    bne $t5 0 exception



# An interrupt:

    and $t5 $t4 0x800    # Check for IP3 (HW 1)

    beq $t5 0 check_trans



# Receiver interrupt:

    lw $t5 0($t0)        # Check receiver ready

    and $t5 $t5 1

    beq $t5 0 no_recv_ready    # Error if receiver is not ready



    lw $t6 0($t1)        # Read character

    li $t7 1



    beq $t6 0xa decr2    # New line (nl)

    bne $t6 0xd next    # Carriage return (cr)



decr2:

    add $s0 $s0 -1        # Decrement line counter



next:

    mfc0 $t4 $13        # Get Cause register

    and $t4 0xfffff7ff    # Clear IP3 bit

    mtc0 $t4 $13



check_trans:

    beq $t7 0 ret_handler    # No char to write yet



    and $t5 $t4 0x400    # Check for IP2 (HW 0)

    beq $t5 0 check_loop



# Transmitter interrupt:

    lw $t5 0($t2)        # Check transmitter ready

    and $t5 $t5 1

    beq $t5 0 no_trans_ready



    sw $t6 0($t3)        # Write character

    li $t7 0



    mfc0 $t4 $13        # Get Cause register

    and $t4 0xfffffbff    # Clear IP2 bit

    mtc0 $t4 $13



check_loop:

    bne $s0 0 ret_handler    # If line counter not zero, get another line



# Done echoing, so terminate program.

    li $v0 10

    syscall            # syscall 10 (exit)



# Return from handler.

ret_handler:

    mfc0 $t4 $12        # Enable interrupts and mask in Status reg

    ori $t4 $t4 0xff01

    mtc0 $t4 $12



    eret            # return to interrupted instruction





exception:

    li $v0 4        # Non-interrupt exception

    la $a0 other_str    # Print message and ignore

    syscall

    b ret_handler



no_recv_ready:

    li $v0 4        # Receiver was not ready after interrupt

    la $a0 no_recv_str    # Print message and ignore

    syscall

    b ret_handler



bad_int:

    li $v0 4        # Interrupt was not from recv or trans

    la $a0 bad_int_str    # Print message and ignore

    syscall

    b ret_handler



no_trans_ready:

    li $v0 4        # Transmitter was not ready after interrupt

    la $a0 no_trans_str    # Print message and ignore

    syscall

    b ret_handler



    .data

other_str:

    .asciiz "Non-interrupt exception\n"

no_recv_str:

    .asciiz "Receiver not ready\n"

no_trans_str:

    .asciiz "Transmitter not ready\n"

bad_int_str:

    .asciiz "Unknown interrupt\n"