Agfdhyk

I'll try to make this short. I am running into an infinite loop issue with my program and even after trying to use the debugger I am having issues with an infinite loop.

The program I am trying to make is an assignment from my instructor, to sort through a table of IP addresses that use .word. I have the program done about 95% the last issue I am having is when I type in 193.0.0.0, I run into this infinite loop, the structure for the loop is the same as the other two, which have no problems. In this program, for a class C domain IP address to match the first 3 numbers of the IP Address need to match user input other wise continue onto the rest of the table.
The specific issue that is happening is when it finds a match for the first number and continues to try and match the second number, and then for the third number. If the second number or third number doesn't match it jumps back to the loop, but then gets trapped there, which shouldn't be happening. At least, from what I can tell it shouldn't be.

The matching works beautifully, it is if it doesn't match that there is a problem, but again I am not sure why I am hoping that a fresh set of eyes will be able to help me see the issue.
Also, sorry for any sloppiness in the code, and for it being kind of lengthy, it is rather simple though. I appreciate the help.

.data



MESSAGE1: .asciiz "Enter an IP addressn"

MESSAGE2: .asciiz "First: "

MESSAGE3: .asciiz "Second: "

MESSAGE4: .asciiz "Third: "

MESSAGE5: .asciiz "Fourth: "

MESSAGE6: .asciiz "The IP address you entered: "

MESSAGE7: .asciiz "."

MESSAGE8: .asciiz "nClass A addressn"

MESSAGE9: .asciiz "nClass B addressn"

MESSAGE10: .asciiz "nClass C addressn"

MESSAGE11: .asciiz "nClass D addressn"

MESSAGE12: .asciiz "nInvalid domain classn"

MESSAGE13: .asciiz "nProgram successfully completed . . .n"

MESSAGE14: .asciiz "n"

MESSAGE15: .asciiz "Matching domain found at line: "

MESSAGE16: .asciiz "Matching domain was NOT found . . . n"

ERROROVER: .asciiz "The entered number is larger than 255.n"

ERRORUNDER: .asciiz "The entered number is smaller than 0.n"











IP_ROUTING_TABLE_SIZE:

    .word   10



IP_ROUTING_TABLE:

    # line #, x.x.x.x -------------------------------------

    .word   0, 146,  92, 255, 255   # 146.92.255.255

    .word   1, 147, 163, 255, 255   # 147.163.255.255

    .word   2, 201,  88,  88,  90   # 201.88.88.90

    .word   3, 182, 151,  44,  56   # 182.151.44.56

    .word   4,  24, 125, 100, 100   # 24.125.100.100

    .word   5, 146, 163, 140,  80   # 146.163.170.80

    .word   6, 146, 163, 147,  80   # 146.163.147.80

    .word  10, 201,  88, 102,  80   # 201.88.102.1

    .word  11, 148, 163, 170,  80   # 146.163.170.80

    .word  12, 193,  77,  77,  10   # 193.77.77.10



.text

.globl main



main:       

    la $a1, IP_ROUTING_TABLE_SIZE

    lw $t9, ($a1)







    li $t7, 255 #top limit



    li $v0, 4

    la $a0, MESSAGE1  #asking for the address

    syscall

FIRST:

    li $v0, 4

    la $a0, MESSAGE2 #first number

    syscall



    li $v0, 5

    syscall



    move $t0, $v0  #saving input for later use.



    bgt $t0, $t7, thi1 # if greater than 255

    blt $t0,$zero, tlo1 # if less than



SECOND:

    li $v0, 4

    la $a0, MESSAGE3 # second number

    syscall



    li $v0, 5

    syscall



    move $t1, $v0  #saving input



    bgt $t1, $t7, thi2 #if greater than

    blt $t1,$zero, tlo2 #if less than

THIRD:

    li $v0, 4

    la $a0, MESSAGE4 #third number

    syscall



    li $v0, 5

    syscall



    move $t2, $v0 #saving input



    bgt $t2, $t7, thi3 #if greater than

    blt $t2,$zero, tlo3 #if less than



FOURTH:

    li $v0, 4

    la $a0, MESSAGE5 #fourth number

    syscall



    li $v0, 5

    syscall



    move $t3, $v0 #saving input



    bgt $t3, $t7, thi4 #if greater than

    blt $t3,$zero, tlo4 #if less than





Address:

    li $v0, 4

    la $a0, MESSAGE6

    syscall



    li $v0, 1

    move $a0, $t0

    syscall



    li $v0, 4

    la $a0, MESSAGE7

    syscall



    li $v0, 1

    move $a0, $t1

    syscall



    li $v0, 4

    la $a0, MESSAGE7

    syscall



    li $v0, 1

    move $a0, $t2

    syscall



    li $v0, 4

    la $a0, MESSAGE7

    syscall



    li $v0, 1

    move $a0, $t3

    syscall



    li $v0, 4

    la $a0, MESSAGE14

    syscall





    j ClassSort









P_EXIT: 



    li $v0, 4

    la $a0, MESSAGE13

    syscall







    jr    $31 #end of module main

################################################################        





ClassSort:



     #check for class A



     li $t5, 127

     blt $t0, $t5, ClassA



     #check for class B



     li $t5, 191

     blt $t0, $t5, ClassB



     #check for class C

     li $t5, 223

     blt $t0, $t5, ClassC



     #check for class D

     li $t5, 239

     blt $t0, $t5, ClassD



     #Invalid otherwise

     bgt $t0, $t5, Invalid



thi1:

    li $v0, 4

    la $a0, ERROROVER #too High

    syscall

    j FIRST

tlo1:

    li $v0, 4

    la $a0, ERRORUNDER #too Low

    syscall

    j FIRST

thi2:

    li $v0, 4

    la $a0, ERROROVER #too High

    syscall

    j SECOND

tlo2:

    li $v0, 4

    la $a0, ERRORUNDER #too Low

    syscall

    j SECOND

thi3:

    li $v0, 4

    la $a0, ERROROVER #too High

    syscall

    j THIRD

tlo3:

    li $v0, 4

    la $a0, ERRORUNDER #too Low

    syscall 

    j THIRD

thi4:

    li $v0, 4

    la $a0, ERROROVER #too High

    syscall

    j FOURTH

tlo4:

    li $v0, 4

    la $a0, ERRORUNDER #too Low

    syscall

    j FOURTH







ClassA:

    li $v0, 4

    la $a0, MESSAGE8

    syscall 



    li $t5, 0 #reset offset

    li $t6, 0 #reset counter

ALOOP:



    la $a0, IP_ROUTING_TABLE      #load table

    add $a0, $a0, $t5             #add current offset



    lw $s0, ($a0)  #load words at offsets

    lw $s1,4($a0)

    lw $s2,8($a0)

    lw $s3,12($a0)

    lw $s4,16($a0)



    beq $t0, $s1, LINENUMBER #branch if match



    addi $t5, 20 #increment offset

    addi $t6, 1  #increment counter



    beq $t6, $t9, NOMAT

    j ALOOP





ClassB:

    li $v0, 4

    la $a0, MESSAGE9

    syscall



    li $t5, 0 #reset offset

    li $t6, 0 #reset counter

BLOOP:  



    la $a0, IP_ROUTING_TABLE #load table

    add $a0, $a0, $t5 #add current offset



    lw $s0, ($a0)  #load words at offsets

    lw $s1,4($a0)

    lw $s2,8($a0)

    lw $s3,12($a0)

    lw $s4,16($a0)





    addi $t5, 20 #increment offset

    addi $t6, 1 #increment counter



    beq $t0, $s1, MATCHSECOND #branch if match



    beq $t6, $t9, NOMAT #branch if no match

    j BLOOP



#############################################################

 #error is  some where here, I think that it is down in MATCHSECOND2

ClassC:



    li $v0, 4

    la $a0, MESSAGE10

    syscall



    li $t5, 0 #reset offset

    li $t6, 0 #reset counter

CLOOP:      

    la $a0, IP_ROUTING_TABLE #load table



    add $a0, $a0, $t5 #add current offset/ the next line in table



    lw $s0, ($a0)  #load words at offsets

    lw $s1,4($a0)

    lw $s2,8($a0)

    lw $s3,12($a0)

    lw $s4,16($a0)





    addi $t5, 20 #increment offset

    addi $t6, 1 #increment counter





    beq $t0, $s1, MATCHSECOND2 #branch if match





    beq $t6, $t9, NOMAT #branch if it reaches the end of the table.







    j CLOOP  #end of ClassC

####################################################

ClassD:

    li $v0, 4

    la $a0, MESSAGE11

    syscall

    j P_EXIT



Invalid: 

    li $v0, 4

    la $a0, MESSAGE12

    syscall

    j P_EXIT



NOMAT:  #no match

    li $v0, 4

    la $a0, MESSAGE16

    syscall



    j P_EXIT



LINENUMBER:

    li $v0, 4

    la $a0, MESSAGE15

    syscall



    li $v0, 1

    move $a0, $s0

    syscall



    j P_EXIT



MATCHSECOND:

    beq $s2, $t1, LINENUMBER



    j BLOOP

################################################

#belong to classC/CLOOP

MATCHSECOND2:

    beq $s2, $t1, MATCHTHIRD #if second number matches, branch to test third

#if it doesn't match loop back to top

    j CLOOP



MATCHTHIRD:

#provide the line of matching address if all three numbers

    beq $s3, $t2, LINENUMBER 

    #if it doesn't match loop back to top

    j CLOOP

 ###############################################

There's flawed logic in the CLOOP (and not only there), when the first digit is "match" and second is "no-match", the check for table size is skipped, thus the loop may get out of IP table data.

Other problems:

     #check for class D

     li $t5, 239

     blt $t0, $t5, ClassD



     #Invalid otherwise

     bgt $t0, $t5, Invalid

This doesn't cover value 240. You can do b Invalid without any test.

Some other suggestions:

If you would preserve $ra (return address from main), you would be able to use jal subroutine to re-use some generic parts of code, like thiX and tloX parts of code .. or the whole request of input values can be like one loop using just arguments to display different prompts and store the values in array (or single register, see below).

And IPv4 addresses are 32 bit values (the reason why "255" is maximum for particular value), and they are exploited as such.

For example sub-network matching uses masking, i.e. if sub-network 160.120.0.0 (that's like value 0xA0780000) has mask 255.255.0.0 (that's like value 0xFFFF0000), then to decide if particular IP a.b.c.d is part of that sub-network, you do

is_part_of_subnet = ((IP & mask) == subnetwork_IP);

Which in assembly may look like

    # fake init for example

    li    $t0, 0xA0780000  # subnet address 160.120.0.0

    li    $t1, 0xFFFF0000  # subnet mask 255.255.0.0

    li    $t2, 0x12345678  # some/any IP address

          # (must start with 0xA078... to trigger positive match)



    # sub-network match-check

    # mask-out parts of IP address which are not significant for test

    and   $t3, $t2, $t1    # this will clear third/fourth value to zero

    beq   $t3, $t0, IP_is_part_of_sub_network

    # IP is not part of subnetwork (first two values are different)

    ...



IP_is_part_of_sub_network:

    ...

Line number in table should be rather calculated value from traversal of the array, than part of the stored value.

After those two things put together, the IP table may be actually compressed down into single word per line, like:

.word 0x925CFFFF  # 146.92.255.255

To make beq test work, you then should mask both values by the class mask to make only the significant parts of IP address survive to the compare...

(I'm not networking expert, so I may have flipped the sub-net mask definition, maybe it's 0.0.255.255, but then the code will flip the mask for the needs of and instruction, or it will use mask like this, but use instead or to set up the bit values in lower positions to 255, from programming point of view you just have to make sure you are using the correct and/or/flip sequence to get the correct results)