MIPS palindrome checker

Question

I am having a hard time figuring out where to start with this project. I am needing to write code in PLP that is a palindrome checker. the task is to write a program that recieves a string of characters via UART, checks if this string is a palindrome, then uses a print function to print either"yes" of "no". I have been given a template that I am to follow when creating the program.

The template project file contains six function stubs that need to be implemented. five are called from the main loop and the sixth is called from "period_check: in the template file it contains descriptions of what each function needs to do and how it should be implemented. I have attempted to fill in some, however I do not think I am on the right track. Please help.

***** I have gotten this much code in, but it does not print out the right output**** it prints no for everything vs no for non palindromes and yes for palindrome.

.org 0x10000000

# Initializations
# NOTE: You may add initializations after line 10, but please do not
# remove or change the initializations to $sp, $s0, $s1, or $s2

li $sp, 0x10fffffc  # Starting address of empty stack
li $s0, 0xf0000000  # UART base address
li $s1, array_ptr   # Array head pointer
li $s2, array_ptr   # Array tail pointer


####################################################################
# Do not make changes to the jump to main, the allocation of
# memory for the array, or the main loop
####################################################################
    j main
    nop

array_ptr:          # Label pointing to 100 word array
    .space 100

main:
    jal poll_UART
    nop
    jal period_check
    nop
    jal space_check
    nop
    jal case_check
    nop
    jal array_push
    nop
    j main
    nop
####################################################################
# ******************************************************************
####################################################################


# The "poll_UART" function should poll the status register of the UART.
# If the 2^1 bit position (ready bit) is set to 1 then it
# should copy the receive buffer's value into $v0 and send
# a clear status command (2^1) to the command register before
# returning (a return statement is already included). In order to
# receive full credit, $s0 must contain the base address of the UART
# and must be used with the appropriate offsets to access UART
# registers and buffers

poll_UART:


    lw $t1, 4($s0)
    li $t2, 0b10
    and $t3, $t1, $t2
    beq $t3, $0, main
    nop
    lw $v0, 8($s0)
    sw $t2, 0($s0)
    jr $ra
    nop


 


# The "period_check" function should check if the current character ($v0)
# is a period ("."). If it is a period then the function should go to the
# label, "palindrome_check". If the character is not a period then it
# should use the included return.

period_check:

    li $t0, 0x2E
    beq $v0, $t0, palindrome_check
    nop

# The "space_check" function should check if the current character ($v0)
# is a space (" "). If it is then it should jump to "main" so
# that it skips saving the space character. If not it should
# use the included return.

space_check:

    li $t4, 0x20
    beq $t4, $v0, main
    jr $ra
    nop

# The "case_check" function should perform a single inequality check.
# If the current character ($v0) is greater than the ASCII value of 'Z',
# which indicates the current character is lowercase, then it should convert
# the value of $v0 to the uppercase equivalent and then return. If the
# current character ($v0) is already uppercase (meaning the inequality
# mentioned before was not true) then the function should return without
# performing a conversion.

case_check:

   li $t5, 0x5A
   slt $t6, $v0, $t5
  li $t7, 1
   beq $t6, $t7, convert

convert:
    addiu $v0, $v0, -32
    jr $ra
    nop


# The "array_push" function should save the current character ($v0) to the
# current location of the tail pointer, $s2. Then it should increment the
# tail pointer so that it points to the next element of the array. Last
# it should use the included return statement.

array_push:

     sw $v0, 0($s2)
     addiu, $s2, $s2, 4
     jr $ra
     nop


# The "palindrome_check" subroutine should be jumped to by the period
# check function if a period is encountered. This subroutine should contain 
# a loop that traverses the array from the front towards the back (using the
# head pointer, $s1) and from the back towards the front(using the tail
# pointer, $s2). If the string is a palindrome then as the array is traversed
# the characters pointed to should be equal. If the characters are not equal
# then the string is not a palindrome and the print function should be used
# to print "No". If the pointers cross (i.e. the head pointer's address is
# greater than or equal to the tail pointer's address) and the compared
# characters are equal then the string is a palindrome and "Yes" should be
# printed. 
# 
# Remember to restore the head and tail pointers to the first element
# of the array before the subroutine jumps back to main to begin processing the
# next string. Also, keep in mind that because the tail pointer is updated at
# the end of "array_push" it technically points one element past the last
# character in the array. You will need to compensate for this by either
# decrementing the pointer once at the start of the array or using an offset
# from this pointer's address.

palindrome_check:


    addiu $s2, $s2, -8 
    move $s3, $s1
    subu $s6, $s2, $s3
    beq $s6, $0, palindrome
    nop
check_loop:
    lw $s4, 0($s3)
    lw $s5, 0($s2)
    bne $s5, $t0, not_palindrome
    nop
adjust_pointers:
    addiu $s2, $s2, -4
    addiu $s3, $s3, 4 
    slt $t8, $s3, $s2
    bne $t8, $t0, check_loop
    nop
    j palindrome
    nop
palindrome:
    li $a0, 1
    call project3_print
    move $s2, $s1
    j main 
not_palindrome:
    li $a0, 0
    call project3_print 
    move $s2, $s1
    j main 
    nop

Erik Eidt · Answer

Ok, this is just my opinion, but you are definitely not on the right track.

The control flow you're showing is problematic.

To see one reason why, try writing this same in C or any other language that you know. You won't be able to do it because of the non-local goto's that's using, where one procedure jumps (without calling) to another procedure.

Further, finding whether an input is a palindrome is not a fixed sequence of one-time steps that are executed on each input character.

You will (1) need to store the characters for later comparison, and (2) need a decision point where you can determine (and print) yes it is, or no it isn't. You don't have any control structure for that.

that recieves a string of characters via UART, checks if this string is a palindrome, then uses a print function to print either"yes" of "no".

Yes, your main should reflect the above description you've been given:

receive a string of characters
checks if this string is a palindrome
print either "yes" of "no"

In other words you might have something like:

int len = input_string();
if ( check_palindrome(len) ) {
    print "yes";
else
    print "no"

Suggest you write it in C or other language you know, then translate that to assembly.

Also consider that we some things we program are functions returning a value rather than procedures that don't return values. Returning a value so that main can take a different course of action (e.g. print yes vs. no) is much better than using non-local goto's to alter the flow of control from within a subroutine.

If your instruction/coursework has given you that main, and is recommending non-local goto's that would be very sad.

I feel for you and your classmates, as this is one of the worst examples of teaching assembly I've seen in a long long time.

array_ptr:          # Label pointing to 100 word array
    .space 100

The label name is misleading. This space is used as an array of words, not a pointer to an array. The storage reserved is 25 words, since .space operates in terms of bytes and words are 4 bytes each. So, the comment is just plain wrong.

The various "functions" called using jal are single use function, so there's really no need for functions in this assignment at all. The "functions" also are going to each other and back to main instead of returning properly like they would in structured programming. So, this is what we call spaghetti code — such code is difficult to reason over and one of the reasons that other languages don't even bother to offer this kind of flow control.

The array being used is storing whole words, when the input elements are only characters, so that's harmless but unnecessary.

   beq $t6, $t7, convert

convert:

This control structure will never choose between two options, it will always convert. Why? Because in the case $t6 is true it will branch to convert: and in the case that $t6 is not true it will fall through to convert:, so same location, will run same code in either case.

You should be able to observe this during debugging.

Debugging Tips

Get to know your data. You should know the address of the array as you debug. You can find this during execution, e.g. look at a register after li ... array_ptr (btw, that opcode should be la, but no matter if it works). Otherwise you can observe the data section and its layout to find that out before running the first instruction.

Single step each line like one would to debug code in any other language, verifying program state between each line. In MIPS assembly, not much program state changes between lines so usually this is pretty simple — usually each instruction only changes one register or one memory location — but you must verify that such change is as you're expecting. Once the first part of the program is properly storing characters into the array, you can use the break point feature to stop at the palindrome check routine and single step only from there on.

Use the smallest possible input first, (in the most degenerate case that would be an empty string, but you may not be handling those so instead) might try a single letter input (should be a palindrome). Once that is working, try two letter input. As I said, first make sure that the character values are being placed into the array properly, and only when you've verified that's working, go on to debug the palindrome check code.

MIPS palindrome checker

1 Answers

Debugging Tips