MIPS (SPIM) : debugging nested sub-routines

Question

I am trying to write a MIPS program that will convert a roman numeral (entered as a string of ascii characters) into its decimal value. There's an I/0 component in that the user is prompted for an input, (limited) validity checks are performed, and then the original string, along with the value is output.

To convert each roman numeral character to its decimal value, I've used a lookup table (ie I get the index of each char in a "roman numeral" array, and then use this index value to get the corresponding value from the "decimal values" array.

Roman numerals are read from L to R, but I cannot simply add each character (ie make the sum purely recursive) because it is possible to have a string like this: XIV, where X = 10, I = 1, and V= 5, and the resulting number = 14 (as opposed to 15). I have logic in place to output this result, but (unfortunately!), I am new to MIPS and to implement the sum logic, I've set up a series of nested sub-routines.

I understand that for a sub-routine that is not arrived at via a "jal" instruction to "know" its return address, you should load the return address on the stack, and pop it at the end of the sub-routine, and then jr $ra. However, some of the math logic in my code relies on branches, rather than "jal" instructions. Somewhere in all this mess, my stack pointers are incorrect, and my code terminates too soon (ie doesn't sum all of the digits in the string).

Any help or pointers would be appreciated. Code is below:

#   This program runs in MIPS, and performs the following functions:
#
#   (1) Reads input Roman Numerals as ASCII text characters
#
#   (2) Calculates the equivalent decimal value
#
#   (3) Writes the calculated value as Arabic numerals to the SPIM
#       console, with appropriate commentary text
#
#   (4) Tests by calculating the values of these Roman numerals:
#       {I:1, V:5, X:10, L:50, C:100, D:500, M:1000}
#
#   (5) Calculates and displays the results of the following Roman numerals:
#       -> LXXIX: 79
#       -> MMXVI: 2016
#       -> MCMXLVIII: 1948
#       -> MDCCCCVIIII: 1909
#
#   NOTE: There is no check for "valid" Roman numerals (i.e., VIIII is not "valid" but will be computed as = 9) 
#   However, subtraction logic is used (i.e. numbers that decrease from L to R = addition; a smaller number to the left
#   of a larger number represents subtraction, such that IX == VIIII == 9.


.data 
        # Lookup tables
        #roman:     .asciiz "IVXLCDMivxlcdm"
        roman:      .asciiz "IVXLCDM"
    #values:        .byte 1, 5, 10, 50, 100, 500, 1000, 1, 5, 10, 50, 100, 500, 1000
    values:     .byte 1, 5, 10, 50, 100, 500, 1000  

    # Input
    input1:     .word 4 
    input2:     .asciiz "\n "
    input3:     .asciiz "\n "
    input4:     .asciiz "\n"

    # Conversion 
    left:       .word 0
    sum:        .word 0
    counter:    .word 0
    temp:       .word 0

    # I/O and program control
    convertAgain:   .word 2
        prompt1:    .asciiz "\nPlease enter the Roman Numeral to convert in all uppercase OR all lowercase letters (i.e. VII or vii): "
        outRecap:   .asciiz "\nYou entered: "
        outFinal:   .asciiz "\nThe equivalent decimal value is: "
        loopdriver: .asciiz "\nWould you like to convert another number? Enter 1 for YES and 2 for NO: "  
        stop:       .asciiz "\n\nProgram complete.\n"

        # Exception handling
        error1:     .asciiz "\nThe Roman numeral you have entered contains invalid character(s). Please try again.\n"


################################################################################
.text

.globl main

main:

        #Prompt user to enter the Roman Numeral to convert
        li $v0, 4 # print prompt1
        la $a0, prompt1 # load address
        syscall

        # Load the word reserved for input, and read in the Roman Numeral string input by the user         
        la $a0, input1 # load contents of input1 (i.e. space allocated to save the input string) into $a0
        la $a1, input1 # load the length allocated for input1 (i.e. 4 bytes or 32 bits) into $a1 to prevent overflow
        li $v0, 8 # read string from user 
        syscall 


       # Go to sub-routines for lookup and conversion
       jal init

        # Display results for the user

        # Load output string 1 to show the user their original input string
        la $a0, outRecap # load contents of outRecap into $a0
        li $v0, 4 # print string 
        syscall

        # Load the string the user entered 
        la $a0, input1 # load contents of input1 into $a0
        li $v0, 4 # print string
        syscall

        # Display the equivalent decimal value
        la $a0, outFinal # load contents of outFinal (string) into $a0
        li $v0, 4 # print string
        syscall

        # Print the sum
        lw $a0, sum
        li $v0,1
        syscall

        # Ask the user whether they want to convert another Roman Numeral (1 if YES; 2 if NO; default is NO)
        la $a0, loopdriver
        li $v0, 4
        syscall 

        # Get user input from keyboard (1 = they want to convert another Roman Numeral; 2 = Exit; default = 2)
        li $v0, 5 # read integer input
        syscall

        # Save the user's input and jump to "Exit" 
        sw $v0, convertAgain
        lw $t0, convertAgain

        bne $t0, 1, Exit # if the user enters a number != 1, go to Exit
        j main  # Go back to start if the user enters 1

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

# Convert Roman Numeral string to integer decimal value   
init: 
    sw $ra, 4($sp)
    sw $a1, 0($sp)
    addi $sp, $sp, -8 # move stack pointer  
    la $t2, input1  #load the address of the string (see by looking at starting index of each the length of each string 
    la $t3, roman
    la $t4, values

# Loop through each char of the input string
loop1:  
    lb $a0, ($t2) # This gets the next byte of the string (from L to R); change the offset to get others (i.e. lb $a0, 3($t0) = I
    beqz $a0, return # If the byte == 0, we have reached the end of the input string; jump to "return"

    li $v0, 11  # print byte to console 
    syscall

    # While string[i] != null:
    jal index

    addi $t2, $t2, 1 # Move to next character in input string 
    jal loop1 # iterate through loop1 again

# Find the index (i.e. offset) of the selected char in the "roman" lookup table
index:  
    lb $t5, ($t3) # load the first byte of romans
    beqz $t5, invalid # if we reach the end of romans and the char isn't found, it's not a valid Roman numeral
    sw $ra, 8($sp)
    addi $sp, $sp, -4 # move stack pointer
    beq $a0, $t5, getVal # once we find the index we want, where the romans value matches our char, need to look it up in values
    jal iter # if we haven't found a match, jump to where we can increment $t3 and iterate through the loop again
#   addi $t3, $t3, 1 # if we haven't found a match, increment $t3 and iterate through the loop again
#   jal index # loop

# Use the index value to look up the char in the "values" array and get the decimal value
getVal: 
    la $t6, roman # Get the address of the array containing Roman numeral characters
    la $t7, values

    sub $t8, $t3, $t6 # Get the index value of the element that matches the byte we are working with [1:7]
    add $t7, $t7, $t8 
    lb $t9, ($t7) # $t9 is the decimal value that corresponds to the letter

    #added new
    sw $t9, 12($sp)
    addi $sp, $sp, -4 # move stack pointer

    jal setup

    addi $sp, $sp, 4 # reset the stack pointer
        lw $ra, 8($sp) # fetch return address
    jr $ra  # go back to main so that results can be displayed


# This is the last part of the index loop
iter:   
    addi $t3, $t3, 1 # if we haven't found a match, increment $t3 and iterate through the loop again
    jal index # loop


setup: 
    # Base case is that the string length == 1, and thus sum = the decimal equivalent of the only Roman numeral character
    lw $s0, sum
    beqz $s0, base # if sum == 0, this is our first pass through the loop, and we just want to add the first char's decimal value to sum and return to get the next char
    jal calcSum

# Base case is our first pass through the loop, where sum == 0. We just want to add the value of the first number in the string to sum, and return to loop1 to get next char.   
base:   #lw $s0, sum
    add $s0, $s0, $t9
    sw $s0, sum

    la $t3, roman
    addi $sp, $sp, 4 # reset the stack pointer
        lw $ra, 16($sp) # fetch return address
    jr $ra  # go back 

    #j return

# Here, we want to pop the previous letter's decimal value off the stack, compare it to current, process sum accordingly, and pop the current char onto the stack for the next iteration    
calcSum: addi $sp, $sp, 8 #reset the stack pointer
    lw $ra, 16($sp)
    lw $t1, 12($sp) # pop the decimal value of the Roman numeral to the left of the current value in the original string    


    la $t3, roman
    bge $t1, $t9, plus # If the decimal value of the previous char in the string is >= the current char, then we can add the current char to the sum
    blt $t1, $t9, minus # if the decimal value of the previous char in the string is < the current char, we should add: current - (2*previous) to the sum

plus:   lw $s0, sum
    add $s0, $s0, $t9 

    la $t3, roman
    addi $sp, $sp, 4 # reset the stack pointer
        lw $ra, 16($sp) # fetch return address
    jr $ra  # go back 


minus: lw $s0, sum
    mul $t1, $t1, 2
    sub $t9, $t9, $1
    add $s0, $s0, $t9

    la $t3, roman
    addi $sp, $sp, 4 # reset the stack pointer
        lw $ra, 16($sp) # fetch return address
    jr $ra  # go back to 

# Go back to main   
return: 
    addi $sp, $sp, 8 # reset the stack pointer
    lw $a0, 0($sp) # 
        lw $ra, 4($sp) # fetch return address
    jr $ra  # go back to main so that results can be displayed

#Exception handling: User input string contains invalid character(s) 
invalid: 
        li $v0, 4   # print string
        la $a0, error1  # load address for error1 (a string)
        syscall

    j main 

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

#System Exit
Exit: 
    # Display exit message
        li $v0, 4   # print string
        la $a0, stop    # load address for stop (a string)
        syscall

        li $v0, 10
        syscall

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

Answer 1

Edit: after a lot of debugging, I managed to get this working. Code is below, in case anyone else has a similar problem, or can give feedback/pointers on how to improve this. Some things I learned: jal instructions need stack pointers above and below the sub-routine call. Bytes weren't a great choice for storing my decimal values, since 500 and 1000 are out of range (both signed and unsigned), and I had to deal with them as special cases.

    .data 
        # Lookup tables
        roman:      .asciiz "IVXLCDM"
    values:     .byte 1, 5, 10, 50, 100, 500, 1000  

    # Input
    input1:     .word 4 
    input2:     .asciiz "\n "       
    input3:     .asciiz "\n "       
    input4:     .asciiz "\n"

    # Conversion 
    left:       .word 0
    sum:        .word 0

    # I/O and program control
    convertAgain:   .word 2
        prompt1:    .asciiz "\nPlease enter the Roman Numeral to convert in all uppercase letters (i.e. VII): "
        outRecap:   .asciiz "\nYou entered: "
        outFinal:   .asciiz "\nThe equivalent decimal value is: "
        loopdriver: .asciiz "\nWould you like to convert another number? Enter 1 for YES and 2 for NO: "  
        stop:       .asciiz "\n\nProgram complete.\n"

        # Exception handling
        error1:     .asciiz "\nThe Roman numeral you have entered contains invalid character(s). Please try again.\n"


######################################################################################################################################################
.text

.globl main

main:

        #Prompt user to enter the Roman Numeral to convert
        li $v0, 4 # Print prompt1
        la $a0, prompt1 # Load address
        syscall

        # Load the word reserved for input, and read in the Roman Numeral string input by the user         
        la $a0, input1 # Load contents of input1 (i.e. space allocated to save the input string) into $a0
        la $a1, input1 # Load the length allocated for input1 (i.e. 4 bytes or 32 bits) into $a1 to prevent overflow
        li $v0, 8 # Read string from user 
        syscall 

        sw $ra, 0($sp) # Push return address onto the stack  
        addi $sp, $sp, -4 # Move stack pointer       

       # Go to sub-routines for lookup and conversion
       jal init

       lw $ra, 0($sp) # Pop return address off the stack
       addi $sp, $sp, 4 # Move stack pointer


# Display results for the user    
display: 
    # Load output string 1 to show the user their original input string
        la $a0, outRecap # load contents of outRecap into $a0
        li $v0, 4 # print string 
        syscall

        # Load the string the user entered 
        la $a0, input1 # load contents of input1 into $a0
        li $v0, 4 # print string
        syscall

        # Display the equivalent decimal value
        la $a0, outFinal # load contents of outFinal (string) into $a0
        li $v0, 4 # print string
        syscall

        # Print the sum
        lw $a0, sum # Load contents of sum into $a0
        li $v0,1 # Print integer
        syscall

        # Ask the user whether they want to convert another Roman Numeral (1 if YES; 2 if NO; default is NO)
        la $a0, loopdriver # Load contents of loopdriver (string) into $a0
        li $v0, 4 # Print string
        syscall 

        # Get user input from keyboard (1 = they want to convert another Roman Numeral; 2 = Exit; default = 2)
        li $v0, 5 # Read integer input
        syscall

        # Save the user's input and jump to "Exit" 
        sw $v0, convertAgain
        lw $t0, convertAgain

        bne $t0, 1, Exit # if the user enters a number != 1, go to Exit    

    move $s0, $zero # Reinitialize
    sw $zero, sum # Reinitialize
    sw $zero, left # Reinitialize

        j main  # Go back to start if the user enters 1

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

# Convert Roman Numeral string to integer decimal value   
init: 
    #sw $ra, 4($sp)
    sw $a1, 4($sp)
    addi $sp, $sp, -4 # move stack pointer  

    la $t2, input1  #Load the address of the string (see by looking at starting index of each the length of each string 
    la $t3, roman # Load the address of the roman lookup table into $t3
    la $t4, values # Load the address of the decimal lookup table into $t4

# Loop through each char of the input string
loop1:  
    lb $a0, ($t2) # This gets the next byte of the string (from L to R); change the offset to get others (i.e. lb $a0, 3($t0) = I
    beq $a0, 10, return # If the ascii char == 0, we have reached the end of the input string; jump to "return"
    beq $a0, 1, return # If the ascii char == 1, we've reached the "start of heading" (i.e. end of line; max = 12 char)

    #li $v0, 11 # print byte to console
    #syscall


    sw $ra, 8($sp) # Push return address onto the stack
    addi $sp, $sp, -4 # Move stack pointer

    # While string[i] != null:
    jal index

    lw $ra, 8($sp) # Pop return address off stack
    addi $sp, $sp, 4 # Move stack pointer 

    addi $t2, $t2, 1 # Move to next character in input string 

    sw $ra, 8($sp) # Push return address onto the stack
    addi $sp, $sp, -4 # Move stack pointer 

    jal loop1 # iterate through loop1 again

    lw $ra, 8($sp) # Pop return address off the stack
    addi $sp, $sp, 4 # Move stack pointer 

# Find the index (i.e. offset) of the selected char in the "roman" lookup table
index:  
    lb $t5, ($t3) # Load the first byte of romans
    beqz $t5, invalid # If we reach the end of romans && char !found, INVALID char

    beq $a0, $t5, getVal # When char == element in romans table, branch to getVal to look it up in decimal table

    sw $ra, 12($sp) # Push return address onto the stack
    addi $sp, $sp, -4 # Move stack pointer

    jal iter # If we haven't found a match, jump to where we can increment $t3 and iterate through the loop again

    lw $ra, 12($sp) # Pop return address off the stack
    addi $sp, $sp, 4 # Move stack pointer 


getVal: 
    la $t6, roman # Get the address of the array containing Roman numeral characters
    la $t7, values

    sub $t8, $t3, $t6 # Get the index value of the element that matches the byte we are working with [1:7]
    add $t7, $t7, $t8 
    lbu $t9, ($t7) # $t9 is the decimal value that corresponds to the letter
    bgeu $t9, 232, adjust1

    j afterGV



adjust1: 

    seq $a2, $t5, 68 # If the character in question == D, set $a2 to 1; else, 0
     mul $t9, $t9, $zero
     beq $a2, 1, adjustD
     addi $t9, $t9, 1000 # Char == M; decimal value = 1000

     j afterGV


adjustD: addi $t9, $t9, 500

    j afterGV

afterGV: sw $ra, 16($sp)
    addi $sp, $sp, -4 # move stack pointer


    jal setup

    addi $sp, $sp, 4 # reset the stack pointer
        lw $ra, 16($sp) # Pop return address off stack
       # lw $t9, 12($sp) # fetch $t9 

    jr $ra  # go back   



# This is the last part of the index loop
iter:   
    addi $t3, $t3, 1 # if we haven't found a match, increment $t3 and iterate through the loop again
    jal index # loop

# Prepare to compare current char w/previous char (i.e. if string is XI, compare I to X)
setup: 
    # Base case is that the string length == 1, and thus sum = the decimal equivalent of the only Roman numeral character
    lw $s0, sum # Load sum into $s0
    beqz $s0, base # If sum == 0, this is our first pass through the loop, and we just want to add the first char's decimal value to sum and return to get the next char

    sw $ra, 20($sp) # Push return address onto stack
    addi $sp, $sp, -4 # Move stack pointer

    jal calcSum # Jump to subroutine that calculates the sum

    addi $sp, $sp, 4 # Reset the stack pointer
        lw $ra, 20($sp) # Pop return address off stack
        jr $ra  # Go back to return address


# Base case is our first pass through the loop, where sum == 0. 
# We just want to add the value of the first number in the string to sum, and return to loop1 to get next char. 
base:   add $s0, $s0, $t9 # Add value of first char + 0 and store in $s0
    sw $s0, sum # Store contents of $s0 in sum
    sw $t9, left # Store contents of $t9 (i.e. current char) as "left" for use in next iteration

    la $t3, roman # Load address of roman array into $t3 (reset pointer)
    addi $t2, $t2, 1 # Add 1 to $t2, so that we can select next char in input string

    jal loop1 # Jump back to beginning of loop 


# Compare current char to previous, and calculate sum accordingly, per rules of Roman Numerals  
calcSum: addi $sp, $sp, 8 # Reset the stack pointer
    lw $t1, left # Pop the decimal value of the Roman numeral to the left of the current value in the original string   
    sw $t9, left # Reset left pointer so it points to current char ("left" of *next* char)

    la $t3, roman # Load address of roman array into $t3 (reset pointer)
    bge $t1, $t9, plus # If value of the previous char  >=  current char, then add the current char's value to the sum
    blt $t1, $t9, minus # If the previous char in the string is < the current char, add: current - (2*previous) to the sum

plus:   lw $s0, sum # Load sum into $s0
    add $s0, $s0, $t9 # Add current char's decimal value to sum
    sw $s0, sum # Store the result in sum

    la $t3, roman # Load address of roman array into $t3 (reset pointer)
    addi $t2, $t2, 1 # Add 1 to $t2, so that we can select next char in input string

    jal loop1 # Jump back to beginning of loop 

minus: lw $s0, sum # Load sum into $s0
    mul $t1, $t1, 2 # Multiply $1 by 2 (need to subtract twice, to get a net impact of -1 * $t1)
    sub $t9, $t9, $t1 # Subtract (previous char - (2* current char)) (i.e. if XIV, do 11 + (5-(2*1)) = 14)
    add $s0, $s0, $t9 # Add this result to the existing sum
    sw $s0, sum # Store the result in sum   

    la $t3, roman # Load address of roman array into $t3 (reset pointer)
    addi $t2, $t2, 1 # Add 1 to $t2, so that we can select next char in input string

    jal loop1 # Jump back to beginning of loop 

# Go back to main   
return: 
    sw $s0, sum # Store sum
        j display # Jump to display to print output for the user

#Exception handling: User input string contains invalid character(s) 
invalid: 
        li $v0, 4   # Print string
        la $a0, error1  # Load address for error1 (a string)
        syscall

    j main 

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

#System Exit
Exit: 
    # Display exit message
        li $v0, 4   # Print string
        la $a0, stop    # Load address for stop (a string)
        syscall

        li $v0, 10  # System exit
        syscall

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