-- program is finished running (dropped off bottom) --

Question

This is a MIPS assembly program. The program to prompt the user for a single 64-bit integer in hexadecimal, convert it, and then print the overflow status, but I get the following error:

-- program is finished running (dropped off bottom) --

Here is my code: I am running the program on MARS 4.5

.data
    prompt: .asciiz "Enter the 64-bit integer in hexadecimal: "
    overflow_label: .asciiz "Overflow = "
    newline: .asciiz "\n"

.text
    # Main program
    main:
        # Prompt user for 64-bit integer
        li $v0, 4                  # syscall for print string
        la $a0, prompt             # load address of prompt
        syscall                    # print prompt

        # Read 64-bit integer
        li $v0, 8                  # syscall for read string
        la $a0, buffer             # load address of buffer
        li $a1, 17                 # maximum number of characters to read (16 hex digits + null terminator)
        syscall                    # read integer

        # Convert input string to integer
        jal convert_hex_to_int     # jump to convert_hex_to_int subroutine

        # Print overflow status
        li $v0, 4                  # syscall for print string
        la $a0, overflow_label     # load address of overflow_label
        syscall                    # print overflow_label

        # Print overflow status
        li $v0, 1                  # syscall for print integer
        move $a0, $t3              # move overflow status to $a0
        syscall                    # print overflow status

        # Exit program
        li $v0, 10                 # syscall for exit program
        syscall                    # exit program

# Subroutine to convert hexadecimal string to integer
convert_hex_to_int:
    li $t6, 0x30                # ASCII value for '0'
    li $t7, 0x0A                # ASCII value for 'A'
    li $t8, 0                    # initialize integer value to 0

convert_loop:
    lb $t9, 0($a0)              # load byte from buffer
    addi $a0, $a0, 1            # move to next character in buffer

    # Check if end of string
    beqz $t9, convert_done      # if byte is null, exit loop

    # Check if character is between '0' and '9'
    sub $t9, $t9, $t6           # convert ASCII to integer
    bltz $t9, check_alpha        # if character is not between '0' and '9', check if it's an alphabet

    # Character is between '0' and '9'
    sll $t8, $t8, 4             # shift current integer value left by 4 bits
    or $t8, $t8, $t9            # combine integer value with new digit
    j convert_loop              # repeat loop

check_alpha:
    li $t9, 0x20                # difference between lowercase and uppercase alphabets

    # Check if character is between 'A' and 'F'
    add $t9, $t9, $t9           # convert ASCII to uppercase
    bge $t9, 0x41, convert_upper  # if uppercase, check if between 'A' and 'F'
    j convert_lower             # if lowercase, convert to uppercase and proceed

convert_upper:
    sub $t9, $t9, $t7           # convert ASCII to integer
    addi $t9, $t9, 0x0A         # adjust value for 'A' to 'F'
    j convert_continue          # continue with the conversion

convert_lower:
    sub $t9, $t9, $t6           # convert ASCII to integer

convert_continue:
    sll $t8, $t8, 4             # shift current integer value left by 4 bits
    or $t8, $t8, $t9            # combine integer value with new digit
    j convert_loop              # repeat loop

convert_done:
    move $v0, $t8               # store result in $v0
    jr $ra                      # return

# Buffer for input string
.data
    buffer: .space 17           # buffer for input string (64-bit integer)

Actually, I supposed to add two 64-bit integers that are signed and entered in hexadecimal format as strings. The program should output the sum in hexadecimal and indicate if there is an overflow. I minumized the prog to only process one entery.