I've been trying to make a function to store a string (passed as parameter to a function) into the heap and make it return the memory address where it's saved. For that, I've made a loop iterating into every char of the string and putting it into an allocated byte. However, I receive the message "invalid program counter value: 0x00000000"

This is what I have so far, any ideas on what might be wrong?

main:
    #Parameters for function
    #a0 (string)
    #a1 (string size)
    
    la $a0, string
    lw $a1, size
    
    jal saveString

    #Registers planification:
    #s0: string copy
    #s1: where space is saved
    #s2:  index pointer for the loop
saveString:
    sw $fp, ($sp)
    sw $ra, -4($sp)
    sw $s0, -8($sp)
    sw $s1, -12($sp)
    sw $s2, -16($sp)
    move $fp, $sp
    addi $sp $sp -20
    
    move $s0, $a0
    
    #allocate memory
    li $v0, 9
    move $a0, $a1
    syscall
    
    bltz $v0, saveString_noMemory
    
    #save pointer with $v0
    move $s1, $v0
    
    #initialize array pointer
    add $s2, $zero, 0

    jal saveString_Loop
    
saveString_Loop:
    beq $s2, $a1, saveString_End

    lbu $t0, ($s0)      #take value of string
    sb $t0, ($s1)       #inserto that value into one bit of allocated memory
    
    addi $s0, $s0, 1    #point to next char of the string
    addi $s1, $s1, 1    #point to next byte of allocated memory
    
    addi $s2, $s2, 1    #i  
    
    move $v0, $t0
    
    b saveString_Loop
    
saveString_NoMemory:
    #todo: error message
    jr $ra
    
saveString_End: 
    lw $fp, ($sp)
    lw $ra, -4($sp)
    lw $s0, -8($sp)
    lw $s1, -12($sp)
    lw $s2, -16($sp)
    move $sp, $fp
    
    jr $ra
    

CodePudding user response：

This seems like a calling convention error. You're saving $ra into the stack but not restoring it after you finish a function. Remember that jal will overwrite $ra. So when you do

jal saveString

you will no longer be able to return to the original function using jr $ra until you restore $ra by loading from the stack.