How can I get the ASCII value of characters in a string in ARM Assembly?

Question

I have to convert the following code into assembly language:

void rotN1(char *str, int n)
{ // NB: str will be modified in-place
  char *p;
  for (p = str; *p != '\0'; p++)
  {
    int currChar = (int)*p;
    if (currChar >= 'a' && currChar <= 'z')
    {
      currChar = currChar + n;
      if (currChar > 'z')
      {
        currChar = currChar - 26;
      }
      *p = (char)currChar;
    }
  }
}

It's basically using a Caesar cipher on a given string (str), and n is the encryption key.

.data
        array:  .string         %[str]
        .equ    len.array,.-array
        .align

.text
        .global  main
main:

        nop

        ldr r2,=array           // pointer
        MOV r0, #0              // initialise loop index to 0
        MOV r1, #len.array      // number of elements 
    MOV r4, %[n]
        Loop:
                ldrb r3, [r2, r0]
            mov r6, # 
            B check_A 
            B check_Z
            ADD r3, r3, r4
            

        ADD r0, r0, #1           //increment loop index
        CMP r0, r1
        BLE Loop


_exit:
        mov r7, #1
        svc 0



check_A:
CMP r3, #97
BLT _exit

check_Z:
CMP r3, #122
BGT _exit

I haven't finished the code but while traversing the array I need to get the ASCII value of that character in each iteration. Is there a way to do that? I have to do this as inline ARM in C.

Answer 1

"...I need to get the ASCII value of that character in each iteration."

You already have them.

ASCII characters each have a value as shown in the link table. Referring to them with their decimal value, characters AZ range from 65 - 90 . az range from 97 - 122 . So, in an array such as:

char msg[] = "Hello";

msg[0] has a value of 72 ,
msg[4] has a value of 111 ,
msg[5] has a value of 0 .

Note, the last conversion is because msg is initialized with a string literal, and string literals in C are defined as a char array terminated with a NULL terminator. Note that not all char arrays are required be NULL terminated, in which case that array would be a simple char array, not a C string.

Also, as you display in your post, an ASCII char surrounded by single quotes represents the decimal value of that character. eg in the expression

char aitch = 'H';     

aitch now has the value 72.