Write an ARM assembly function that takes a string and two characters and returns a pointer to a new modified string. The first character is the character to replace and the second is the character to replace it with. The C program below changes the character 'i' to an 'X'

The C language program is:

#include

extern newsubst( char * string, char this, char that ) ;

void main( int argc, char * argv[] )

{

            char * result ;

            char this = 'i' ;

            char that = 'X' ;

            char string[] = "Another string to modify" ;

            result = newsubst( string, this, that ) ;

            printf( "Original: %s\nModified: %s\n", string, result ) ;

}

The input to the ARM assembly language function is a pointer to the first element of the string in register a1. The character to look for is in a2 as a byte in the low part of the register and the character to replace it with is in a3 as a byte in the low part of the register. Remember a character is 1 byte long. A string in C is terminated with a null byte (one equal to zero).

To determine the length of the string you can use the library subroutine strlen. The input to this subroutine is a pointer to the string in a1. The subroutine returns the length of the string not counting the terminating null byte in a1.

To obtain space for the new string you can use the library subroutine malloc. The input to this subroutine is a number of bytes of space to obtain in a1. The subroutine returns a pointer to the allocated space in a1.