Simple OS kernel -- screen output long string array issue - operating-system

Question

I am following the steps in: "Writing a Simple Operating System from Scratch". I got the basic kernel up and running, but got a strange error when I try to output a very long string.
The following code output all "0"s. But when I use a shorter "msg" string, then the screen prints "x".
I wonder if anybody can help me, thanks much! (I am testing it in Bochs in windows 8).
Here is the kernel.c
void start ()
{
// Create a pointer to a char , and point it to the first text cell of
// video memory (i.e. the top - left of the screen )
unsigned char *video_memory = ( unsigned char*) 0xb8000;
// At the address pointed to by video_memory , store the character 'X'
// (i.e. display 'X' in the top - left of the screen ).
// this string: the outputs are "0"s.
const char msg[] = "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzAbcdefghijkAbcdefghijklmnopqrstuvwxyzA";
// this string: output are "x"s.
//const char msg[] = "abcdefghijklmnopqrstuvwxyzabcdefghijk";
int i = 0;
int offset = 0;
while(i<85)
{
// Test if msg points to the right memory
if(msg[i]==0)
{
video_memory[offset] = '0';
}
else
{
video_memory[offset] = 'x';
}
video_memory[offset+1] = 0x02; // Green color
i = i + 1;
offset = offset + 2;
//set_cursor(offset);
}
}