error 65

I'm new to embedded c programming... I have an error *** error 65: access violation at C:0x0300 : no 'execute/read' permission*** while running the following program.... can anyone clarify my doubt...

Program:

#include <reg51.h>
void main(void)
{ unsigned char z;
z=P1;
z=z&0x3;
switch (z)
{ case(0):
{ P0=0;
break;
} case(1):
{ P0=1;
break;
} case(2):
{ P0=2;
break;
} case(3):
{ P0=3;
break;
} }
}