A computer system such as a smart card includes first, second and third memory areas. The first memory area may be a general code area, and stores therein at least one general application program including at least one subroutine call to the second memory area. The second memory area may be a vector table area, and stores therein at least one subroutine call to the third memory area. The third memory area may be a security code area, and stores therein at least one security application program that is only callable by a subroutine call from the second memory area. A processing system is configured to execute a general application program in the first memory area, including a subroutine call to the second memory area, and to unlock the third memory area only upon executing a subroutine call from the first memory area to the second memory area.