How to dynamic set default value to parameters?

Hi,

There are two parameters of radio buttons on the interface. I want to set default valuse to parameters according to sy-sysid. That is:

(1) if sy-sysid = 'S1'

p1............. 'X'

p2.............

(2)if sy-syid = 'S2'

p1............

p2............ 'X'

how to write the code?

Thanks