Abstract: A computer system includes a display, a graphics chip capable of controlling the display and modulating light signals emitted from a plurality of sub-blocks in a main block on the display sequentially according to a predetermined rule, a digitizer comprising a light signal detecting module which is capable of detecting a light signal emitted from the display and sending a corresponding signal when the light signal emitted from the display is detected, a light signal processing unit for processing signals sent from the light signal detecting module and generating a detection succeeded signal when the result of the processing shows the modulation of the light signals emitted by the display according to the predetermined rule is detected by the light signal detecting module, and a positioning unit for determining the position of the digitizer on the display according to the predetermined rule.