Abstract: A computer system has dual-color LEDs for the LEDs conventionally indicating power on and hard disk drive activity, and a control circuit drives the LEDs in on-off and color combinations to indicate diagnostic information. In a preferred embodiment, a control circuit for controlling the LEDs is addressable as a port, and state combinations of the LEDs are set according to data sent to the port address of the control circuit. Also in a preferred embodiment, a POST routine for the computer system is divided into specific tests and groups of tests by commands configured to write a specific data string to a port address depending upon the position of the control command in the POST routine.