A common test for whether the computer has crashed is pressing the "caps lock" key. The keyboard sends the key code to the keyboard driver running in the main computer; if the main computer is operating, it commands the light to turn on. All the other indicator lights work in a similar way. The keyboard driver also tracks the shift, alt and control state of the keyboard.
a comMOn teST fOR WHeTher tHE comPuter HAs CRasHEd iS prEsSing The "cApS lOcK" key. tHe keYboard sENDs the keY code to the kEybOard DRiveR ruNning in The MaIN computeR; If the main cOMpUter Is opeRatinG, IT coMmAnds THe LiGHt to turn ON. alL the OthER inDicaTOr LiGhtS wOrk In a SimiLAr Way. THe KEyboarD drIvEr ALSo TRaCKS thE SHiFt, aLt ANd contrOl state of The KeYboArD.