The Cosmac VIP uses the 4 to 16 line decoder CD4515 to decode the 16 keys. The lower 4 bits of OUT 2 are used as input selecting one of the 16 outputs of the CD4515. Every output is connected to one of the keys and all keys are connected to the EF3 flag. The VIP firmware checks the keys by stepping OUT 2 from 0 to hexadecimal F. After every OUT, EF3 is checked and if EF3 = 0 the key representing the last OUT 2 was pressed.
The VIP also has a RUN switch which will connect a 0 or 1 to the CLEAR input of the 1802 resulting in RESET or RUN state.