Gunfight

Gunfight, a two-player competitive gunslinging game, is one of four built-in programs in the Bally Arcade's system ROM.

The Bally Arcade patent includes an excellent pseudo-code flow chart for Gunfight. Though not every game will function the same way, it does lay out a set of best practices for initialization, game states, the game loop, input handling, and interrupts.

        Get Maximum Score
        Clear RAM
        Set vertical blank, horizontal color boundary, interrupt mode
        Set colors
        Play Streets of Laredo        
STRND:  Start round
        Initialize bullets and timers
        Set up screen
        Display scores
        Display "Get Ready"
        Put up proper number of Cacti, Trees, and Wagon
        Set up vectors so cowboys walk out
        Start interrupts
        Pause until cowboys walk out
        Erase "Get Ready"    
LOOP:   Call SENTRY (check for a change of input)
        Call DOIT
        If bullet hit anything...
                Kill object
                If cowboy killed...
                        Set Death Flag
        Got to LOOP
        
        DOIT:
                If time up for round
                        Exit
                        Go to STRND
                Else
                If Death Flag set
                        Exit
                        Go to STRND
                Else
                If Player 1 or Player 2 Pot moved
                        Update new arm angle
                Else
                If Player 1 or Player 2 Joystick moved
                        Update new velocity
                Else
                If key depressed
                        Coffee break
                Else
                If Player 1 or Player 2 trigger pulled
                        Fire bullet
                Else
                If 1 second has elapsed
                        Update new time
                ENDIF
        Exit
        
        Interrupt Routine:
                Bump all time bases
                Erase all active bullets
                Vector bullets
                Write bullets to new location
                Set each bullets' hit flag if it hit something
                Erase next object in write queue
                Vector that object
                Write that object to a new location
                Put object back in queue
                Schedule next interrupt
        Exit