Programming adventures in VIC-20.

Main loop

A text adventure (like any other game) is a big loop that runs forever until the player exits the game.

Main loop of Pursuing Tom Ram is simple. Here is the code.

  
;--- Main --------------------------

Game_Begin
        ; Init variables
        jsr Init
        ; show intructions of the game
        jsr Instructions
        
LocLoop
        jsr PrintContent

MainLoop
        ; Reset stack
        ldx #$ff
        txs

        jsr InputCommand; get command
        jsr EvalCommand
        jmp MainLoop

  

Complex adventures should have a more complex main loop. For example, you may add a subroutine to perform actions after player writes a verb.

Main loop

When I jump to a subroutine with JSR, the processor stores the address for coming back of the subroutine (with RTS) in the stack. Sometimes I do not call RTS,. Instead, I force the jump with (JMP). While the player plays, the stack is growing with useless memory addresses If the stack grows beyond limits, the computer crashes.

I have to options. I could be careful enough to execute a RTS every time I call a JSR or I could clean the stack when it is safe.

I did not find the way to execute a RTS at every time I jump with JSR, so I chose the second option. At the beginning of the main loop, I know there is no JSR jump waiting to return, so it is a safe place to clear the stack.

Processor knows where the stack is because it stores the address in a register called S. so I set the original address of the pile in S at the beginning of the main loop.

Two last words. First, the stack is managed by 6502, not commodore computers, so almost every machine worth a 6502 works with the sack in the same way. Second, the address of the stack begins at $199, but processor adds $100 to the address, so I store $99 in the S register.