Commodore 64 Text Adventures: Objects and Actions

We are writing a text adventure for the C64 using commodore BASIC and we got pretty far, but it is not finished so … What should we do next?

As the code grows I am so happy that I chose to go with the Commodore 64 Visual Studio extension. This extension still produces valid C64 BASIC code but allows you to omit line numbers and use labels instead. If you are not using it currently you really should check it out.

If you do need the line numbers, though, I still got you – go get that version on Github.

Commodore 64 BASIC Programming Series

Part 1: Introduction to Commodore BASIC

Part 1.5: Installing CBM Prg Studio on Mac/Linux

Part 2: Commodore BASIC Commands GOSUB and FOR Loops

Part 3: If/Then, Game Logic and Cursor Movement

Part 4: The Magic of POKE

Part 5: C64 DOS Commands

Part 6: Working with Data Files

Part 7: Programming C64 with Visual Studio Code

Part 8: C64 BASIC String Manipulation

Part 9: C64 Text Adventure Game

Part 10: Objects and Actions in Commodore 64 Text Adventures

It’s not a fully playable game yet:

Up to now the player can move around and grab objects, but not interact with them.
We do not have help text.
Winning condition so the game can be completed.
Mysteries and hidden spaces.
We can win so how does the user lose.
Let’s pretty up the introduction screen.

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Play the Adventure in Your Browser

You can try out the adventure as it stands in your browser using my C64 emulator here.

As I write this I am working on my version of the 8bitworkshop browser based code editor and emulator experience to include Commodore BASIC as well as assembler and C.

I think if I succed it will make it easier for people to follow these tutorials and create their own really cool C64 (and more) games and programs.

You can now follow the tutorials and edit the code right in your web browser with the Online Retro IDE

– No downloads, configuration, etc necessary, and it is free!

Interacting with Game Objects

Our variables do include game objects, and we can place them in rooms.

This is 1980s BASIC so we are limited to data structures we can use, so we went with having several lists (arrays):

The object name.
Object description.
Which room an object is in. If it shows as in room 0 then it is in the players inventory.

Now we need to be able to use objects.

if left$(i$,4) = "use " then gosub useobject

Initially we are able to just copy what we did for get and drop so we pick up the name and find the object’s number from the list.

How could or should we build interaction into this data setup?

We could have on/off flags for our objects, remember how we set up the game map?

IF MID$(EX$(PL),1,2)<>"00" THEN PRINT ". NORTH"

In the same way we could say “if it is 0 then the light is off, 1 then the light is on“.

For my XC-BASIC text adventure engine this is the route I went. You can play it here. It didn’t get as far as a full game unfortunately because XC-BASIC 3 came out and was incompatible with XC2. One day I might come back to it and get it working again.

After some consideration, I don’t think we should do anything complicated. At least not now.

There is a point where abstraction just creates additional complexity and extra work (especially given the tools in this version of the language).

In general, it really depends if you are writing a game or a game engine.

If we don’t go the clever data-driven route it has a benefit that it gives us the complete flexibility to just come up with whatever logic we want for any scenario. We do not have to know ahead of time how you might need things to work. Drop a piano on the character, electrocute them in a wet bathroom, send them to the vacuum of space – it’s your game!

Therefore you get something like these:

objectactions:
rem actions for using objects
if f=1 and pl=3 then print "boom! the furnace explodes, filling the room with fire and smoke!":gosub waitkey:gosub gameover: rem matches in furnace room
if f=1 and pl=2 then print "suddenly the furnace roars to life, filling the room with heat and light!":ex$(2)="010604000300":m=m-1: if m<=0 then print "You are out of matches":ol(f-1)=-1: return : rem remove matches from inventory
if f=1 and pl <> 2 then print "you strike a match and light it, illuminating the room for a moment.":m=m-1: if m<=0 then print "You are out of matches":ol(f-1)=-1: rem remove matches from inventory
if f=2 and pl=4 then print "click! the door has unlocked!":ex$(4)="020005000000":ol(f-1)=-1: rem remove key from inventory
if f=2 and pl<>4 then print "you try to use the key, but it doesn't fit any locks here."
RETURN

In the first scenario, lighting the matches (f=1) in the service hatch (pl=3) ignites a gas leak, blowing up the furnace.

On the other end of the drama scale, using a key can unlock a door (or not).

Replacing Parts of Strings in C64 BASIC

Again, as mentioned earlier, we could work on parts of the fixed length strings but for convenience in my example we just write in a new copy of the data that reveals a new exit.

Here is how we can add the room into the data more surgically. Remember to read if there is an south door we do mid$(ex$(pl),5,2) . We look at the character position and get two characters back.

To change just the correct characters in a C64 BASIC string is involved:

10 a$="abcdefgh"
20 print a$
30 a$=left$(a$,3)+"x"+mid$(a$,len(a$)-4,4)
40 print a$

In C64 BASIC you must reconstruct the string to replace a part of it

Later versions have MID$ for assignment as well as retrieval — In C64 BASIC you must reconstruct the string to replace a part of it

In later versions of Commodore BASIC it turns out you can do the MID$ in reverse and write characters at that position mid$(ex$(pl),5,2)="05" – much more convenient.

That way ex$(4)="020000000000"becomes 020005000000

💬 Questions or comments? Head over to the community to discuss!

Adding New Screens (Help Text, Winning, and Intro)

Adding a basic screen of text is as easy as setting up the subroutine (either a numbered line or preferably in the Visual Studio approach, a label: and gosub to it:

help:
rem show help screen                          :                                       :
gosub newscreen
print "you have woken up in a dark and damp    basement, with no memory of how you got here."
print ""
print "your head is pounding and you feel      disoriented. you need to find a way out."
print ""
print "enter the command you want to use:"
print "  North, South, East, West, Up, Down"
print "  (n, s, e, w, u, d)"
print ""
print "  get

Commodore 64 Text Adventures: Objects and Actions

Commodore 64 BASIC Programming Series

Play the Adventure in Your Browser

Interacting with Game Objects

Replacing Parts of Strings in C64 BASIC

Adding New Screens (Help Text, Winning, and Intro)

More Advanced `IF` Logic in C64 BASIC

Keep an Eye on Memory

Full Adventure Code Now (So Far)

Commodore 64 Text Adventures: Objects and Actions

Commodore 64 BASIC Programming Series

Play the Adventure in Your Browser

Interacting with Game Objects

Replacing Parts of Strings in C64 BASIC

Adding New Screens (Help Text, Winning, and Intro)

More Advanced IF Logic in C64 BASIC

Keep an Eye on Memory

Full Adventure Code Now (So Far)

More Advanced `IF` Logic in C64 BASIC