Saturday, 23 July 2016

Fort Django

A little reflection on the making of a Commodore 64 game. It's the first C64 machine-code game project I've really finished. All right, it was made with cc65 C with inline assembler but what I mean it's not made in BASIC. Significantly, it's the first 8-bit game I've made public in some way.

I guess making an 8-bit game is something I would have liked to do for a long time. Fort Django was started in 2014, and after a long pause I found the energy to make it into a release.

Djangooooooooohh....!
The game

You guide the character with a joystick in port 2. You can run, climb, crouch, jump and shoot. Shoot down the baddies, collect money bags and find the exit. The descending bonus timer means the faster you can collect the next bag the more $ you can get.

The game is very short and not at all hard. The only challenge comes from trying to be faster, for example it's possible to break the $10000 barrier on completion.

Get that bag, shoot that baddie
Inspired by Saboteur! from Durell, I at first thought about making a beat 'em up oriented game. As I needed to scale down the project I found it would be simpler to turn the game into a shooter with a western theme. The map is very small, but then again I like short games such as Saboteur! and Bruce Lee, as they have a strange kind of replay value. Much like with Saboteur!, I wanted to ensure there was a definite ending to the game and score could not be milked forever.

Making of

I created the game with cc65 C compiler, using inline-assembler for speed critical parts such as the sprite routines. The C64 has eight sprites, eight 8-bit addresses for the horizontal coordinate (0-255) and one address that holds the highest bits for all the X coordinates, so the sprites can also reach the right hand part of the screen. (256-320)

For a beginner it can be a bit tricky to decode these 9-bit sprite coordinates, especially in pure assembler. I have eight separate 16-bit memory locations for storing the X coordinates. These are then broken down into the hardware sprite coordinate values. This approach is a compromise between ease of use and speed.

The figure below shows how the 16-bit X coordinates are stored in $C000/$C001, $C008/$C009, $C010/$C018 byte pairs (the grey stuff in the middle). The less significant byte of these 16-bit values can be copied directly to the $D000, $D002, $D004... but the high bit is taken from the lowest bit of the most significant byte of the 16-bit values and combined into a value that is stored in $D010.


This is done once in a frame, so the high-bit issue can be forgotten in other parts of the code. The handiness of this is only really apparent in C, where you can then move the sprite x coordinate around with:

*(unsigned*)0xc000=*(unsigned*)0xc000+1;

Comparisons between coordinates become easier, too.  This checks if sprite 7 is right of the sprite 0:

if(*(unsigned*)0xc038>*(unsigned*)0xc000){do_stuff();}

Basically all the "collision detection" is built from this type of statements instead of the hardware sprite collision address, and as such is not pixel perfect. In these box-collision cases it's better to be lenient toward the player and a bit biased against the enemies.

The "16-bit" coordinate ought not to exceed 511, because only one bit is taken from the more significant byte.

The sprite Y coordinates are 8-bit values anyway and can be handled directly with the $D001, $D003, $D005... hardware addresses.

The whole X coordinate copying is achieved with the code below, starting from the less significant byte copying and ending with the high-bit construction. Obviously other locations than $C000- can be used for the coordinate storage.

     lda $C038
     sta $D00E
     lda $C030
     sta $D00C
     lda $C028
     sta $D00A
     lda $C020
     sta $D008
     lda $C018
     sta $D006
     lda $C010
     sta $D004
     lda $C008
     sta $D002
     lda $C000
     sta $D000

     lda $C039
     clc
     rol a
     ora $C031
     rol a
     ora $C029
     rol a
     ora $C021
     rol a
     ora $C019
     rol a
     ora $C011
     rol a
     ora $C009
     rol a
     ora $C001
     sta $D010

This is a starting point for a fairly generic solution, but of course it can be adapted for any particular needs. For example, the top sprite coordinates of the dudes in the game are copied and transformed from the bottom part coordinates, which changes the above routine a bit. Also, if less sprites are used why bother going through all the eight?

About the graphics

The graphics are made with PETSCII editor. Not only the background tiles, but the game map and even the sprites have been edited there. It goes to show that the PETSCII editor multiframe-editing is surprisingly powerful way for controlling this type of game "assets".
The game map tiles.
Another PETSCII screen was wasted for defining the movement rules for the above tiles. These are used for building a movement table every time the player enters a room, just as the room is drawn from tiles. How all these sprite, tile and movement table elements exactly relate to each other is a bit too intense to explain here, especially as they are not that well thought out.
The tile movement rules. @=space, A=block, B=platform, C=ladder
The map was also edited in the PETSCII editor. The 40x25 area is divided into 5x3 tile elements, giving 64 simple screens. Coloring indicates enemies and money bags. Not everything is absolutely visible in the picture below, as some spaces can be "colored" too. The game engine allows only certain combinations and positions for the enemies and objects.
The game map.
The map memory area is also used for indicating whether objects or enemies are removed, from the 1 (white=enemy) and 2(red=bag) status into 255 and 254. After the game is over these are changed back into 1 and 2.

Editing a multicolor sprite in PETSCII editor. The sprite export is not a standard feature!

What next?

I dropped many game elements I toyed with at some point. For instance, the chests could have contained items, and the doors could have potentially led to other areas in the fort.

The gold bags were a fairly late addition when I felt that only shooting bad guys would be far too minimal. From adding the bags it was a small leap to increase the jumping elements in the game. The bags also inspired the time-based "bonus dollars" mechanic. Still there might have been a bit more to do.

Code-wise, a music routine would have been nice but in the beginning I was a bit scared to sync the animation with a music interrupt. The sound effects are also sparse due to my inexperience with SID.

Yet, all additions would have also expanded the complexity of the game and the testing time exponentially. I'm glad I could finish it as it is. With this experience I already have some ideas about how to approach this type of project better.

Links

You'll need a Commodore 64 or an emulator to play the game.

Direct download
Alternative link
Page at CSDb
A cracked version in a T64 format

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