My games for VIC-20

In this repsitory you'll find some games I wrote between 1982 and 1984 for the
VIC-20. At that time I was 15, so the purpose was mainly for fun and learning.

The games were written originally in MOS-6502 machine-language directly in memory,
using a self-developed disassembler (use of an assembler was not possible due
to memory constraints). This means in particular the code was not relocatable,
so all variable and jump addresses were hard-coded. I had calculate all the
branch offsets manually, and re-calculate when moving or inserting code. I achieved
this by maintaining a copy of the code on paper, and cutting the paper with a
scissor when inserting code. On paper I also could maintain comments to the
code. As example I scanned two of about 20 pages of the snakes game.

The code you find here is typed-in newly using the hex-dump also contained on
these paper trails. Afterward I ran the image through the
"dxa" disassembler
and reverse-engineered its meaning to allow extending the original sparse
comments. The result of this process is checked in as initial revision in git.
(Corresponding to the above example would be label "l12e0" and following in the
initial commit.)
Afterward I cleaned up the code (such as introducing symbolic names
for variables and functions in place of the hex labels generated by the
disassembler), further improvement of documentation, bug-fixing
and a few adaptions and extensions noted below. As a result the games now play
nicely in the excellent VICE VIC-20
emulator.

Dino-Eggs game

In this game the world consists of 4 levels in a cave between which the
player can traverse via ladders and by jumping. The objective is picking
up Dino eggs buried in the ground and sometimes hidden by stones. To
keep the Dino Mum away, the player has to keep a fire going, which needs
to be kindled by wood to be found in the cave. All the while the player
has to watch out for snakes crawling across the levels and hidden in
the ground beneath stones.

I should note I didn't completely finish developing this game in the 80s.
There are sketches showing I had planned for spiders crawling on the ceiling
of the cave and occasionally lowering themselves (this explains the empty
space remaining at the top of the current game); also there's no sound yet. Also
the code implementing the snakes somehow got lost (except for the jump to the start
of that handler). But I rewrote that code now as I remember it, and fine-tuned
some details in the game controls, so that it's nicely playable.

For more details see the Dino-Eggs documentation.

Snakes game

The game simulates a bunch of snakes wriggling around on a field, between which
the player needs to run around and pick up items. Of course the player also has
the option of killing the pest.

I do not recall where I got the idea for this game; I guess I saw a similar
game maybe running on an Arcade machine in a movie or TV show. What I liked
best about it is the movement of the snakes. I recall I had some trouble coming
up with a game objective built around those cool snakes.

For more details see the Snakes documentation.

10-Miles-Runner

This is likely one of the first machine language programs I wrote. It's an
extremely simplified variant of the Space Invaders theme. In this game, an
alien space ship (symbol) is toggling between left and right side of the
screen, firing at the player at the bottom of the screen. With each iteration,
the ship is coming a little closer to the player. The player has to evade the
ship's fire by moving left or right, and try hitting the ship with his own
missiles before the ship reaches him at the bottom.

For more details see the 10-Miles-Runner documentation.

I have no idea where the name of this game came from. The code is
short at just under 350 lines of assembly. Apparently this was a direct
translation from an earlier version written in BASIC. The game does not even
use user-defined characters for the objects shown on screen. I just included it
here, as it shows my learning curve, starting with this initial attempt,
(actually followed by a Pac-Man derivate not included here as the
code for generating the maze is missing), then the already much larger Snakes
game, and finally the Dino-Eggs game.

How to run the games

First install the VICE VIC-20 emulator.
My games require 8kB memory extension, so you need to configure VICE once
accordingly: Start xvic, then press F12, select "Machine settings" then check
"Block 1 (8kB)"; quit the menu by pressing Escape. Then save these settings via
F12 -> "Settings management" -> "Save current settings" and quit the emulator.
Note if this is not done, the games will crash, or at best you'll see only a
black screen.

Now you can run the games by simply specifying the name of one of the game
executables (which you find in the prg directory of this repository) on the
xvic command line. If the program does not start automatically after having
been loaded, type RUN into the VICE window and press the RETURN key.