Posts tagged retro

C64 Programming - Invader Fractal #2

:: C64, 6502, retro

In the last post we discovered how to decode 15-bit symmetric invaders into a chacrater set. This time around we will make them fractal!

The invaders from the last post occupy exactly one character each. The aim is to now select an invader at random, and then draw it one size bigger, using other random invaders as its “pixels” like this

Then, the process can be repeated again, drawing an even bigger invader which is formed by drawing even bigger “pixels” which are composed of the size 2 invaders, like this!

C64 Programming - Invader Fractal

:: C64, 6502, retro

I have recently been getting into programming the Commodore 64. It’s lots of fun to work in such a restricted environment, where you have to use various hardware tricks to achieve your goals. Every machine cycle and byte of memory counts!

I have not programmed in 6502 assembler or the C64 before. I am using the popular C64 emulator WinVice and an assembler called DASM. Having messed around a bit and learnt the basics of the instruction set and the hardware/memory layout, I programmed a couple of effects such as the basic text scroller. I have an idea in mind for my first demo, and part of it revolves around the invader fractal. I have implemented this in various langauages (F# and D being the most recent) and figured it would be a nice fit for the C64.


The invader fractal is a very simpe idea, based on the observation that the classic space invaders are symmetrical. Given a 5x5 grid, we can observe that the middle column is static, whilst columns 4 and 5 are columns 1 and 2 flipped around:

This means we can store the information for each row in just 3 bits, multiplied for each row gives us 15 bits to encode an entire invader. 15 bits gives a total of 2^15=32,768 unique invaders. “Real” space invaders are a little bit bigger than 5x5, but we will stick with their smaller cousins.