Apple ][ 128B Entropy Demo



Sourcecode and Disk Image

You can get the disk image here: entropy.dsk (140k), 31 May 2018)

You can get the sourcecode here in the basic/two-liners directory of my dos33fsprogs github repository:
git clone
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For full details see the size optimization notes in the source code.


Back in the day people would challenge themselves to write one or two line BASIC programs that did impressive things. One of my favorite is "Entropy" by Dave McKellar from Toronto.

This two-line BASIC program can be found on the Beagle Brother's Apple Mechanic Disk.

I thought it would be interesting to see if I could convert it to 6502 assembly language.

The original code

2 FOR I=1 TO 99: HGR2: FOR E=.08 TO .15 STEP .01:
	FOR Y=4 to 189 STEP 6: FOR X=4 to 278 STEP 6:
	SCALE=(RND(1)<E)*RND(1)*E*20+1: XDRAW 1 AT X,Y:

What it Does

Line 1 sets up an Applesoft BASIC shape table. The Apple II lacks any sort of sprite or graphics acceleration, but Applesoft provides a software vector drawing engine. The data statements setup vector art for a simple box shape.

Line 2 loops across the screen, drawing a box scaled to size 1, but randomly scaling it up to size 2 leading to some interesting patterns. It is drawn with XDRAW which draws the XOR (inverse) of what's already there. When it hits the end of the screen, it starts again, this time erasing things. But again randomly boxes of different sizes are drawn leading to interesting patterns. After iteration 4 boxes of size 3 are also drawn. And after iteration 8 it clears the screen and starts again.

This sound simple, but it leads to some neat patterns.

128B 6502 Assembly Demo

I thought it would be neat to see how small I could make this in assembly language. Currently I have it down to 118 bytes (the executable is 122 bytes because DOS33 includes size/address filesystem metadata in the file itself).

It started as a direct port of the BASIC version, it probably can be made smaller. The assembly code calls directly into the BASIC firmware in various places.

There were some challenges. One is that Applesoft BASIC has no integers: all numbers are stored in 5-byte floating point. This made it hard to do compact math, though the main issue was dealing with the random numbers. As an aside, the Applesoft random number generator is pretty awful and you can actually find multiple academic articles written at the time complaining about it.
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