2018-05-29 04:01:23 +00:00
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Apple II Entropy Demo (128B)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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BASIC version -- Dave McKellar
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6502 Assembly Version -- Deater (Vince Weaver)
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Webpage
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=======
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http://www.deater.net/weave/vmwprod/entropy_demo/
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Background
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==========
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Back in the day people would challenge themselves to write one or two line
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BASIC programs that did impressive things.
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One of my favorite is "Entropy" by Dave McKellar from Toronto.
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This two-line BASIC program can be found on the Beagle Brother's
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Apple Mechanic Disk.
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I thought it would be interesting to see if I could convert it to 6502
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assembly language.
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The original code looks like this:
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1 ROT=0:FOR I=1 TO 15: READ A,B: POKE A,B: NEXT: DATA
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232,252,233,29,7676,1,7678,4,7679,0,7680,18,7681,63,
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7682,36,7683,36,7684,45,7685,45,7686,54,7687,54,7688,63,
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7689,0
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2 FOR I=1 TO 99: HGR2: FOR E=.08 TO .15 STEP .01:
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FOR Y=4 to 189 STEP 6: FOR X=4 to 278 STEP 6:
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SCALE=(RND(1)<E)*RND(1)*E*20+1: XDRAW 1 AT X,Y:
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NEXT: NEXT: NEXT: NEXT
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Line 1 sets up an Applesoft BASIC shape table. The Apple II lacks any sort
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of sprite or graphics acceleration, but BASIC provides a software vector
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drawing engine. The data statements setup vector art for a simple box shape.
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Line 2 loops across the screen, drawing a box scaled to size 1, but randomly
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scaling it up to size 2 leading to some interesting patterns. It is drawn
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with XDRAW which draws the XOR (inverse) of what's already there. When it
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hits the end of the screen, it starts again, this time erasing things.
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But again randomly boxes of different sizes are drawn leading to interesting
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patterns. After iteration 3, boxes of size 3 are also drawn. And after
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iteration 8 it clears the screen and starts again.
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This sound simple, but it leads to some neat patterns.
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128B 6502 Assembly Demo
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=======================
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I thought it would be neat to see how small I could make this in assembly
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2018-05-31 16:47:28 +00:00
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language. Currently I have it down to 118 bytes (the executable is 122
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2018-05-29 04:01:23 +00:00
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bytes because DOS33 includes size/address filesystem metadata in the file
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itself).
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It started as a direct port of the BASIC version, it probably can be made
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smaller. The assembly code calls directly into the BASIC firmware in various
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places.
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There were some challenges. One is that Applesoft BASIC has no integers:
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all numbers are stored in 5-byte floating point. This made it hard to do
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compact math, though the main issue was dealing with the random numbers.
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As an aside, the Applesoft random number generator is pretty awful and you
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can actually find multiple academic articles written at the time complaining
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about it.
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