312 lines
13 KiB
HTML
312 lines
13 KiB
HTML
<html>
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<head>
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<title></title>
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</head>
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<body>
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<h2>Flame Demo (64-byte assembly)</h2>
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The first attempt was to see if we could get some assembly language going,
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so a good thing to try is my <a href="../apple2_fire/">64B Flame Demo</a>.
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<center>
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<img src="flame.png">
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</center>
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The first attempt uses the traditional method of entering assembly
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language from BASIC via Applesoft: you have data staements holding the
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raw 8-bit data for the 6502 Machine Language, you poke them to a free
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area of memory (often $300 (which is 768 in decimal)) and then you call
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to it.<br><br>
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Note this code is longer than 256 chars long which means while it is
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a valid Applesoft program, you can't type it in on an actual Apple II
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as it will overflow the keyboard input buffer. You can however tokenize
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it and load from disk, which must be what the Apple II bot does.
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<br><br>
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<pre>
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1 FOR X=768TO832:READ A:POKE X,A:NEXT:CALL 768:DATA 44,80,192,44,82,192,32,112,252,169,255,145,42,136,16,251,169,22,133,37,32,34,252,160,39,104,240,5,10,240,4,144,2,73,29,72,48,10,177,40,41,7,170,189,57,3,145,40,136,16,230,198,37,16,221,48,205,0,187,0,170,0,153,0,221
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</pre>
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This worked, but then 4am got me thinking about more compact ways to try
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and fit larger programs. I looked into the way that Sellam Abraham did things
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with his "Mesmerizer Exorciser" Kansasfest 2020 Hackfest entry, but it turns
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out that is a bit too big to fit in a tweet.<br><br>
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My fist attempt loads a short assembly language program that then decodes
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a string. The string is machine code encoded with two values, the first
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shifted left by three then xored with the second to get the value.
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<br><br>
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Why not just include raw 8-bit values? For one on twitter we're going
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to assume you can only pass in ASCII text, so that limits you to 7-bit values.
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In addition you can't send control characters (the first 32 values) and
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on Apple II+ you can't send lowercase, further reducing things. In the
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end you are sort of limited to around 6-bits (64 values) and have to
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construct the machine code from that.
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<br><br>
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Also note that by now I had remembered that Applesoft ignores all spaces
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so you can leave them out to get more room.
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<pre>
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1 FOR X=768TO789:READ A:POKE X,A:NEXT:CALL768:DATA162,0,189,106,8,48,12,10,10,10,93,171,8,157,0,12,232,208,239,76,0,12
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9 "MBPMBPLFW]WZMYJW]JXLLLW\LEVHIVHZHAKANI^MMH]_OIZMYJTPLJSNQH_H]H[HSD@@DB@@@DAGAB@@CAFEE@BD@G@@EB@D@BAE@@BA@AGBEADA@@@FFE@E@E@C@B@A@E"+
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</pre>
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<hr>
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<h2>Circles Demo (101-byte assembly)</h2>
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<center>
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<img src="flame.png">
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</center>
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This is an unreleased demo of mine based on part of Hellmood's "Memories"
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Demo<br><br>
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In this case we are using 4+4 bit encoding, so it takes two bytes to represent
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each byte. This is a bit wasteful but it's a lot easier to write a 4+4
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decoder than a more efficient 6+2 one. Applesoft in particular makes
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this really difficult as it has no shift or bitwise logic functions.
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It's not easy to write decoders in assembly either, so multiplication
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in Applesoft it is. The A=(A-INT(A)) code is the way to get a
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remainder (modulus) in Applesoft.
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<br><br>
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Note in this case I abandoned using a string. It has some problems,
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the worst being that you can't include a quote character (ASCII 34).
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Using a REM remark (comment) statement takes slightly more room but
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everything after it is ignored.
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<br><br>
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You might ask how we load values from a REM statement. In Applseoft the
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whole program is loaded into memory, REM statements and all. Programs
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are loaded at $800 in tokenized format, but it's fairly easy to poke
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around and find where lines get loaded in memory and then use PEEK to load
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the values in.
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<pre>
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2 GR:FOR X=0TO100:A=(PEEK(2245+X/2)-32)/4:IFJTHENA=(A-INT(A))*4
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3 J=NOTJ:POKE768+X,(PEEK(2144+X)-32)*4+A:NEXT:CALL768:REM+4PY_H&A\RH%A\B&N3 ""?2 29_*'222A^JN8 A,I\B( ^I\( ^I^B( ^I\( ^Y\B$SY\R$P,N)('&&%%$$##""!!!! 5 ="""". ! &.)+)!.*#!'$!( ' . !, ) "."!!+&"""')#)$ $,
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</pre>
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<hr>
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<h2>Autumn (117 byte demo)</h2>
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<center>
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<img src="autumn.png">
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</center>
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This is a slightly shorter version of my <a href="../seasons_demo/">
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Seasons</a> demo (without color cycling) that was entered in the Outline
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Online 2020 Demo Party.
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<br><br>
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This is using 6+2 coding, with the 6 bit values (with 32 added to get the
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bits up in visible ASCII) first followed by the packed remaining 2 bits
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in chunks of three. Qkumba figured out the crazy use of the exponent
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to handle the proper shifting of the 2-bit values.
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<br><br>
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Because it uses a lot
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of multiplies and shifts, this is fairly slow to decode which is why
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it has a directive to skip 9 seconds. It should skip more but there
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were only 4 free bytes in the tweet so we couldn't delay more.
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<br><br>
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<pre>
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{B9}
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1REM(V\I\I\B.Y\A\FY\R@:A\9\&B9\A\F9\R@:A\9\9]9]9]9]L'Y^T Y^I\:@A
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\D Y\.J Y\A\J Y\A\I^")^1^A^*!" JI\,HI\*\TG([]I\I\I\(5]3 P")90'6"F)=8KBI.)H%1@&1F1WA6%=:S50@"E,\
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2FORI=0TO116:C=INT((PEEK(2171+I/3)-32)/4^(I-INT(I/3)*3)):POKE768+I,((PEEK(2054+I)-32)*4)+C-INT(C/4)*4:NEXT:CALL768
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</pre>
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<h2>Spaceship -- Applesoft Shapetable</h2>
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<center>
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<img src="ship.png">
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</center>
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After managing to fit Autumn in, I gave up on assembly language and went
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back to coding in Applesoft. One thing Applesoft has going for it is
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"Shapetables" which are a software vector drawing library included
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in the Applesoft BASIC ROMs.
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<br><br>
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To use shapetables, you map out the vectors. You can only do
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draw+move UP/DOWN/LEFT/RIGHT and pen-up move (no-draw) UP/DOWN/LEFT/RIGHT.
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Each operating is three bits, you can pack two or sometimes three directives
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per byte and there's a header with the number of shapes and the offset
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of the shapes.
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<br><br>
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Typically you'd POKE the shapetable into memory via data
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statements (there's no easy way to
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load them, though oddly there is a dedicated command to loading them from
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casette tape). Poking wastes a lot of space, so I thought maybe we
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can stick it in a REM statement like before. There's a trick here, you
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need to have your shapetable be valid ASCII. To do that you need the second
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value in each byte to not be a not-draw instruction, and also you can't
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have a third value either.
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<br><br>
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Once you have your shapes, you can DRAW them at a location, or XDRAW
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to xor draw (which makes it easier to draw and erase). There is ROT
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to rotate and SCALE to scale.
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<br><br>
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This example like 2 tells Applesoft where the shapetable is by POKEing
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in the locations to the proper zero page address. The shape table
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in the REM depends on being on line 5, because we depend on the
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actual layout of the BASIC program.
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<br><br>
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This points to address $814. If you look there in memory you find the values
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<pre>29 08 05 00 B2 37 ...</pre>
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The 29 08 (little endian) is actually part of the linked list of the BASIC
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program in memory pointing to the next line. The 05 00 is the line number.
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And B2 is the token for REM followed by the data. By pointing at the
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29 we say we have 29 shapes, but that doesn't matter as we only use
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1. The next value is ignored. The next is the offset to
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the first shape (from the beginning). Since this is 5 it skips the REM
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and goes to the data.
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<br><br>
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The rest is just regular Applesoft BASIC, it draws some random stars,
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moves the spaceship, and draws some flames and then repeats.
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<br><br>
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<pre>
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2POKE232,20:POKE233,8
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5REM7:'%%,5..>'<29'
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6HGR2:FOR X=1 TO 100:HCOLOR=7:HPLOT RND(1)*280,RND(1)*192:NEXT
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7SCALE=5:FORR=0TO16:ROT=R:GOSUB9:GOSUB9:NEXT:FOR X=100TO270:GOSUB9
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8HCOLOR=5:HPLOTX-30,86+RND(1)*16TOX-10,91:X1=X+SQR(X/25):GOSUB9:X=X1:NEXT:GOTO6
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9XDRAW1ATX,91:RETURN
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</pre>
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<h2>Nyan Cat -- Shape Tables and Page Flipping</h2>
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<center>
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<img src="nyan.png">
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</center>
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I wanted to do something wiht page-flipping and shape tables, possibly
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the only two graphics features the Apple II can do better than other
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8-bit computers. I was thinking something like my
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<a href="../shapetable_party/">Shapetable Party</a> demo.<br><br>
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In the end trying to make complex ASCII shape tables was too much.
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But then oddly I had a dream where I made a HGR version of Nyan Cat.
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So here it is, a program that brought a lot of joy.<br><br>
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Not much exciting about it, the poptart-cat shapetable is done more or
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less like in the previous example. It uses HGR:HGR2 to clear both
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graphics pages and leaves things on PAGE2. The rainbow and cat are
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first drawn on PAGE2 (the routine at line 8 draws them). Then
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they are drawn again at a slightly different offset on PAGE1.
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The POKE 230,32 tells the Applesoft routines to draw to PAGE1
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($2000) instead of PAGE2 ($4000). Ideally we'd switch back to page1
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so we could see both being drawn, in fact when watching it there's
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an annoying pause while PAGE1 is drawn and you can't see it. However
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I was out of characters to do that.
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<br><br>
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As some have pointed out, I used tail-call optimization here, where instead
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of GOSUB9 one lsat time in line 8 I fall through to line 9 and let the
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return from there return from the GOSUB to line 8. This is something
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I do a lot in 6502 assembly and it's fun to do it in BASIC too.
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<br><br>
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The actual program is like 7 which just flips pages rapidly.
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The V=0 call was added to slow the animation a bit, it roughly is as slow
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as the GOTO7. I tried putting slower things (like SQR() square root)
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to slow it even more, but you have to put things twice or the animation
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is unbalanced and there wasn't enough room for two.
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<br><br>
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It was a bit more of a pain than you think to get the HGR colors to
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plot in rainbow order. Also in size coding like this you run into issues
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where it takes less characters to have things to the upper left part
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of the screen as the coordinates can be less than 3 digits.
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<br><br>
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<pre>
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2POKE232,20:POKE233,8
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5REM$,.,6>???$$--5
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6ROT=0:SCALE=5:P=49236:HGR:HGR2:GOSUB8:Q=1:POKE230,32:GOSUB8
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7POKEP+1,0:V=0:POKEP,0:GOTO7
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8C=5:Y=80:XDRAW1AT134,102+Q*2:GOSUB9:C=1:GOSUB9:C=6:GOSUB9:C=2
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9HCOLOR=C:FORZ=YTOY+5:FORX=0TO13:Q=NOTQ:HPLOTX*8,Z+QTOX*8+7,Z+Q:NEXTX,Z:Y=Z:RETURN
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</pre>
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<h2>Double Hires Pattern</h2>
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<center>
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<img src="dhgr.png">
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</center>
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I was interested in seeing if I could get some better colors going.
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I have done demos that do
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<a href="../megademo/">Vapor Lock<a> and
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<a href="../lores-escape">mid-screen race-the beam mode switching</a>
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but I was pretty sure the Linux-based emulator used by the Apple II bot
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can't do that type of cycle-counted effects.<br><br>
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The emulator did seem to be emulating Apple IIe, which opened things
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up for double hi-res graphics. Those are tricky to do and you can't
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easily program them from Applesoft. I actually tried at first but had
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some issues fitting in a reasonable amount of room, so assembly language
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it is.<br><br>
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This was just a first test but it made a neat pattern. I had a lot of trouble
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just getting horizontal lines drawn. To do that in double-hires you
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have to draw an increasingly rotated-by-one bit version of the color
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across 4 bytes on two different bank-switched graphics pages, a huge pain.
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<br><br>
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<pre>
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{B10}
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1REM(X\(V\C7PC#PC PJ(AYJ A_J(Y_I_(* Y_I_RPT[3$ H H ($]H)C5P(1 C5P(1 B$\8I_D)R@)_8 XP)%%1&9Z)OF!S+ !7 ' #1GA#
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2FORI=0TO77:C=INT((PEEK(2132+I/3)-32)/4^(I-INT(I/3)*3)):POKE768+I,((PEEK(2054+I)-32)*4)+C-INT(C/4)*4:NEXT:CALL768
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</pre>
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<h2>Double Hi-res Rasterbars (126 byte assembly)</h2>
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<center>
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<img src="rasterbars.png">
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</center>
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My goal was to get two colors of double hi-res rasterbars going.
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It turns out this is much harder than it sounds. I did get one
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rasterbar moving fairly easily, but getting a second independent bar
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was trouble. Even then it was 136 bytes which was too big for the loader.
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<br><br>
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So I size optimized the code like crazy, including using the "BIT" trick
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to avoid a jump and various other things and got it down to 126 bytes.
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The pattern ended up being a bit nonsensical, as I had to use an EOR (xor)
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to calculate the location of the second bar, but there is more than one
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color.
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<br><br>
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Then I optimized down the BASIC loader. It's still qkumba's 6+2 loader
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but I found some parenthesis I could remove, did some algebra and orders-of-operation
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changes to remove some more parenthesis, and did some tricks with
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integer variables. Applesoft does everything in floating point, but
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you can specify 16-bit integer values which will truncate, which removes
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the need for an INT (though qkumba later noticed that might not be
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necessary).
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<br><br>
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<pre>
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1REM(X\(V\C7PC#PC P1YJ (+ J!(+ F,!R0L"HR+HBC* RTYA_F2$JJ!A_I_R!D 2 * &9_JO= A_F2Y_H H ($]H)C5P(; C5P(; B$\:JQ_$R8I_D)R@)_8 $(, W[_XP)%%:0'TP!((Z !!=5!V-AY-4"0P%T!T0@QT9X0.Y
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2FORI=768TO894:C%=(PEEK(1924+I/3)-32)/4^(I-INT(I/3)*3):POKEI,C%+4*(PEEK(1286+I)-32-INT(C%/4)):NEXT:CALL768
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</pre>
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<h2>The Future</h2>
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So am I done for now?<br><br>
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There are still a few tricks remaining. There is some deep magic you can
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do if you call into the Applesoft interpreter direct from assembly language.
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Lots of articles on this from the 1980s.<br><br>
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Applesoft supports calls like USR and & besides CALL to call into
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assembly language. The most useful is & which just does a plain
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jump to $3F5. If you load your assembly properly you can use this to
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save another few characters by getting rid of the CALL768. (Also if
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you load your code at $3F5 you can watch it being decoded as it writes
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to the $400 text page).
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<br><br>
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So maybe I will come back to this again, but for now there's other
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projects I really need to finish. It's been fun!
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<hr>
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<a href="../demos/">Back to Apple II Demos</a>
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</body>
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</html>
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