How MD201512 works

To be honest, there’s nothing going on in MD201512 that hasn’t been well documented online previously but we’ll go through it from top to bottom just because we can…

The picture is in AFLI format, that’s high resolution bitmap but with 8,000 bytes of colour RAM rather than the regular 1,000 and a routine which exploits a “feature” of the vertical smooth scroll, forcing the VIC-II to fetch colour data one every scanline instead of once every eighth. The image was drawn by prof4d (who is no doubt wondering why I’m so obsessed with his work since he was the artist behind the picture “borrowed” for MD201509 as well) and originated on the Spectrum. It was chosen primarily for being “wintery” without having anything specifically Christmassy going on and, more importantly, for featuring a snowman.

Conversion was handled by yet another cheap and cheerful, half-completed Windows-based utility which takes SCR files (which are essentially the raw binary of a Spectrum SCREEN$) and reworks the data into a format the C64 can deal with; said converter completely ignores the bright bit of each attribute cell but I’d rather fix that manually. The application of extra colour during conversion to AFLI was reasonably stubtle – by my standards at least – so there’s more PAL blending going on than anything else and those striped areas like the upper part of the sky should appear as solid colours on a real C64.

After the picture is out of the way the code gets to the scrolling message and this is a variation on the main part of MD201509 where the background colour is split several times across each scanline; what makes it different is that the unrolled loop producing those splits deliberately takes sixty four cycles rather than the sixty three available on a PAL scanline so the result is that the splits are “skewed” and produces that nice, almost plasma-like effect.

Finally there’s the sprites in the lower border and these are just moving around horizontally whilst colour registers changed once per scanline, recycling the colour table already in use for the scroller. There’s literally no time left on each scanline despite the “loop” already being unrolled, have a look for a comment saying “split the sprite colours” for this routine and it’s literally just load and store commands using all three registers with no padding between each line; yes it could have been optimised but there was no necessity to do so since it did what I wanted already!

How Septic works

Because it’s rather simple in the graphical department there aren’t many points of interest to that side of Septic apart from mentioning that the Apple II’s graphics are “backwards” compared to other 8-bits with the lowest used bit of a byte being the leftmost pixel; display RAM is arranged in an “interesting” manner too and it only uses seven bits per byte for pixels as well with the eighth selecting one of two colour pairs for the artifact-powered colour display. I wrote a very cheap and cheerful Windows bitmap to Apple II picture converter a while back which can sort of handle artifacted colour too, but that is currently nowhere near user friendly enough that I’d consider releaseing it for public consumption.

The character set didn’t even need the “specialist” tools though, it was just drawn seven pixels wide with one pixel gaps in reverse order, flipped horizontally in Promotion and shovelled into my regular bitmap to raw binary converter which can, amongst other things, write the data out in rows; the first 64 bytes of the binary file contain the top pixel line of each character in turn, the second 64 are the second line and so on. Look for the partially unrolled bmp_draw in the source code and it should hopefully demonstrate more clearly why having the font in this format is useful.

So having put the minimal graphics code aside we get to the meat of Septic, that one channel music driver. When making sound, the Apple II’s processor has to sit there and click the speaker at regular intervals so one relatively simple-to-follow means of achieving that is a loop along the lines of…

		ldx duration
		ldy pitch
sound_loop	lda speaker
		dey
		bne sound_loop
		dex
		bne sound_loop-$02

…where the LDA makes the speaker click, the Y register is used to time the gap between those clicks and X is a duration counter governing how many times the central loop is executed. The above code will work, but there’s a problem; the higher the pitch, the quicker the central loop runs and the shorter the duration of the note. To make something workable with this method, each possible frequency requires it’s own duration value for one “beat” of the music driver which would in turn be multiplied by the the number of beats that the note is actually going to play for!

So instead the sound loop in Septic was based on a web-found listing that popped up in a discussion at the Atari Age forums which has a better approach. Originally I disassembled the program which was POKEd into memory and meant to be called from BASIC to get some source code, but Anders Carlsson offered up his own, improved version so I migrated my driver over to his instead. That routine, called be_snd_on_play in includes/be_driver.asm, has a central loop which runs for whatever the duration is and within this loop a second timer counts down to zero and, when it hits that value, the speaker is clicked and the timer reset. All my code does after that is wrap a simple, one channel music driver around the sound generator and define a series of labels to make music entry easier. That data looks like this chunk of includes/be_mps.asm:

be_pattn_03	!byte g_2,dur01
		!byte g_1,dur05
		!byte off,dur04
		!byte g_3,dur01
		!byte g_2,dur05
		!byte off,dur04
		!byte g_2,dur01
		!byte g_1,dur05
		!byte off,dur04
		!byte g_3,dur01
		!byte g_2,dur05
		!byte off,dur04
		!byte g_2,dur01
		!byte g_1,dur05
		!byte off,dur04
		!byte g_3,dur01
		!byte g_2,dur05
		!byte off,dur04

		!byte g_2,dur01
		!byte g_1,dur04
		!byte ax2,dur01
		!byte ax1,dur04
		!byte g_3,dur01
		!byte g_2,dur01
		!byte g_2,dur01
		!byte g_1,dur05
		!byte off,dur02
		!byte eod

The label g_3 means a G in octave three, ax1 is an octave one A sharp, off rather unsurprisingly stops the current sound being played, dur04 is a duration of four “beats” (the actual meaning of which can which can be tweaked in the music driver itself) and eod is used to mark the end of the current pattern or track data. Assembling and testing Septic‘s code is more fiddly than the C64 or Atari 8-bit releases I’ve previously pushed to GitHub because the newly assembled binary needs to be manually pushed into a disk image but, in theory at least, it should be possible for someone else to pick up the driver, plug new data into it or possibly expand on what’s there.

There are a metric bucketload of improvements that could be made of course – options like multiple channels or more complex sounds are certainly possible – but I don’t have anywhere near the musical ability to do the required tuning to keep everything sounding right and currently lack the hardware for testing more involved sound engines. More importantly, the “plan” was to provide Apple II folks with something relatively simple to make music with that could perhaps be built upon, so hopefully that slightly tenuous goal was met.

How MD201511 works

Since MD201511 was thrashed out in a hurry this’ll be a relatively short post, but here we go for the sake of completeness and so I can “show off” just a little… not about the Zybex-style starfield of course, that’s old hat and I’m probably the only C64 coder who hasn’t written one previously! It uses a block of thirty two characters in the font, taking 256 bytes of memory so STA $4C00,X can write anywhere within that block, subtracting eight from X moves one character to the left and it wraps around at the lefthand edge. Those characters are tiled across and down the screen with a “random” start position for each line to give some variety and a colour fade is similarly applied. Those logos over the top are nothing special either of course, although each three by three sprite copyright symbol only takes eight sprites because a little work went into designing them with the central sprite empty!

The primary effect is bang in the middle of the screen where, for a mere seven scanlines, the code is vertically splitting the horizontal scroll register to have seven independently moving areas; it doesn’t do the full eight scanlines to avoid the badline like MD201509‘s colour splits don’t. I did try just having the ROL scroller but the “join” between areas looked rough so the static text areas went in to make things look slightly neater. I’ve also been hinting in a less-than-subtle way that something “odd” is going on but, so far at least, nobody has said that they’ve noticed so here’s how MD201511 looks if the border colour is changed…

…and there’s only a “notch” at the left where the borders come in to mask the scrolling rather than on both sides. Since the code is writing to $D016 across the line it can switch from 38 column mode at the start to 40 by the end, meaning that seven scanline area is actually using 39 columns and everything else on screen leaves the lefthand column free so that it all lines up!