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.