A 512 byte intro for Nordlicht 2022 (party theme: “Yarr, Matey! — Celebrate our 10th anniversary with us!”), where it ranked 2nd place in the competition.

This time, it’s 512 bytes uncompressed, i.e. raw code – which means 476 usable bytes on Amiga systems, as 36 bytes are required to have a valid executable file.

Uses the fact that you can easily open windows under AmigaDOS by using special file names like CON:10/20/64/32 (containg window position and size) to create a really coarse pixel grid. I thought all the tiny windows looked funny, too.

After opening, the windows are filled using ANSI escape sequences which can overwrite the window borders under AmigaDOS.

  dc.b	$9b,'43m'  ; background color 3
  dc.b	$9b,'0x'   ; absolute left X = 0
  dc.b	$9b,'0y'   ; absolute top Y = 0
  dc.b	$9b,'4t'   ; 4 rows
  dc.b	$9b,'8u'   ; 8 columns
  dc.b	$9b,'0 p'  ; switch cursor off
  dc.b	$0c        ; ASCII form feed, clears and fills
                   ; with background color

Sadly, these escape sequences are Kickstart 1.x only. The newer console.device implementations require a special command to set the background color used for filling and won’t let you overwrite the window border all the way.

Different skull designs; the “high-res” one with 16×20 windows/pixels wasn’t possible as the OS would crash somewhere around the 150th window.

I did not have space for sound data or to generate waveforms, so I simply used the chipram at address 0 for both channels. With a sample length of 64 words, it sounds like a sawtooth oscillator, and with a length of 32, it kind-of sounds like an organ – what a lucky coincidence!

Typical memory contents under Kickstart 1.3 from position 0: 4 bytes Guru marker (would contain 'HELP' instead of $00000000 when a software failure occurs), 4 bytes exec base and the CPU exception vector table addresses with 4 bytes each.

As waveforms, in action:

On top of that, Monkey Island always works!

Size-coding hack: The skull is drawn by exclusive-or-ing the background color with the current pixel color in the ANSI escape sequence written to all the windows. After the skull is fully drawn, the intro keeps iterating over the skull endlessly, but self-modifies itself to only draw into the first window (bottom-right) with different strings (outputting the parrot and ArrrTW) and applies the exclusive-or to the parrot’s beak now; the character $3c = '<' then is either unchanged (color 0) or gets modified to $3d, yielding a '=' and thus opening and closing the beak (<, =, etc.) – nice arrangement of the character codes there! :)

Since this 512 byte intro is uncompressed, we can directly study the anatomy of the binary:

OS overhead

Amiga executable hunk structure

Code

68000 machine code

Music data

Three bytes per record: position, voice 1 frequency, voice 2 frequency

Strings

Escape sequences, the parrot and window file name format string. exec.library’s RawDoFmt is used to format CON:%d/%d/64/32/ into CON:0/0/64/32/ etc.

Skull bitmap

Half a byte (nibble) per color index of each cell in the 10×8 raster.

The colors are read backwards when drawing, and only the last 70 nibbles are included in the binary.

The skull bitmap starts with ten 0-colored blocks and we cheekily assume that the memory region after our binary is filled with zeroes. This way, we can start reading from 5 bytes (10 nibbles) beyond our data to draw the first ten cells with a color of 0.

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