The cartridge

It's 16384 bytes, and that's it. No loader, no blocks, nothing to wait for: the MSX maps the cartridge at 0x4000-0x7FFF and what's there is what's there, permanently. That makes the byte budget far cleaner than it usually is, because there aren't two different things living at the same address at different moments: one single snapshot of memory, no overlaps.

Where it comes in

The first sixteen bytes are the header the BIOS reads:

41 42 10 40 00 00 00 00 00 00 00 00 00 00 00 00
'A' 'B'  \_ INIT = 0x4010

The two letters are the signature telling the machine there's an executable cartridge here, followed by four two-byte vectors. Only the first is filled in: the other three —the ones that would add statements to BASIC or declare a device— are zero. This cartridge is a game and nothing else.

So the BIOS finishes bringing the machine up and calls 0x4010. And from there it never comes back.

What the startup does

Not much, and all of it deliberate:

• turns interrupts off and fills the BIOS's 512 bytes of hooks with ret, so none of them does anything;
• installs its own on the timer hook, the one the BIOS calls on every screen refresh;
• puts the stack at 0xE400 and clears the 2 KB just above it, which is where the whole game state is going to live;
• writes the graphics chip's registers, silences the sound chip and zeroes video memory;
• and drops into an empty two-byte loop.

That empty loop is the main program. From then on the entire game runs inside the interrupt, fifty or sixty times a second, and the thread that started the machine never does anything again.

Where the state lives

There isn't a single variable in the cartridge, because it's ROM. Everything the game keeps track of lives in the MSX's RAM from 0xE000 upwards, which is why the listing is full of addresses starting 0xE0: they aren't cartridge data, they're variables.

The ones read most:

0xE000	the game state, 0 to 15
0xE001	the step within that state
0xE003	the frame counter, which serves as a clock for everything
0xE005	the latch stopping the interrupt from treading on itself
0xE009	what's being pressed, with the previous frame's at 0xE008
0xE040	the high score, with the score at 0xE043
0xE078	the penguin's four sprites
0xE100	the speed
0xE112	the obstacle slots

The screen, laid out backwards

The game doesn't settle for what the BIOS leaves behind: it writes the graphics chip's eight registers itself, from an eight-byte table it carries. And the layout it ends up with is the opposite of the usual one.

0x0000	the colours
0x1800	the sprite patterns
0x2000	the patterns
0x3800	the name table
0x3B00	the sprite attributes

The normal arrangement has patterns low and colours high; here it's the other way round, and that matters more than it sounds, because drawing the cartridge with the bases swapped doesn't produce an error: it produces a picture, and a convincing one at that, since a colour table read as artwork looks a lot like artwork. That's why this repository's tools take the bases from the registers the game writes, and not from a constant typed in by hand.

The mode is the 256x192 graphics one with unmagnified 16x16 sprites, and the border ends up dark blue.

Three banks, and sixteen flat-colour tiles

In this mode the screen splits into three thirds, each with its own set of 256 tiles. The cartridge sets all three up identically to begin with: the same typeface and the same graphics written three times over, 1792 bytes of pattern and another 1792 of colour in each third.

And there's one detail that pays for itself: the first sixteen tiles of each third are flat colour squares, one per palette entry. They're built by hand, uncompressed, by writing the colour number eight times over. With those, painting the sky or the ice costs no artwork at all: you fill the row with the tile of whatever colour you want.

And everything else is compressed

Not a single graphic sits there raw. There's a 61-byte decompressor reading a format you can follow at a glance —one byte says whether what follows repeats so many times or is so many bytes taken as they come— and four separate doors into it depending on where the destination has to come from, or on whether the picture needs mirroring on the way in.

Of the cartridge's 16 KB, 10437 bytes are data and 5947 are code.

The full layout

Not one byte without an owner. Here's what's there, top to bottom:

0x4000-0x4010	the header
0x4010-0x44DF	startup, interrupt, controls and the state machine
0x44DF-0x4787	video, strings, decompressor and the panel
0x4787-0x4843	the per-stage scenery tables
0x4843-0x4A01	the title banner, video memory access and the map
0x4A01-0x4B01	the map artwork, the route and the ten stages
0x4B01-0x53E1	the penguin, collisions, the track and the obstacles
0x53E1-0x55D9	the bends and the seven horizons
0x55D9-0x5839	the base names, the flags and the on-screen labels
0x5839-0x588A	the title screen and the demo recording
0x588A-0x6BE9	the typeface and all the artwork, compressed
0x6BE9-0x7241	the 92 pieces the obstacles are built from
0x7241-0x7519	the scenery tree
0x7519-0x78C1	the finish, the fish, the speed and the background sprites
0x78C1-0x79C9	the frames of the creature from the holes
0x79C9-0x7B37	the sound player
0x7B37-0x7EB7	the notes, the durations and the sound streams
0x7EB7-0x8000	the padding up to 16 KB