The terrain is an array in Data Memory rather than something read back out
of the map, and that is the whole reason this is cheap: the ground under
the lander is one index into 128 bytes, where asking the screen would be a
transfer through the controller every frame.
The column is the world position over eight, masked to the moon's 128. The
surface is that column's row times eight - three turns left of the shift
register, since a row is at most 24 and 192 fits in the low half. The feet
are the lander's top plus its eight pixels.
WHAT DECIDES IS THE SPEED AT THE MOMENT IT ARRIVES. Both of them, and both
have to be gentle: three quarters of a pixel a frame downwards and half of
one sideways. Sideways is the tighter on purpose, because a landing that
was soft downwards and sliding is a lander on its side - which is the
interesting half of the difficulty, and the half the drift bar was blind
about until it existed.
Two fixtures say it works, and they differ only in what was held: one
holds nothing and falls the whole way, the other pulses the thruster six
frames in sixteen and survives. Same terrain, same seed, same keys.
Also: the gamepad did nothing, and the reason is that the four direction
buttons are the D-PAD. A lot of controllers made this century have one
nobody uses - the thumb goes on the stick, which reports as an axis rather
than a button - so a pad that was plugged in and working correctly did
nothing at all. The stick counts as held past halfway now. Untested here,
because there is no controller in this environment and the suite runs
headless; Voyager also says at startup which controllers it can see, so a
pad that still does nothing can be told apart from one nothing noticed.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
A moon has no air, so a sideways drift never stops by itself and stopping
one means cancelling the velocity EXACTLY. That is not hard to do; it is
hard to do blind, which is what it was - a number nothing on the screen
said anything about.
So sprite one is a bar whose width is the drift. It runs right from the
middle of the screen for a rightward one and left for a leftward one, so
which way is as plain as how fast, and stopped is the one state with
nothing drawn at all. The whole of it is a target width written once a
frame; the device stretches one tile into it and the program draws
nothing.
Sideways thrust is one a tick rather than two. At two, the smallest
correction available was twice the size it needed to be and overshooting
was the normal outcome.
WHICH ZERO MEANS NOTHING TURNED OUT TO MATTER. A target width of nought is
the NATURAL width, not an empty sprite - so a bar with no drift in it came
out eight pixels wide, sitting at the middle of the screen, saying
"stopped" in the same shape it says "drifting slightly". What draws
nothing is a SIZE of nought, which is the other zero in the other byte.
Both meanings are deliberate and documented and it still caught me out
inside a week of writing them down.
The check for it earned its place by failing on that before it was found,
which is the best evidence a check can offer.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
A held thruster burns every tick it is held for, which is the whole reason
the pad exists: the console can only say a key went down, so a thruster
driven by it could be pumped and never leaned on.
The burn happens on the same tick gravity does, and for the same reason -
a sixteenth of a pixel is the smallest step this arithmetic takes, and
applied sixty times a second it is an enormous acceleration. On the tick,
thrust and gravity are two numbers whose RATIO is the whole feel of the
thing. Position still moves every frame; only the acceleration is stepped,
and nothing can see that.
Two against gravity's one, so climbing and falling are the same speed.
Three was the first try and it left the moon after about a second of
holding.
If there is a pad the console's arrows are ignored, because under a window
the same keypress reaches both - the pad as a level, the console as a byte
- and a thruster that fired twice for one press would be a mystery to
anybody tuning it. q still quits, since a pad has no letter for it. With
no pad the arrows still burn once a press, which is the most that can be
done down a wire.
And break.sh now rebuilds the disk images as well as the binaries. Half
the things worth breaking here are SplitBit assembly rather than C, and
those live on the fixture disks - so an edit to a .asm file changed
nothing the suite could see, and the tool reported that nothing caught the
break. That is the exact lie it was written to prevent, turning up in a
new place. With the disks rebuilt it catches this one: the lander falls
between the two captures instead of climbing.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
CALL eighth
OUTA 0x36
RET puts A back the way it found it, so the column origin was written the
high byte of the position that had been passed in, and the answer the
subroutine had worked out went nowhere. The fine register was computed
inline with OUTQ and was correct, which is exactly what it looked like
from the outside: smooth scrolling within a cell that never advanced one.
Q is the only register that crosses a RET. Every other answer in this
program already came back in it; this one had been written as if A would
do, and A very nearly does, which is what makes it worth a comment rather
than a fix.
Gravity was Jupiter's. A sixteenth of a pixel per frame per frame is the
smallest step this arithmetic can take and it crossed the screen in a
second, so it is applied one frame in six instead - which divides the pull
by six and costs a byte and a compare. The alternative was a finer unit
for velocity than for position, and that means a shift every time one is
added to the other, twice a frame, for ever.
And the check that catches all this now looks 1.5 million cycles in rather
than twelve. The first number came from assuming a program that saves a
whole screen takes a long time to start; it does not, and by twelve
million the lander had flown seven hundred frames and left the picture.
A capture near the beginning is worth more than a tuned one - there is
less between it and the start that can move.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
A lander over a moon that wraps. Landing, crashing, fuel, cargo and bases
are not here - this rung exists to answer whether it FEELS right, because
everything after it is bookkeeping and none of it is worth building on a
lander that is no fun to fly.
The moon comes for free. The map's column origin is a ring in hardware, so
128 cells is 1024 pixels of surface with no edge and no seam to cross.
Position and velocity are sixteen bit in SIXTEENTHS OF A PIXEL, and the
unit is the design: gravity is a small number added to a velocity and a
velocity is a number added to a position, with no multiply or divide
anywhere. 1024 pixels is 16,384 sixteenths, which is 2^14 - so going all
the way round is an AND with 0x3FFF rather than a comparison, and it is
never wrong at the seam.
The lander never moves sideways. The world scrolls under it and it sits at
the middle of the screen, which is a byte a frame instead of two and is
also what makes the wrap invisible: there is no moment where it jumps.
One key is one burn. The console says which key went down and there is no
such thing as a key coming up, so a thruster cannot be held - a press adds
to the velocity once. That is a property of the machine rather than a
choice this program made, and it reads as pumping the engine.
Four bugs found by running it, all worth keeping written down:
B CANNOT BE A LOOP COUNTER here. Every comparison is an INIB, so the
count was overwritten by whichever bound was last tested and the loop
reset itself for ever. Counters that outlive arithmetic live in memory.
A subroutine answers in Q, and the AND after it read A. The moon came
out flat because it was testing the height against 1 instead of the
random number.
Row minus height, not height minus row: they are equal at the surface,
equal does not borrow, and the surface row has to be ground.
And the shift register has A as its HIGH half. Written the other way,
the view scrolled by 256 cells for every one it should have.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
AnachronautandClaude Opus 5