Lunar Porter, rung one: it flies

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

Programs/CosmOS/README.md

@@ -726,6 +726,7 @@ from every assembly file in it. Several are old programs written for the bare ma
 | Break | Stops itself twice with SWI osBreak, so that the registers can be seen changing between one stop and the next. |
 | Grid | The first program to use the screen as a screen. Redefines a tile above the font, fills all 128 map rows, and scrolls it diagonally a pixel at a time. |
 | Sprite | Moves a ball across the shell's own text, writing not one byte of the map to do it. It leaves the sprite in the table on the way out, because clearing them is the system's job - see below. |
+| Lander | Lunar Porter, rung one: a lander over a moon that wraps. The map's column origin is a ring in hardware, so 128 cells of it is 1024 pixels of surface with no edge and no seam. Position and velocity are sixteen bit in sixteenths of a pixel, which is what makes going all the way round an AND rather than a comparison. |
 | Depth | Four pillars at four distances and a ball walking past them, behind the near ones and in front of the far ones. The ball is sprite nought and every pillar is numbered after it, so table order puts it in front of all four - what actually decides is the depth buffer, asked a column at a time. |
 | Flip | Draws a whole screen into the bank that is not being shown, waits, and then shows it in one byte out of one port. It writes nothing else at all - not a tile, not a colour - so it does not ask for the screen to be saved, and the line it printed is still there when it comes back. It deliberately does not put the displayed screen back either, because that is the system's to restore: a program that faulted while flipped could not have. |
 | Edit | A line editor. |