Let a meteor move independently

A meteor should keep approaching whether the player moves, hesitates or holds both keys. Give it its own position, then update that position on the same clock as the ship.

This checkpoint adds meteor_x, meteor_y and active. Restart puts the meteor at (116,24) and sets active to 1. The ship still starts at (116,160). They happen to share an X coordinate; they do not share a position variable.

At this stage contact does nothing. Leave the ship still and let the meteor pass through it. Move on a second run and check that neither the meteor’s X nor its downward speed follows you. R restarts both objects.

Update the state, then the picture

After moving the ship, the loop checks active. Zero means there is no meteor to update. For an active meteor it calls meteor to XOR away the old image, adds three to meteor_y, and either retires it or draws at the new position.

meteor supplies its own X/Y, meteor_sprites and twelve rows to the drawing helper. The table still reserves sixteen rows per shift block, so the existing 64-byte offset calculation remains valid. The companion file now includes both artworks.

A Y coordinate of 174 or greater retires the object: active becomes zero, phase becomes 3, and CLEAR SPACE appears. We remove it before drawing there. That keeps the whole twelve-row image within the play area above the control line.

Calculate an arrival

The meteor travels from y=24 towards retirement at y=174: a distance of 150 pixels. At three pixels per update, that is fifty updates. At 25 updates per second, it lasts two seconds after it is first drawn.

To test a different speed, replace add a,3 immediately after ld a,(meteor_y) with add a,2. Do not change the ship’s ld e,2. Predict seventy-five updates, or three seconds, then run. The two speed values belong to different objects.

The counter and active flag decide what exists. A leftover picture is a drawing error, not proof that the object is still active. Conversely, clearing pixels without clearing active would leave an invisible object that keeps updating. Those two mistakes will matter once contact can end the run.