Exercise Notes

Chapter 1: Building Skyfall

Columns 3 and 5 are catches; 2 and 6 are misses. The calculation subtracts PadX from DropX and accepts results below 3. A drop at column 2 underflows to 255, which the unsigned comparison rejects along with result 3 for column 6.

Chapter 2: Reading Tetro

ApplyGravity copies the piece into the board and stores the full-row mask in ClearMask. It moves the falling overlay away, starts ClearHold, and the render shows the marked rows in white. Gravity pauses while the mask is set. When the one-shot fires, FinishClear removes the rows, updates the totals, clears the mask and spawns the next piece. A blocked spawn enters GameOver.

Chapter 3: The TMS9918 Profile

During frame N, MoveRight stores 125 in state and PlaceMoth copies it to the sprite shadow, setting SpriteDirty. VRAM still contains 124. The GlimCommit at the start of frame N+1 copies the shadow to VRAM, and the VDP then shows x=125.

Chapter 4: Building Rushlight

The fly's centre is (128,96), and dividing each coordinate by eight gives grid cell (16,12). If the lantern occupies that cell, Gather blanks the old tile, increments Score, reduces Pace when it is above its floor, chooses a new lantern cell and renders the replacement.

Chapter 5: Two Displays, One Language

The cellular automaton fits matrix8x8: its world is an 8x8 set of cells that can be redrawn together. The garden fits tms9918: its tile grid preserves the scenery while sprites move independently at pixel coordinates. Persistence and coordinate system determine the choice.