Module window 

The window: a second tilemap laid over the background.

It has no scroll of its own. Wherever it is placed, it draws from its grid’s top left corner, so moving what it shows means rewriting the grid. Staying put is what suits it to a status bar or a text box while the background scrolls underneath.

Position

The hardware register is the screen column plus seven, so WX of 7 is the left edge. set_position takes the screen column and adds it, which also puts the off-screen values out of reach: WX below 7 starts the window left of the screen and is where the hardware behaves least predictably. Reach for mmio::WX directly if that is wanted.

Writing mid-frame

WX, WY and the enable bit are least glitchy written during VBlank, or during HBlank where they must change mid-frame, which is what the Access here is for. set_map is not among them: it takes effect from the next tile fetched and has nothing to go wrong.

Use hide where the window has to come and go within a frame, not the enable bit. The PPU turns the window on for a frame when WY first matches LY, and on the Game Boy Color clearing the enable bit undoes that: the window then stays away until WY matches again, which for the rest of the frame it cannot. See https://gbdev.io/pandocs/Window.html.

Constants

X_OFFSET

What WX is offset by: WX of 7 is screen column 0.

Functions

enabled

Whether the PPU draws the window at all, from LCDC bit 5.

hide

Take the window off screen without disturbing the enable bit.

map

Which grid the window reads, from LCDC bit 6.

position

Where the window’s top left corner sits on screen, as (x, y).

set_enabled

Set LCDC bit 5.

set_map

Point the window at a grid.

set_position

Put the window’s top left corner at (x, y) on screen.