Module obj 

Objects: the 40 sprites the PPU draws over the background.

Each is four bytes in OAM at 0xFE00, described by OamEntry. Position is offset so that a sprite can sit partly off screen: y of 16 and x of 8 put it at the top left corner, and either axis at zero hides it.

Object tiles always read as Base8000, whatever the background is set to. In Tall the low bit of the index is ignored and the pair is drawn as one 8 by 16 sprite. See https://gbdev.io/pandocs/OAM.html.

What the hardware drops

Only PER_SCANLINE objects are drawn on any one scanline; the rest of that line’s are skipped. Which ones survive is by x on the original Game Boy, lower first, and by OAM index on the Game Boy Color. Nothing reports the loss.

Writing OAM

Writing entries directly costs a blanking window per byte. The usual way is a OamShadow page in work RAM, edited whenever, handed to the hardware in one OamDma once a frame. See https://gbdev.io/pandocs/OAM_DMA_Transfer.html.

Structs

OamDma

The transfer routine, once it is in HRAM and callable.

OamShadow

A page of entries for OamDma to hand over.

Enums

Size

How tall an object is, from LCDC bit 2. It applies to all of them at once.

Constants

DMA_LEN

Bytes OamDma::install needs. Keep in step with the ram_fn below.

ENTRY_COUNT

Objects OAM holds.

PER_SCANLINE

Objects the PPU draws on one scanline.

SPARE

Bytes past the entries that the alignment pays for anyway.

Functions

enabled

Whether the PPU draws objects at all, from LCDC bit 1.

hide

Move an object off screen, which is the only way to stop it being drawn.

set

Write one entry.

set_enabled

Draw objects, or stop.

set_size

Set the size every object is drawn at.

size

The size every object is drawn at.