231. Ppu status flag constants

File to update

emulator/ppu/ppu.py

Constants to add

VBLANK_STARTED      (Will be used in next test)
SPRITE_ZERO_HIT     (Will be used later...)
SPRITE_OVERFLOW     (Will be used later...)

Why this step exists

PPUSTATUS is the CPU-readable PPU status register at $2002. It is one byte, and its important hardware status flags live in the high bits:

bit:  7 6 5 4 3 2 1 0
      V S O - - - - -

Meaning

V = VBlank started
S = Sprite 0 hit
O = Sprite overflow

Why constants

Magic numbers such as 0x80, 0x40, and 0x20 work mechanically, but they hide the hardware meaning. Named constants make later PPU behavior easier to read:

self.status &= ~VBLANK_STARTED

is clearer than:

self.status &= ~0x80

Important scope

This step only names the status bits. It does not implement sprite 0 hit or sprite overflow behavior yet. Those require rendering, sprite evaluation, OAM, and scanline timing.

Suggested implementation pseudocode

VBLANK_STARTED = 1 << 7
SPRITE_ZERO_HIT = 1 << 6
SPRITE_OVERFLOW = 1 << 5

Why 1 << 7:

Shifting 1 left by 7 positions creates a byte with only bit 7 set

1 << 7 == 0b1000_0000 == 0x80