Trait PpuBus 

pub trait PpuBus {
    // Required methods
    fn ppu_read(&mut self, addr: u16) -> u8;
    fn ppu_write(&mut self, addr: u16, value: u8);

    // Provided methods
    fn ppu_read_sprite(&mut self, addr: u16) -> u8 { ... }
    fn chr_phys(&self, _addr: u16) -> Option<u32> { ... }
    fn chr_reads_are_pure(&self) -> bool { ... }
    fn nametable_unfolded(&self) -> bool { ... }
    fn peek_nametable(&mut self, _addr: u16) -> Option<u8> { ... }
    fn write_nametable(&mut self, _addr: u16, _value: u8) -> bool { ... }
    fn peek_ex_attribute(&mut self, _v: u16) -> Option<ExAttribute> { ... }
    fn bg_split_state(
        &mut self,
        _scanline_y: u16,
        _coarse_x: u16,
    ) -> Option<BgSplitState> { ... }
    fn notify_a12(&mut self, _level: bool) { ... }
    fn notify_scanline_start(&mut self) { ... }
    fn notify_vblank(&mut self) { ... }
    fn nametable_address(&self, addr: u16) -> u16 { ... }
}

Bus interface the PPU sees.

In production the lockstep bus in rustynes-core routes:

• CHR reads/writes ($0000-$1FFF) → mapper.
• Nametable reads/writes ($2000-$3EFF) → PPU’s own CIRAM, with the mapper-supplied mirroring offset via PpuBus::nametable_address.
• A12 transitions → mapper.

In tests, a small in-memory PpuBus impl owns 8 KiB of CHR-RAM and a dummy mirroring map.

Required Methods

fn ppu_read(&mut self, addr: u16) -> u8

Read a byte at addr. The PPU passes addresses in the full $0000-$3FFF window; the bus is responsible for routing CHR ($0000-$1FFF) and nametables ($2000-$3EFF) appropriately.

fn ppu_write(&mut self, addr: u16, value: u8)

Write a byte at addr.

Provided Methods

fn ppu_read_sprite(&mut self, addr: u16) -> u8

Read a byte from the pattern-table window ($0000-$1FFF) on behalf of a sprite tile fetch. MMC5 in 8x16 sprite mode uses a different CHR bank set ($5120-$5127) for sprite fetches than for BG; other mappers default to the same path as Self::ppu_read.

fn chr_phys(&self, _addr: u16) -> Option<u32>

HD-pack tile identity: the ABSOLUTE post-banking offset into CHR-ROM for a pattern-space address (Some(offset)), or None when CHR is RAM (or the mapper doesn’t expose it). tile_index = offset / 16 keys Mesen CHR-ROM <tile> replacements; None routes to the CHR-RAM content-hash path. Default None; only consulted on the HD-pack fetch path.

fn chr_reads_are_pure(&self) -> bool

v3.1.0 (T-SPRITE-LIMIT) — whether a CHR read through Self::ppu_read / Self::ppu_read_sprite has no effect beyond returning the byte. The “disable sprite limit” option makes extra, display-only pattern reads, and only where this is true, so the option can never change emulation. Default true; the core forwards the mapper’s answer (Mapper::chr_reads_are_pure).

fn nametable_unfolded(&self) -> bool

Whether $3000-$3EFF is independent cartridge RAM rather than a mirror of $2000-$2EFF (the mapper’s nametable_unfolded). When it is, the PPU passes those addresses unfolded to Self::peek_nametable and Self::write_nametable. Default: false.

fn peek_nametable(&mut self, _addr: u16) -> Option<u8>

Optionally synthesize a nametable byte for addr ($2000-$3EFF).

When the bus returns Some(v), the PPU uses v directly and skips its CIRAM read. MMC5 uses this for fill mode and ExRAM-as-nametable. Default returns None.

fn write_nametable(&mut self, _addr: u16, _value: u8) -> bool

Optionally absorb a nametable write directly into mapper storage.

Returns true if consumed; PPU then skips its CIRAM write. Default returns false.

fn peek_ex_attribute(&mut self, _v: u16) -> Option<ExAttribute>

Optional per-tile extended attribute + CHR-bank override for the BG tile currently being fetched (loopy-v passed in v). MMC5 in $5104 mode 01 (ExGrafix) returns Some(...) here. Default returns None.

fn bg_split_state( &mut self, _scanline_y: u16, _coarse_x: u16, ) -> Option<BgSplitState>

Optional vertical split-screen override for the BG fetch group about to start at (scanline_y, coarse_x). MMC5 with split enabled ($5200 bit 7) returns Some(...) here for tile columns that fall within the alt region. Default returns None.

fn notify_a12(&mut self, _level: bool)

Notification of a PPU A12 line transition (rising or falling). The PPU calls this on every transition, with level = true for high. MMC3 / MMC5 use this internally for IRQ counter clocking.

fn notify_scanline_start(&mut self)

Notification that the PPU is starting a new rendered scanline (visible or pre-render). MMC5 uses this to drive its scanline IRQ counter. Default no-op.

fn notify_vblank(&mut self)

Notification that the PPU has entered vertical blank. MMC5 uses this to clear its “in-frame” flag. Default no-op.

fn nametable_address(&self, addr: u16) -> u16

Resolve a logical nametable address in $2000-$3EFF to a CIRAM offset in 0..0x800 under the mapper’s currently-selected mirroring.

Default impl uses a vertical-mirroring fallback so this trait remains drop-in for ad-hoc test buses; the lockstep bus in rustynes-core overrides this to delegate to Mapper::nametable_address.

Dyn Compatibility

This trait is dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementors