Window Edge Cases

Three edge cases of the window trigger model (window control): the inert right-edge zone, multiple activations on one scanline, and the NUKO/PANY one-dot slip.

The right edge

WX = 167 never fires: PX terminates at 167 on the same dot XUGU fires, and TADY resets PX before any further advance — no NUKO rise is possible.

WX ∈ [~162, 166]: the cascade fires but is observationally inert. Measured at WX=166 (dmg-sim measurement, patched gbmicrotest win0_a ROM; zero variance across 368 firing scanlines, all values sim-dots from AVAP↑):

Mode 3 length is identical to the no-window baseline on the same trace. The MOSU-driven NYXU pulse executes (BG counter resets, steps 1–7 of the sequence all fire, NOPA captures), but XYMU sets ~0.5 dots after MOSU↑ and the combinational POKY-clear switches the pipeline off before any SACU edge can advance the fresh fetch — structurally executed, observationally inert. The +6-dot penalty term simply does not apply once MOSU lands inside the [WODU↑, XYMU↑] closing window.

Right-edge window + sprite collision (WX=166, sprite at OAM X=167, gambatte m2int_wxA6_spxA7 ROM, dmg-sim measurement): on steady-state firing scanlines the window cascade adds nothing (inert as above) and the sprite adds exactly +6.000 dots — the right-edge collapse of the single-sprite penalty (sprite pipeline). The one-off first firing scanline (LCDC.5 just enabled, NOPA still 0) measures +17.0 dots — the standard 11-dot first-sprite penalty plus the 6-dot core, induced by the MOSU↑ NYXU pulse re-firing the fetch on the cascade's maiden activation only.

Multi-activation within one scanline

A mid-Mode-3 LCDC.5 1→0 write raises XOFO and clears PYNU on the same dot; NOPA captures the cleared PYNU one ALET edge later, re-opening NUNY's gate. PYCO and NUNU are not reset by XOFO — they keep propagating any NUKO pulse regardless of LCDC.5. PYNU's nor_latch inputs are level-sensitive, so what happens at a subsequent LCDC.5 0→1 restore depends on NUNU's state at the XOFO↓ edge:

1. NUNU=0 at restore — the match fired long before the disable and has drained. PYNU stays 0; a new activation needs a fresh NUKO rise, which under fixed WX is impossible same-line (PX is monotonic) — the CPU must rewrite WX ahead of PX. This is the structurally-new "multi-fire" case: Mealybug m3_lcdc_win_en_change_multiple (WX rewritten 24 → 120 between the toggles) produces two independent MOSU pulses per scanline (dmg-sim measurement: second activation at NUKO↑ +132.013, PYNU↑ +132.996).
2. NUNU=1 at restore — the match fired inside the disabled window and the restore lands within the ~1-dot NUNU-high pulse (≈ [NUKO↑+1.0, NUKO↑+2.0]). PYNU sets combinationally at XOFO↓ — a deferred completion of the first activation, not a new one. Mealybug m3_lcdc_win_en_change_multiple_wx exercises exactly this: on LY=16 and LY=44 the restore lands inside the NUNU pulse and MOSU fires within a gate delay of XOFO↓; on LY=22 the match falls between the off-pulses and the standard sequence runs untouched (dmg-sim measurement).

The window line counter (VYNO → VUJO ripple) clocks on PYNU falling edges. A scanline with N intra-line LCDC.5 1→0 transitions advances it N+1 times (once per disable, once at the end-of-line ATEJ). Under WX-only rewrites with LCDC.5 held high it advances exactly once per scanline: PYNU's set input saturates (already 1) and NUNY stays closed (NOPA holds), so a second NUNY/MOSU pulse is impossible without a PYNU fall.

The NUKO/PANY slip

NUKO has exactly two netlist consumers: PYCO (the capture chain above) and PANY — the BG drain-detector input (BG pipeline). The second consumer produces the subtlest window behaviour on the DMG.

PANY pulses high for ~1 dot at every tile-boundary drain. A NUKO pulse landing inside that window splits it across RYFA's capture edge: PANY is forced low at NUKO↑, released at NUKO↓, and then falls naturally one dot late — so RYFA captures the late half, and the entire SEKO → TEVO → NYXU chain slips by one dot for that boundary. The slip delays both the BG-shifter parallel load (unconditionally — this branch has no window gating) and the window tile-X increment (only when in_window=1).

• WX-rewrite form (Mealybug m3_wx_4_change family): a mid-Mode-3 WX rewrite that lands PX==WX inside a PANY window produces the documented pair-shuffle pattern at recurring tile boundaries. Measured: three perturbed scanlines per batch with NUKO arriving in the last sub-dot of the PANY window; pulses outside the window do nothing; and the simulated render is bit-identical to the hardware reference — the mechanism is fully on-die (dmg-sim measurement).
• Armed-but-disabled form (no rewrite needed): NUKO is gated by REJO but not by LCDC.5. With the window armed then disabled (REJO=1, LCDC.5=0) and WX ≡ 7 (mod 8) placing PX==WX at a tile boundary, the natural sweep fires the same slip — the BG shifts one pixel right from that column to the line's end, every scanline until VBlank clears REJO, while the window itself never renders (PYNU is held reset, so the wx_clk branch stays silent). Measured end to end: the boundary's PANY/RYFA/SEKO/TEVO/NYXU all slip exactly +1.000 dot (dmg-sim measurement).