Window Control

The window subsystem decides when the BG fetcher switches from background to window tiles within a scanline, and triggers a full fetch-pipeline reset at the handoff. It comprises a WY-match decoder with a frame-held latch (REJO), a per-dot WX-match decoder (NUKO), a four-stage trigger chain (PYCO → NUNU → PYNU → NUNY, three capture stages feeding the combinational NUNY) gated by LCDC.5, a hit-and-latch condition (RYDY), a window-mode DFF (NOPA), and the window-trigger pulse (MOSU) that drives NYXU alongside AVAP and TEVO.

The WY-match frame latch (REJO)

wy_match (= LCDC.5 ∧ LY==WY) → SARY (TALU-clocked) → REJO.s, with REJO.r = REPU = mode1 ∨ ppu_reset2

Three properties carry all the behavioural weight:

1. LCDC.5 lives inside the decode. PALO's NAND5 includes ff40_d5, so wy_match is force-low whenever the window is disabled — SARY can only capture 1 during scanlines where LCDC.5 is held high.
2. REPU holds REJO cleared through every VBlank. The set chain only becomes effective at REPU's fall at VBlank exit: REJO sets there if SARY is already 1, else it waits for a later in-frame wy_match capture. Under a fixed WY, a missed set at VBlank exit locks REJO at 0 for the whole frame (LY cannot re-cross WY except via the 153→0 wrap, which lands back inside VBlank).
3. Once set, REJO cannot be disarmed mid-frame. A WY rewrite that drops wy_match cannot clear the NOR latch; only the next VBlank's REPU does.

The capture clock hclk is TALU — one rising edge every 4.000 dots at a fixed phase relative to AVAP (measured anchors at ≈ +0.99, +4.99, +8.99, +12.99, +16.99 … dots; SCX-invariant, since SCX gates SACU, not the TALU distribution). A fresh wy_match↑ therefore reaches REJO within 0.083 dots of combinational delay plus a race to the next TALU edge — uniform in [0, 4.000] dots.

The VBlank WY-rewrite trap (gambatte window_late_wy family, dmg-sim measurement): a test inherits WY=0, enables LCDC.5 mid-VBlank, and rewrites WY=$FF before VBlank exits. The LY 153→0 wrap briefly raises wy_match and SARY captures 1 — but REPU still holds REJO at 0. The WY=$FF write drops wy_match; SARY recaptures 0 on its next TALU edge, ~1.4 ns before mode1 falls. At VBlank exit SARY=0, REJO stays 0, and the window never fires all frame. The whole family's hardware-pass outcomes follow from this SARY/REPU edge order.

Mid-Mode-3 fresh arming (gambatte late_wy_FFto2 sub-cluster at SCX ∈ {0, 2, 3}, dmg-sim measurement): the WY=$02 write lands at LY=2 AVAP+15.495 dots; wy_match rises +0.080 dots later; the next TALU edge at +16.992 captures it and REJO sets at +16.997 — identically across all three SCX variants. Whether the same-scanline cascade fires then depends on whether the one-dot PX==WX window is still open when REJO arms:

In all three, LY=2 renders without a window and LY=3 fires the standard cascade (REJO holds across H-Blank). PX is monotonic within a scanline: once past WX, no same-line re-fire is possible without a WX rewrite. The general rule: the cascade fires same-line only if REJO's set (write dot + 0.083 + TALU race) lands before the PX==WX window closes — and increasing SCX widens that budget dot-for-dot, since the window shifts with the fine-scroll delay while TALU's phase does not.

The 10-step activation sequence

flowchart LR
    PX["PX == WX"] -- "NUKO<br>(gated by REJO)" --> PYCO
    PYCO -- "ROCO↑<br>(needs POKY=1)" --> NUNU
    NUNU -- "MEHE↑" --> PYNU["PYNU<br>window-armed latch"]
    PYNU -- "∧ nopa_n" --> NUNY["NUNY<br>win_start"]
    NUNY --> MOSU["MOSU → NYXU<br>fetch-pipeline reset"]
    NUNY --> PUKU
    PUKU --> RYDY["RYDY<br>window hit"]
    RYDY -- "self-latch" --> PUKU
    PYNU -- "ALET↑" --> NOPA["NOPA<br>window mode"]
    NOPA -- "closes NUNY" --> NUNY
    PORY -- "clears" --> RYDY

With WIN_EN=1, REJO=1, and the fetcher idle (POKY=1):

1. PX reaches WX → NOGY drops → NUKO rises.
2. PYCO captures NUKO on the next ROCO edge.
3. NUNU captures PYCO on the next MEHE edge.
4. PYNU sets (window armed).
5. NUNY = AND2(PYNU, nopa_n) rises → win_start → MOSU rises.
6. PUKU drops; RYDY = NOR3(0,0,0) rises (window hit).
7. NOPA captures PYNU on the next ALET edge (window mode active).
8. NUNY falls → MOSU falls.
9. PUKU stays low — RYDY's own feedback holds it.
10. RYDY remains latched until PORY rises during the restarted BG fetch's cascade — the SUZU falling edge that tells the fetcher to load window tile data.

RYDY's fan-out (each via the shared SYLO/TOMU triple-inversion): SOCY halts CLKPIPE (BG pipeline), TUKU blocks the sprite trigger (sprite pipeline), and SUZU (via TUXY) drives TEVO's window-restart arm.

MOSU drives the same 7-dot pipeline reset as AVAP. Measured on a window-firing scanline (dmg-sim measurement):

MOSU, AVAP, and TEVO each drive NYXU independently through the NOR3 — they do not combine or extend each other.

One interaction: at WX=0 with SCX & 7 = S > 0, the first window pixel lands +S later still (+10.037 at SCX=3). MOSU fires at the POKY floor before the line's fine scroll has been paid, and the post-restart pixel start absorbs it; at WX ≥ 1 the pipe is already running and +7.037 holds at any SCX.

The per-WX timing model

Measured across the full gbmicrotest window sweep (dmg-sim measurement, win[0-15]_a ROMs; all values sim-dots):

• MOSU↑ from AVAP = 6.989 + WX for WX ≥ 1 — one dot per WX step.
• WX=0 fires at the same +6.989 floor: the POKY gate on PYCO's clock blocks anything earlier (PX==0 is true from reset, but no capture clock runs until the first fetch completes).
• Window penalty: +6.000 dots, invariant across WX ∈ [0, 15] — exactly one BG-fetch restart.

The joint (WX, SCX) generalisation: SCX enters only through the PX==WX match dot (PX advances on SACU, which starts (SCX & 7) dots later); the capture chain downstream of NUKO has no SCX dependence, and the WX=0 POKY floor is SCX-invariant:

MOSU↑(WX, SCX) = 6.989                     if WX = 0   // POKY floor, SCX-invariant
               = 6.989 + WX + (SCX & 7)    if WX ≥ 1   // PX-driven

Measured joint anchors: (WX=0, SCX=3) → +6.989 (floor confirmed SCX-invariant); (WX=10, SCX=0) → +16.989; (WX=10, SCX=3) → +19.989 — the formula is exact at all three (dmg-sim measurement, gbmicrotest win{0,10}_scx3_a ROMs, 1,290 firing scanlines each, zero variance).

Sprite-coincident MOSU delay. A sprite fetch overlapping the capture window freezes ROCO (PYCO's capture clock) with the rest of CLKPIPE; NUKO stays high (PX is frozen too), and the capture fires on the first ROCO edge after TAKA clears — the whole cascade shifts 6.000 dots later. Measured at the collision band of Mealybug m3_lcdc_win_map_change (WX=7, sprite X=7 at LY=56–63): MOSU↑ at +19.989 vs the +13.989 control — the 5.515-dot TAKA window plus post-freeze ROCO alignment (dmg-sim measurement, zero variance across 56 collision scanlines).

Whether the sprite cost appears as "MOSU later" or "Mode 3 longer at the tail" depends only on whether TAKA overlaps the capture window; total Mode 3 picks up the same +6 dots either way.

Edge cases

Three self-contained edge-case studies — the right-edge inert zone, multiple window activations on one scanline, and the NUKO/PANY one-dot slip — are collected in Window edge cases.