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 gated by LCDC.5, a hit-and-latch condition (RYDY), a window-mode DFF (NOPA), and the window-trigger pulse (MOSU). The trigger chain is PYCO → NUNU → PYNU → NUNY: three capture stages feeding the combinational NUNY. MOSU drives NYXU alongside AVAP and TEVO.

What's where

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. These offsets are identical 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. Increasing SCX widens that budget dot-for-dot, because 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, because 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, since PX is frozen too. The capture fires on the first ROCO edge after TAKA clears, so 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.