Mode Transitions

Mode transitions are the state-machine edges between Modes 2, 3, 0, and 1. Each is driven by a specific signal event: AVAP↑ starts Mode 3; the WODU → VOGA → WEGO chain sets XYMU to end it; the RUTU → NYPE distribution ends the scanline into Mode 2 or Mode 1; the CATU/ANEL chain re-initialises the Mode 2 machinery at each boundary; MYTA fires at frame end. This chapter walks each edge.

At a glance

Mode 2→3: AVAP↑ resets XYMU directly — a 0.483-dot pulse that is not ALET-rising-aligned (it reacts to the falling edge).
Mode 3→0: WODU → VOGA → WEGO → XYMU in 0.436 dots — Mode 3 ends within the same dot WODU fires; baseline total 173.481 dots.
Scanline end: RUTU → NYPE splits across two TALU edges — POPU (Mode 1) first, MYTA (FRAME_END) and MEDA one period later.
The CATU/ANEL chain turns RUTU into the ATEJ line-end pulse feeding five subsystems — and the LCD-on path bypasses it entirely, which is why the first scanline has no Mode 2 STAT phase.

Mode 2 → Mode 3

AVAP — the scan-complete pulse (OAM scan) — drives XYMU's reset directly: AVAP↑ clears XYMU to Q=0 and Mode 3 begins. AVAP's four netlist consumers:

Cell	Type	Role
ASEN	or2	OR2(ATAR, AVAP) — clears the scan-active latch BESU
NYXU	nor3	NOR3(AVAP, MOSU, TEVO) — BG fetch counter reset (BG pipeline)
POME	nor2	NOR2(AVAP, POFY) — per-scanline synchroniser of the ST sync loop (LCD output)
XYMU	nor_latch	reset pin — Mode 3 starts

Pitfall: AVAP is not ALET-edge-aligned

It rises ~0.018 dots after an ALET falling edge (BYBA's XUPY-clocked capture) and falls ~0.483 dots later when DOBA captures on the subsequent ALET rising edge — the 0.483-dot pulse straddles one ALET rising edge. Snapping AVAP to the nearest ALET rising edge produces half-dot bookkeeping errors; the reactive edge is the falling one.

The transition lands at dot 80 of the scanline (Mode 2's 80-dot decomposition is in OAM scan), and the 7.026-dot startup cascade to the first pixel — zero variance across scanlines, no first-scanline transient — is in the BG pipeline.

Mode 3 → Mode 0

WODU → VOGA → WEGO → XYMU set

Gate	Role	Type	Clock / Trigger
WODU	Mode 0 condition	and2	XENA, XANO (STAT interrupts)
VOGA	H-Blank capture DFF	dffr	ALET rising — primary here
WEGO	XYMU set driver	or2	TOFU, VOGA
XYMU	Rendering-mode latch (active-low Mode 3 indicator)	nor_latch	Set: WEGO; Reset: AVAP

The sequence: PX reaches terminal count (XUGU decode), no sprite match (FEPO=0), WODU = AND2(XENA, XANO) rises — in the ALET-low phase, 0.063 dots after the ALET falling edge. The same-dot ALET rising edge, 0.435 dots after WODU↑, latches WODU into VOGA; WEGO rises 315 ps later and XYMU sets simultaneously (dmg-sim measurement):

Event	Δ from WODU↑ (dots)	Δ (ps)
WODU↑	0.000	0
VOGA↑	+0.435	+106,107
WEGO↑	+0.436	+106,422
XYMU↑ (Mode 3 ends)	+0.436	+106,422

Mode 3 ends within the same dot WODU fires — a half-dot pipeline delay set by WODU's ALET-low-phase rise and VOGA's same-dot capture. The baseline duration decomposes exactly: AVAP → WODU = 173.045 dots (167 pixels through the pipe after the 7-dot startup), WODU → XYMU = 0.436, total 173.481 dots at SCX=0 with no sprites or window.

Mode 0 → Mode 2 / Mode 1

The scanline ends through LINE_END (RUTU) and its NYPE redistribution (line counters carries the cell detail):

LX reaches 113; SANU fires.
RUTU captures on the SONO edge and holds for one full TALU cycle.
LY increments (MUWY toggles on RUTU's rise).
NYPE captures RUTU on the next TALU rising edge — half an M-cycle after RUTU's capture. NYPE's Q clocks POPU; NYPE's Q_n clocks MYTA and MEDA — splitting the distribution across two TALU edges one period apart.
POPU captures XYVO on NYPE's rise: LY < 144 → Mode 2 begins; LY ≥ 144 → Mode 1 (VBlank).
One TALU period later, MYTA captures NOKO (FRAME_END) and MEDA captures NERU (LY=0). MYTA's later edge is the source of the LYC=153 race window (STAT interrupts).

In parallel, RUTU drives the scan-counter reset chain below.

The CATU/ANEL chain

The OAM scan counter is reset at each scanline boundary by a two-DFF-plus-combinational chain transforming RUTU into the ANOM reset pulse:

flowchart LR
    RUTU["RUTU<br>LINE_END"] -- "∧ NOT(vblank)" --> CATU["CATU<br>dffr · XUPY"]
    CATU -- "Q sets" --> BESU["BESU<br>scan-active latch"]
    CATU -- "Q" --> ANEL["ANEL<br>dffr · NOT(XUPY)"]
    CATU -- "ABAF = NOT(CATU)" --> BYHA["BYHA<br>oa21"]
    ANEL --> BYHA
    BYHA --> ATEJ["ATEJ<br>line-end pulse"]
    ATEJ --> ANOM["ANOM<br>scan-counter reset"]
    ATEJ --> TADY["TADY<br>PX reset"]
    ATEJ --> SECA["SECA<br>TAKA set"]
    ATEJ --> AZYB["AZYB<br>slot-counter reset"]
    ATEJ --> ABAK["ABAK<br>store resets"]

Stage	Cell	Type	Clock	Inputs	Drives
1	CATU	dffr	XUPY	D = AND2(SELA, ALES); reset = ABEZ	ABAF, ANEL, BESU
2	ANEL	dffr	AWOH = NOT(XUPY)	D = CATU; reset = ABEZ	BYHA
3	BYHA	oa21	—	ABAF (= NOT(CATU)), ANEL, ABEZ	ATEJ
4	ATEJ	not_x2	—	BYHA	ANOM + 5 more consumers
5	ANOM	nor2	—	ATEJ, ATAR	scan counter resets, BALU

Only CATU and ANEL are clocked — one half-XUPY pipeline stage between them; the rest is combinational. CATU's data decomposes to RUTU AND NOT(vblank) (SELA is RUTU through two buffers; ALES = NOT(XYVO)) — the chain fires only outside VBlank. Both DFFs are held in reset while the LCD is off.

The steady-state sequence: RUTU rises mid-XUPY-cycle; CATU captures it one dot later (243,059 ps = 0.996 dots, picosecond-identical across 431 boundaries — dmg-sim measurement); ANEL captures CATU half an XUPY cycle after that; BYHA/ATEJ/ANOM pulse the counter reset between the two captures. Total: CATU's capture asserts the reset on its own edge (via the ABAF arm), ANEL's capture releases it (via the second BYHA arm), and the counter ticks 0→1 on the next XUPY rising — one XUPY cycle after CATU.

CATU's clk-to-Q is directly measured: 988 ps after the XUPY edge, zero variance across 860 scanline boundaries (dmg-sim measurement) — slightly slower than BYBA's 902 ps, matching the two cells' relative load parameters.

ATEJ's other consumers matter. Beyond ANOM, the line-end pulse drives TADY (the PX-counter reset — BG pipeline), SECA (the TAKA set net — the H-Blank TAKA re-assert in the sprite pipeline), AZYB (the sprite-store slot-counter reset), and ABAK (the per-slot store resets). One pulse, five subsystems.

ANOM's BALU arm. While the reset pulse is asserted, BALU = NOT(ANOM) goes high and forces BEBU=1 — masking AVAP during the boundary transition. The full interaction lives with the AVAP detector in OAM scan.

The LCD-on bypass

When LCDC.7 goes 0→1, the first scanline enters its scan by a different mechanism — CATU and ANEL are not involved:

During LCD-off, ATAR=1 holds ANOM=0 (counter in reset) and ABEZ=0 holds CATU/ANEL at 0; RUTU is 0.
At the LCD-on edge, ATAR falls and ANOM releases immediately — combinationally, no chain propagation. CATU's data is still 0 (no RUTU pulse exists yet), so the chain stays silent.
The counter simply starts advancing on the next XUPY edge.

Consequences on the first post-LCD-on scanline:

BESU never sets (its set input is CATU.Q) — the STAT mode bits read Mode 0 through the nominal scan window and the Mode 2 interrupt does not fire;
the scan itself runs — counter 0→39, FETO, BYBA, AVAP, Mode 3 all normal;
the sprite store is never populated (CARE stays unarmed).

From the second scanline on the full chain operates — RUTU fires at the first LINE_END and everything proceeds steady-state. The first-frame observability consequences are catalogued in LCD-on → first WODU.

Mode 1 → Mode 2 (frame start)

LY reaches 153: NOKO=1; MYTA captures it on NYPE's falling-edge distribution.
LAMA = NOR2(LYHA, MYTA) goes low — LY resets to 0.
NERU (the LY=0 NOR8) rises.
MEDA captures NERU on the same NYPE_n edge family — driving only the LCD vertical-sync pad (LCD output); MEDA touches no scan-state or mode-control logic.
The first visible line's OAM scan begins through the normal RUTU/CATU machinery at the next scanline boundary.

The LY=153→0 wrap's CPU-visible fine structure ("LY reads 153 for only a few dots") is timed edge-by-edge in CPU-visible timing at mode boundaries.