Startup and Fine Scroll

Every scanline's Mode 3 opens with the same deterministic cascade from the scan-complete pulse (AVAP) to the first pixel clock — and fine scroll rides on it, holding the pixel clock off for exactly SCX & 7 extra dots. This page covers the fine-scroll counter, the startup pipeline stage by stage, and the measurements that pin both.

The fine scroll counter

A 3-bit ripple counter that discards SCX & 7 pixels at Mode 3 startup by delaying SACU's enable. The counter runs at CLKPIPE cadence (via ROXO = NOT(SEGU)) while ROXY holds SACU high; a two-stage match pipeline (PUXA on ROXO, NYZE on MOXE) captures the match, and POVA then clears ROXY.

The match pipeline:

• Per-bit XNORs read ff43 (SCX) live — there is no SCX snapshot at Mode 3 entry.
• POHU = AND4(ROXY, match bits): the ROXY term makes the comparator self-terminating — it collapses as soon as POVA clears ROXY.
• PUXA latches POHU on ROXO↑; NYZE mirrors PUXA on the next MOXE↑; POVA = AND2(NYZE.Q_n, PUXA) is high exactly between those two edges — a single-dot pulse, by construction unable to re-fire within the scanline (PUXA/NYZE reset to 0 outside Mode 3, guaranteeing NYZE.Q_n=1 at entry).

The startup pipeline: AVAP to first SACU

Each scanline's Mode 3 begins with a 7-dot cascade from AVAP↑ to the first SACU rising edge — 7.026 dots at SCX=0, with every stage at a deterministic offset, zero variance across scanlines (dmg-sim measurement, purpose-built scx0_steady ROM, n=1,612):

The "AVAP at 0.000" row is the dot-count reference, not an ALET-edge alignment — AVAP transitions on the ALET falling edge preceding the table (it derives from BYBA's XUPY-clocked capture); the downstream DFFs are ALET/MYVO-edge aligned. The cascade is identical on every scanline including the first (whose 2-dot shortening lives entirely in Mode 0 — see Register writes). The window's MOSU trigger feeds NYXU through the same NOR3 and produces the same 7-dot pipeline (Window control). The NYXU pulse itself runs 0.492 dots when AVAP drives it and 0.495 dots from TEVO (0.503 from MOSU).

Fine-scroll startup suppression

The cascade (NYKA → PORY → PYGO → POKY) fires at the same offsets regardless of SCX; only ROXY's release moves. Measured at three SCX values (dmg-sim measurements, purpose-built scx{0,3,7}_steady ROMs, n=1,151–1,612 scanlines each, zero variance):

The model is exact: 1.000 dot per SCX & 7 step; first SACU at 7 + (N & 7) + 0.026; Mode 3 core duration 173 + (N & 7) + 0.045.

Edge-by-edge startup waveform

The internal transitions, measured at SCX ∈ {0, 3, 7} (dmg-sim measurement, purpose-built scx{0,3,7}_steady ROMs; AVAP↑ = 0.0000; PASO re-arms for good shortly after ROXO's first rise):

Common cascade (SCX-independent):

Per-SCX match pipeline:

Each SCX increment shifts the block by exactly 1.0000 dots with identical internal shape. The counter self-stop at SCX=7 does not suppress ROXO (PUXA's clock path is independent of PECU), so the SCX=7 match fires with the same geometry.

The tail-of-Mode-3 drain

The drain detector fires on a fixed 8-dot cadence at +13.997, +21.997, …, +165.997, +173.997 dots from AVAP — independent of SCX. Mode 3 ends at +(173.045 + N). The 21st drain edge at +173.997 therefore lands:

• at SCX=0: after Mode 3 ends (by 0.952 dots) — suppressed: the VOGA/WEGO transition and the mode3 reset on PYGO/RENE/RYFA clear the cascade first;
• at SCX&7 ≥ 1: inside Mode 3 — SEKO fires, TEVO → NYXU → a final parallel-load of the in-flight fetch's tile data.

The 21st load completes within the closing dot but produces no observable LD output — the SACU count is already at 167, so the freshly loaded content never meets another CP-gated edge before H-Blank. Both planes load from the same tile index — there is no hybrid-fetch path here (dmg-sim measurement, AGE m3-bg-scx-nocgb ROM: post-load shifter content 0b11111111 on both planes where tilemap column 21 holds an all-$FF tile).

Late SCX writes during the discard

The live-SCX rule, measured to its edge (dmg-sim measurement, gambatte ly0_late_scx7_m3stat_scx3_{1,2} ROMs — SCX=3 held, late SCX=7 write during the first-scanline discard):

In _2 the write becomes visible 0.025 dots after the capture edge — and the already-cleared ROXY cannot re-open (POHU collapsed at ROXY↓). The rule is purely "the ff43 value at the count=N ROXO↑ wins"; both hardware outcomes follow from it.

Startup invariance under stacked sprites

The whole cascade is configuration-invariant: with 5+5 sprites stacked at two X positions chosen so (X + SCX) mod 8 == 0 (the wilbertpol intr_2_mode0_timing_sprites_scx1_nops worst case), every BG-fetch and cascade transition between AVAP↑ and first SACU↑ lands at an identical sub-dot offset across SCX ∈ {0..4}, and the sprite-chain edges (FEPO/TEKY/SOBU/ RYCE/TAKA/WUTY/SUDA) are likewise SCX-invariant (dmg-sim measurement). The transitions nearest integer-dot boundaries sit 83–469 ps clear of them, deterministically — real-silicon process variance cannot produce per-configuration selectivity from a uniform per-die offset.