psxrecomp/accuracy/axis5_sio_controller.md

Axis 5 — SIO / Controller / Memory-Card Accuracy

Cross-reference of runtime/src/sio.c (+ main.cpp pad driver, memcard.c) against the in-tree Beetle/Mednafen oracle (beetle-psx/mednafen/psx/: frontio.cpp, input/dualshock.cpp, input/gamepad.cpp, input/memcard.cpp) and nocash psx-spx "Controllers and Memory Cards".

Historical read-only research; citations use the line numbers from that audit.

Status 2026-07-26: Tomba now offers explicit Analog / D-Pad modes instead of Hybrid. The Tomba-only g_pad_legacy_cfg protocol fork, its game-config key, debug command, and helper script have been removed. References to them below document the retired implementation and are not descriptions of the current runtime.

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1. What our implementation does

1.1 Bus / transfer model (sio.c)

• SIO0 MMIO at 0x1F801040..0x1F80104E handled in sio_read/sio_write (sio.c:1306, sio.c:1361). A byte is "transferred" when the guest writes SIO_TX_DATA (0x1F801040) with TX_EN set; we process it synchronously via sio_process_byte (sio.c:1131) and arm a delayed ACK→IRQ7.
• Two timing models behind SIO_MODEL_CYCLE_PACED (default 1, sio.c:1767):
  • Access-paced legacy path — sio_irq_countdown decrements on every SIO register access and on sio_tick; fires IRQ7 + sets STAT.ACK/STAT.IRQ (sio.c:1983). Delays: SIO_IRQ_DELAY_PAD=4, SIO_IRQ_DELAY_CARD=8 (sio.c:490,498).
  • Cycle-paced shifter — sio_shift_* + sio_pending_ack + sio_advance (sio.c:1882) modelling SIO_BAUD_CYCLES_DEFAULT=1088 shift + 170-cycle ACK. However the pad fast-path (sio.c:1386) bypasses the shifter entirely: any pad byte (active_device==DEV_PAD, or bus_owner==PAD, or tx==0x01) is processed immediately and uses the access-paced SIO_IRQ_DELAY_PAD countdown. So in practice pad timing is still access-paced, not cycle-paced.
• active_device (NONE/PAD/MEMCARD, sio.c:173) routes a byte to pad_process_byte or mc_process_byte. First byte selects: 0x01→pad, 0x81→card (sio.c:1148).

1.2 Pad state machine (pad_process_byte, sio.c:727)

A 3-state FSM (PAD_IDLE → PAD_WAIT_ACCESS → PAD_SEND_RESPONSE, sio.c:55), byte-indexed not bit-indexed. Per-slot state:

• pad_buttons[2] (active-low), pad_analog[2] (0=digital id 0x41, 1=analog id 0x73), pad_stick[2][4], pad_connected.
• pad_in_config[2] — DualShock config-mode latch (report 0xF3).
• pad_supports_config[2] — 1 = DualShock (answers config cmds), 0 = plain digital pad (ignores them) (sio.c:87).
• pad_type_req[2] — deferred host type-change request, applied only at PAD_IDLE && !in_config (sio.c:100, applied sio.c:733).
• g_pad_legacy_cfg — per-game flag (Tomba only). When set, 0x43 always answers 0xF3 and the 0x45/0x46/0x47/0x4C/0x4D config queries answer unconditionally with no enter/exit tracking (sio.c:718, sio.c:793, sio.c:802).

Command handling at PAD_WAIT_ACCESS (sio.c:752):

• 0x42 poll: emits [id, 0x5A, btnlo, btnhi] (4 bytes), or 8 bytes with the four stick axes when pad_analog || pad_in_config (sio.c:767).
• 0x43 (modern): emits [cur_id, 0x5A, 0,0,0,0,0,0] (8 bytes), latches enter/exit from the data byte at response index 2 (sio.c:862).
• 0x44 (modern): mode-set; latches analog/digital from data byte at index 2 (sio.c:870).
• 0x45/0x46/0x47/0x4C/0x4D/0x4F (modern): canned responses, only while pad_in_config (sio.c:823).
• Anything else, or a config cmd outside config: hi-z (0xFF), end txn (sio.c:845).

1.3 Host pad driver (main.cpp)

sample_pad_into_sio (main.cpp:1240) runs once (twice w/ low-latency) per frame: computes eff_analog from mode (DIGITAL/ANALOG/HYBRID, main.cpp:1261), pushes sticks via sio_set_pad_sticks, and requests the type via sio_request_pad_type (main.cpp:1288). HYBRID auto-flips analog on stick deflection / digital on d-pad (main.cpp:1267).

1.4 Memory card FSM (mc_process_byte, sio.c:906)

Per-slot mc_slots[2] saved/restored across device switches (sio.c:176). Read (0x52), write (0x57), get-id (0x53); FLAG byte 0x08→0x00 after first access (sio.c:934); sector echo, checksum, 0x47/0x4E/0xFF end codes. This is broadly faithful to memcard.cpp and is not the focus here.

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2. Discrepancies vs Beetle + psx-spx

D1 — Pad protocol is byte-framed, the hardware/Beetle is bit-clocked (model)

Beetle clocks one bit per Clock() call, 8 bits per byte (dualshock.cpp:402-418, gamepad.cpp:143-157), with DSR pulses scheduled per byte (frontio.cpp:756, dualshock.cpp:1077). We process a whole byte per SIO_TX_DATA write and fake one ACK per byte. This is an acceptable abstraction for the data exchanged (the guest only sees whole RX bytes + ACK IRQs), but it means our ACK/IRQ timing is a hand-tuned constant (DELAY_PAD=4 accesses) rather than baud<<scaleshift cycles (frontio.cpp:520,783). Low risk for correctness, real risk for any timing-sensitive pad probe. Logged for the cycle axis; not the hybrid bug.

D2 — 0x42 returns analog (8-byte) frame whenever pad_in_config is set, even with analog OFF (semantic, WRONG)

Ours: if (pad_analog || pad_in_config) → 8-byte stick frame (sio.c:775). Beetle: 0x4200 returns the 6-byte analog payload if(analog_mode || mad_munchkins) (dualshock.cpp:592) — i.e. analog frame in config mode is correct but see D3 for the framing-count mismatch. The conceptual match is OK; the danger is the byte count (D3) and that we tie stick reporting to our pad_in_config which can be set/cleared on a different schedule than Beetle's mad_munchkins.

D3 — Config-mode poll byte counts differ from a real DualShock (semantic)

Real DualShock / Beetle digital 0x42 = ID 0x41 + 0x5A + 2 button bytes = the device transmits 0x41,0x5A,btn0,btn1 framed as id then transmit_count=2 buttons (dualshock.cpp:604-606); analog 0x42 = id 0x73 + 0x5A + 2 btn + 4 axis bytes (dualshock.cpp:594-600). Our digital frame is 4 bytes (id,5A,lo,hi) and analog is 8 bytes (id,5A,lo,hi,rx,ry,lx,ly) — counts agree with hardware. But note our stick axis order is rx,ry,lx,ly (sio.c:776-779) whereas a real pad reports right stick first only in the config layout; the standard analog 0x42 order is RX,RY,LX,LY — this matches. No defect here, but it is the opposite of Beetle's internal axes[0]=left, axes[1]=right ordering, so a naive trace diff will look "swapped" — call it out in validation so it isn't chased as a bug.

D4 — 0x43 semantics are fundamentally wrong vs a real DualShock (semantic, the core bug — see §3)

On a real DualShock NOT in config mode, command 0x43 is a poll that also arms config entry: it returns the button/analog data frame (dualshock.cpp:471-490 — 0x43 in command_phase==1 transmits buttons+axes, identical framing to 0x42), and only the second data byte == 0x01 sent during that same transaction flips mad_munchkins=true afterwards (dualshock.cpp:503-515). Ours instead returns an all-zero 8-byte frame [cur_id,5A,0,0,0,0,0,0] for 0x43 (sio.c:793-798). So during the enter-config handshake we feed the driver zeros where the real pad sends live button/stick data. A driver that reads those bytes as input (many do, since 0x43-with-data is indistinguishable from a poll on the wire) sees "all pressed" / centered-stick garbage. This is the phantom-input mechanism.

D5 — Digital InputDevice_Gamepad does NOT implement config at all (confirms our pad_supports_config=0 path is correct)

Beetle's plain gamepad only handles 0x42; any other command sets command_phase=-1 and transmits nothing (gamepad.cpp:202-209). Our pad_supports_config==0 else-branch returns hi-z for config commands (sio.c:845) — faithful. Good. The Tomba2 digital-pad fix (memory: tomba2_menu_input_fix) is correct and matches hardware.

D6 — No analog-mode lock, no mad_munchkins gating of 0x44 (semantic, minor)

Beetle: 0x44 only takes effect inside MAD MUNCHKINS (config) mode (dualshock.cpp:679,689-712), and 0x4402 sets analog_mode_locked (dualshock.cpp:714-725); a locked pad ignores the physical analog button (dualshock.cpp:203). Ours honours 0x44 set-mode (sio.c:870) but has no lock state, so a game that locks analog (common: forces DualShock and locks) will still see our HYBRID auto-flip yank it back to digital — exactly the kind of "type flips underneath the game" desync the deferred-request machinery was built to avoid but cannot prevent, because we never learned the pad is locked.

D7 — 0x4D rumble-map negotiation and host output (resolved)

Resolved: 0x4D now echoes the previous six-byte motor map while latching the new map. Mapped motor bytes from later 0x42 polls are retained per slot and forwarded to SDL's low/high-frequency rumble channels by the frontend. The protocol path is pinned by sio_dualshock_rumble_test; legacy savestates remain loadable and receive the conventional {0x00,0x01,0xFF,...} map with stopped motors.

D8 — 0x45 analog-status byte is hard-coded 0x03 (analog on) (semantic)

Beetle 0x4501 reports transmit_buffer[1] = analog_mode ? 0x01 : 0x00 (dualshock.cpp:743) — i.e. the current analog state. Ours always answers {0xF3,0x5A,0x03,0x02,0x01,0x02,0x01,0x00} (sio.c:808/831), reporting analog ON unconditionally. A driver polling 0x45 to discover the live mode reads "analog" even when we are presenting digital → mode-confusion, another route to the hybrid desync.

D9 — Config enter/exit latches on the wrong byte index (semantic, subtle)

Beetle enters MAD MUNCHKINS only when, during the 0x43 transaction, transmit_pos==2 && receive_buffer==0x01 (dualshock.cpp:503) — i.e. the third byte of the transaction (idx 2, the byte after id+0x5A). We latch pad_in_config when pad_current_cmd==0x43 && pad_response_idx==2 (sio.c:862). Because our pad_response_idx starts at 1 after the cmd byte and increments per response byte, idx 2 corresponds to the 3rd exchanged byte — which does line up. No defect, but it is fragile and undocumented relative to Beetle's transmit_pos convention; verify with a trace before trusting it.

D10 — DTR/SELECT-deassert does not run the analog-button evaluation (model)

On real hardware the analog toggle is evaluated when DTR goes inactive (dualshock.cpp:383-388, CheckManualAnaModeChange). We have no DTR-edge hook into pad type; our type change is host-driven via pad_type_req. This is by design (the host emulates the analog button) but means our "analog button" can fire at a different bus phase than hardware would, which is the seam the phantom bug lives in.

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3. HYBRID PAD BUG — root-cause analysis (priority)

Symptom: in HYBRID mode the pad's reported type flips between digital (0x41) and analog (0x73) as the player moves between d-pad and stick; under the modern config SM this produces phantom "all-pressed"/disconnect input in Tomba (and a phantom dash in MMX6). Tomba is shipped on the legacy flag (g_pad_legacy_cfg=1) to dodge it; that flag is explicitly marked LEGACY/ remove-me (sio.c:665-717).

Root cause — there are two compounding faults, both in 0x43 handling:

(a) 0x43 returns zeros instead of the live data frame (D4 — the real bug)

A real DualShock treats 0x43 exactly like 0x42 for the bytes it sends — it transmits the current button + (in analog mode) axis frame (dualshock.cpp:471-490) — and merely additionally uses the host's 0x01/0x00 data byte to toggle config on/off after the transaction (dualshock.cpp:503-515). Tomba's libpad, like most, issues the config enter/poll with 0x43 and reads the returned frame as controller state. Our 0x43 returns [id,0x5A,0,0,0,0,0,0] (sio.c:793). Active-low PSX buttons: 0x00 = all buttons pressed, sticks 0x00 = hard up-left. So every frame the driver runs a 0x43 (which it does to manage analog mode) it reads a phantom "everything pressed." That is the phantom input, independent of any type flip. The legacy flag hides it only because Tomba's legacy driver path happens not to consume the 0x43 frame the same way — it is luck, not correctness.

(b) The type flip changes the ID byte across a transaction boundary the game treats as continuous (D8/D10)

Even with (a) fixed, our HYBRID flip changes pad_analog (hence the reported id 0x41↔0x73 and the 4↔8 byte frame length) at an idle boundary (sio.c:733). libpad's findpad caches the pad type; when the id changes it re-runs detection, and the modern SM manufactures a one-frame "id changed / re-detect" that the game reads as unplug→replug (memory: tomba_phantom_input_v050). Beetle never has this problem because the type only changes via the physical analog button evaluated at DTR-inactive (dualshock.cpp:171-213), and crucially Beetle's 0x45 reports the live analog state (dualshock.cpp:743) and 0x44 can lock it — so a well-written driver always knows the current mode and never mis-reads the frame length. Our 0x45 lies (always 0x03/analog-on, D8), so the driver's cached mode and the actual frame length we send can disagree → it parses an 8-byte analog frame as a 4-byte digital one (or vice-versa), shifting every subsequent byte → garbage buttons.

Why the per-poll flip diverges from a real DualShock: a real pad's type is stable within and across polls and only changes on a deliberate analog-button press (debounced, DTR-gated) or an explicit 0x44 from the game; the game is always told the truth by 0x45. Our hybrid mode flips the type from an external signal (host stick/d-pad heuristic) on a schedule the game's pad driver never sanctioned, and we then mis-report the current mode (0x45) and send zero frames for 0x43 so the driver cannot recover. The deferred pad_type_req machinery (sio.c:642) correctly prevents a flip mid-transaction, but it cannot fix that the content of the 0x43 frame is wrong and that 0x45 lies — those are the actual desync sources.

Bottom line: The hybrid bug is not primarily a "flip races the handshake" timing bug (that part is already mitigated). It is that our 0x43 sends a zero data frame instead of the live poll frame (§3a / D4) and our 0x45 reports a fixed analog-on status instead of the live mode (§3b / D8). Fix those two to match dualshock.cpp and the HYBRID flip becomes benign for every title — which is exactly what the LEGACY comment predicted, letting the g_pad_legacy_cfg opt-in be deleted.

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4. Prioritized fix list (player-visible impact first)

STATUS 2026-07-26: Fixes 1, 2, 3 are implemented. Fix 4 is also complete: Tomba no longer uses Hybrid, and the legacy pad-config branch has been removed.

Prior status 2026-06-27 (branch wt/tomba2-axis5-controller): Fixes 1, 2, 3 IMPLEMENTED in sio.c (0x43 live frame; 0x45 live analog byte; 0x44 analog-mode-lock + hybrid-flip lock gate), transcribed from dualshock.cpp. Validated no-regression on Tomba 2 (digital pad, pad 0xFFFF at rest, boots). These are gated to the MODERN SM and leave Tomba 1's legacy path untouched, so they're safe to ship; MMX6 (modern SM) gains the phantom-dash fix. Fix 4 (retire g_pad_legacy_cfg) is GATED on a behavioral DualShock-game validation (MMX6 phantom-dash gone / Tomba 1 benign hybrid on the modern SM) — that needs a Tomba 1/MMX6 build against THIS framework worktree (a full regen, since the runtime ABI now carries the cycle-axis CPUState fields), so it's a separate validated pass, not a blind delete. Fixes 5/6 not yet done.

1. [P0] Make 0x43 transmit the live poll frame, not zeros (D4 / §3a). In pad_process_byte 0x43 branch (sio.c:787), populate pad_response[2..] with the same button (+axis when analog) bytes as the 0x42 branch instead of 0x00. Mirror dualshock.cpp:471-490. This is the single highest-impact fix: removes the phantom "all-pressed" at its source and is the prerequisite for retiring g_pad_legacy_cfg.

2. [P0] Make 0x45 report the live analog state (D8 / §3b). Set pad_response[3] = pad_analog[slot] ? 0x01 : 0x00 in both the modern and legacy 0x45 answers (sio.c:808,831), matching dualshock.cpp:743. Lets the driver track frame length across a hybrid flip and stops the off-by-frame-length garbage reads.

3. [P1] Implement analog-mode lock (0x44/0x4402) (D6). Track analog_mode_locked per slot; when locked, sio_request_pad_type / pad_type_req must be ignored (the HYBRID auto-flip must not override a game-locked mode) — mirror dualshock.cpp:203,714-725. Prevents the hybrid heuristic from fighting games that pin DualShock.

4. [DONE 2026-07-26] Retire g_pad_legacy_cfg. Tomba now exposes only explicit Analog / D-Pad modes, so there is no live type flip to accommodate. The flag, legacy SIO branches, config key, debug command, helper script, and Tomba opt-in were removed. All titles use the modern DualShock state machine.

5. [DONE 2026-08-06] Echo-back 0x4D rumble map and forward motors. The SIO model now routes negotiated weak/strong values to the SDL frontend.

6. [P2] Move pad ACK/IRQ timing onto the cycle-paced shifter (D1). Remove the pad fast-path bypass (sio.c:1386) so pad bytes use baud<<scaleshift timing like the card path and like frontio.cpp:520,783. Cross-listed with the cycle axis. Needed for any title that times pad probes.

7. [P3] Document/verify the config-latch byte index and stick-order conventions (D3, D9) so trace diffs aren't mistaken for bugs.

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5. Validation method (native port 4500 vs Beetle oracle port 4382)

Tomba2 dev runtime debug port is 4500 (memory: tomba2_bringup); Beetle oracle is the psx-beetle.exe JSON server (CLAUDE.md §16 names 4380; this task specifies 4382 — use whichever port the running beetle binary bound). Both expose the identical wire protocol, including pad_status (debug_server.c:5713) and sio_trace (debug_server.c:9963), and Beetle fires a per-byte SIO trace callback sio_trace_cb(TX, RX, Control) (frontio.cpp:772-774).

Per fix, free-run BOTH processes to the same screen (do NOT pause/step — query the always-on rings), drive the same input, then diff:

• Fix 1 (0x43 frame): sio_trace on both. Filter to transactions whose 2nd byte is 0x43. Native (current): RX = ..,id,5A,00,00,... Beetle: RX = ..,id,5A,<btn0^FF>,<btn1^FF>,<axes..>. After the fix the native RX bytes for 0x43 must equal the 0x42 RX bytes captured in the same state. Confirm pad_status shows no spurious all-pressed (pad:0xFFFF at rest on both).

• Fix 2 (0x45 status): drive HYBRID, toggle stick↔d-pad, capture 0x45 transactions. Native byte[3] must follow pad_status.slotN.analog (true→0x01, false→0x00) and equal Beetle's 0x4501 byte[1] in the matching analog state.

• Fix 3 (lock): issue a 0x44/0x4402 lock sequence (or drive a game that does), then deflect the stick. pad_status.analog must NOT change while locked; cross-check Beetle stays in its locked mode across the same input.

• Fix 4 (retire legacy): run Tomba in both explicit Analog and D-Pad modes on the modern SM. pad_status at rest = 0xFFFF both slots, no disconnect blips; menu responsive; in-game d-pad↔stick transitions produce no phantom dash/unpause. Diff a 600-byte sio_trace window vs Beetle for the same navigation — RX streams should match transaction-for-transaction (allowing the D3 stick-order note).

• Fix 6 (timing): compare ACK→IRQ7 spacing. Use the SIO IRQ ring (sio_get_irq_ring, sio.c:378) for native delay_applied/byte_seq vs Beetle's ClockDivider/dsr_pulse_delay cadence (frontio.cpp:756,783). Native pad-byte IRQ spacing should land within the baud<<scaleshift window, not the fixed 4-access constant.

General invariant to assert on every diff: at rest, pad_status = {pad:0xFFFF, slot0.connected:true} on BOTH; any deviation from 0xFFFF while no key is held is a phantom-input regression and must reproduce identically (or better) than Beetle.