M CHANGES → CHANGES

@@ -51,6 +51,7 @@ - GBA SIO: Fix copying Normal mode transfer values
  - GBA SIO: Fix Normal mode being totally broken (fixes mgba.io/i/1800)
  - GBA SIO: Fix deseralizing SIO registers
  - GBA SIO: Fix hanging on starting a second multiplayer window (fixes mgba.io/i/854)
+ - GBA SIO: Fix Normal mode transfer start timing (fixes mgba.io/i/425)
  - GBA Timers: Fix toggling timer cascading while timer is active (fixes mgba.io/i/2043)
  - GBA Video: Latch scanline at end of Hblank (fixes mgba.io/i/1319)
  - GBA Video: Fix Hblank timing

M src/gba/sio/lockstep.c → src/gba/sio/lockstep.c

@@ -367,17 +367,22 @@ }
 
 static uint32_t _slaveUpdate(struct GBASIOLockstepNode* node) {
 	enum mLockstepPhase transferActive;
-	int attachedMulti, attached;
+	int attached;
+	int attachedMode;
 
 	ATOMIC_LOAD(transferActive, node->p->d.transferActive);
-	ATOMIC_LOAD(attachedMulti, node->p->attachedMulti);
 	ATOMIC_LOAD(attached, node->p->d.attached);
 
-	node->d.p->siocnt = GBASIOMultiplayerSetReady(node->d.p->siocnt, attachedMulti == attached);
+	if (node->mode == SIO_MULTI) {
+		ATOMIC_LOAD(attachedMode, node->p->attachedMulti);
+		node->d.p->siocnt = GBASIOMultiplayerSetReady(node->d.p->siocnt, attachedMode == attached);
+	} else {
+		ATOMIC_LOAD(attachedMode, node->p->attachedNormal);
+	}
 	bool signal = false;
 	switch (transferActive) {
 	case TRANSFER_IDLE:
-		if (!GBASIOMultiplayerIsReady(node->d.p->siocnt)) {
+		if (attachedMode != attached) {
 			node->p->d.addCycles(&node->p->d, node->id, LOCKSTEP_INCREMENT);
 		}
 		break;
@@ -479,15 +484,32 @@ value = GBASIONormalClearSi(value);
 		}
 		if (value & 0x0080) {
 			if (!node->id) {
-				// Internal shift clock
-				if (value & 1) {
-					ATOMIC_STORE(node->p->d.transferActive, TRANSFER_STARTING);
-				}
 				// Frequency
+				int32_t cycles;
 				if (value & 2) {
-					node->p->d.transferCycles = GBA_ARM7TDMI_FREQUENCY / 1024;
+					cycles = 8 * 8;
+				} else {
+					cycles = 64 * 8;
+				}
+				if (value & 0x1000) {
+					cycles *= 4;
+				}
+
+				enum mLockstepPhase transferActive;
+				ATOMIC_LOAD(transferActive, node->p->d.transferActive);
+
+				if (transferActive == TRANSFER_IDLE) {
+					mLOG(GBA_SIO, DEBUG, "Lockstep %i: Transfer initiated", node->id);
+					ATOMIC_STORE(node->p->d.transferActive, TRANSFER_STARTING);
+					ATOMIC_STORE(node->p->d.transferCycles, cycles);
+
+					if (mTimingIsScheduled(&driver->p->p->timing, &node->event)) {
+						node->eventDiff -= node->event.when - mTimingCurrentTime(&driver->p->p->timing);
+						mTimingDeschedule(&driver->p->p->timing, &node->event);
+					}
+					mTimingSchedule(&driver->p->p->timing, &node->event, 0);
 				} else {
-					node->p->d.transferCycles = GBA_ARM7TDMI_FREQUENCY / 8192;
+					value &= ~0x0080;
 				}
 			} else {