From 71bbd10ad0ba87e25e256b617e0b3446abd37fb6 Mon Sep 17 00:00:00 2001
From: libretroadmin <libretroadmin@users.noreply.github.com>
Date: Sun, 14 Jun 2026 15:30:30 +0000
Subject: [PATCH] tengen: auto-reload + trigger-on-reach-0 for RAMBO-1 IRQ
 (#544)

The Tengen 800032 (RAMBO-1) IRQ counter on real hardware auto-reloads
from its latch when it would otherwise underflow, and the IRQ
triggers when the counter REACHES zero (going from 1 to 0) rather
than on the underflow step that follows.

Upstream's earlier model triggered on underflow (count 0 -> 0xFF)
and then continued decrementing through 0xFE, 0xFD, ... until the
game wrote a reload register ($C000/$C001/$E000/$E001 with
rmode==1).  For titles that don't write the reload register on
every IRQ - Skull & Crossbones in 2-player mode is the documented
case (libretro docs Compatibility entry), but Hard Drivin', Klax,
Rolling Thunder, Road Runner, Xenophobe etc. all use the same
mapper - subsequent IRQs after the first underflow are silently
swallowed, displacing the status-bar split point and producing the
screen-shake / glitch reported in #544.

Backport reference: negativeExponent's libretro-fceumm_next commit
7f277a8 ("Cleanup mapper 64/158") plus the Mesen RAMBO-1 model.
This patch is the minimal algorithmic delta against upstream:

  - RAMBO1IRQHook (CPU-cycle mode, IRQmode==1): trigger on
    IRQCount reaching 0, auto-reload from IRQLatch on the next
    clock after the trigger.
  - RAMBO1HBHook (scanline / A12-approximation mode, IRQmode==0):
    same change.  rmode is still set on IRQ fire so existing
    register-reload semantics on $C000/$C001/$E000/$E001 are
    preserved bit-for-bit - the writes do exactly what they did
    before, the only behaviour change is what the IRQ hook itself
    does between writes.
  - IRQLatch == 0 edge case handled (reload yields 0, which is the
    reach-0 trigger condition, so IRQs fire on every clock - the
    documented hardware behaviour at latch=0).

Crucially, this does NOT change:
  - SFORMAT chunk layout (REGS/CMDR/MIRR/RMOD/IRQM/IRQC/IRQA/IRQL
    chunks all still present, same byte sizes, same order).  Old
    savestates continue to load correctly.
  - The write-side dispatch (RAMBO1_Write) for $A000, $8000,
    $8001, $C000, $C001, $E000, $E001 - all behave exactly as
    before, including the rmode==1 conditional reload.
  - Mapper 158 (TENGEN 800037, Alien Syndrome Unl) wiring - it
    shares the RAMBO1 IRQ hook and gets the same fix for free.
  - PRG/CHR/mirroring sync logic.
  - Power-on and StateRestore behaviour.

Build verified: tengen.c -O2 / -O3 / aarch64 -O2 clean.
---
 src/boards/tengen.c | 42 ++++++++++++++++++++++++++++++++++++------
 1 file changed, 36 insertions(+), 6 deletions(-)

diff --git a/src/boards/tengen.c b/src/boards/tengen.c
index 6abf140..aaf49c5 100644
--- a/src/boards/tengen.c
+++ b/src/boards/tengen.c
@@ -48,9 +48,28 @@ static void FP_FASTAPASS(1) RAMBO1IRQHook(int a) {
 		smallcount += a;
 		while (smallcount >= 4) {
 			smallcount -= 4;
-			IRQCount--;
-			if (IRQCount == 0xFF)
-				if (IRQa) X6502_IRQBegin(FCEU_IQEXT);
+			/* Mesen / _next reference: the Tengen RAMBO-1 IRQ counter
+			 * auto-reloads from the latch when it would otherwise
+			 * underflow, and the IRQ triggers when the counter
+			 * REACHES zero (going from 1 to 0) rather than on the
+			 * underflow step that follows.  Upstream's earlier model
+			 * triggered on underflow (count 0 -> 0xFF) and then
+			 * continued decrementing through 0xFE, 0xFD, ... until
+			 * the game wrote a reload register.  Match the hardware
+			 * model: trigger on reach-0, and on the following clock
+			 * reload from latch automatically so the counter cycles
+			 * continuously without needing per-IRQ register writes. */
+			if (IRQCount == 0) {
+				IRQCount = IRQLatch;
+				if (IRQCount == 0 && IRQa) {
+					X6502_IRQBegin(FCEU_IQEXT);
+				}
+			} else {
+				IRQCount--;
+				if (IRQCount == 0 && IRQa) {
+					X6502_IRQBegin(FCEU_IQEXT);
+				}
+			}
 		}
 	}
 }
@@ -58,9 +77,20 @@ static void FP_FASTAPASS(1) RAMBO1IRQHook(int a) {
 static void RAMBO1HBHook(void) {
 	if ((!IRQmode) && (scanline != 240)) {
 		rmode = 0;
-		IRQCount--;
-		if (IRQCount == 0xFF) {
-			if (IRQa) {
+		/* See comment in RAMBO1IRQHook above for the auto-reload /
+		 * trigger-on-reach-0 model.  Same change applies here for the
+		 * scanline-clocked (A12 approximation) IRQ path. */
+		if (IRQCount == 0) {
+			IRQCount = IRQLatch;
+			/* Latch == 0 means fire on every clock - reload yields
+			 * 0 again, which is the reach-0 trigger condition. */
+			if (IRQCount == 0 && IRQa) {
+				rmode = 1;
+				X6502_IRQBegin(FCEU_IQEXT);
+			}
+		} else {
+			IRQCount--;
+			if (IRQCount == 0 && IRQa) {
 				rmode = 1;
 				X6502_IRQBegin(FCEU_IQEXT);
 			}