From 34f1c644f599a2a0319d0edeeef5dd929ec7fa2c Mon Sep 17 00:00:00 2001
From: Connor McLaughlin <stenzek@gmail.com>
Date: Sun, 1 Mar 2020 17:07:17 +1000
Subject: [PATCH] GPU: Update GPUSTAT if raster has passed to the next line

Fixes games which poll GPUSTAT, including The Next Tetris (Europe).
---
 src/core/gpu.cpp | 12 ++++++++++++
 src/core/gpu.h   |  3 +++
 2 files changed, 15 insertions(+)

diff --git a/src/core/gpu.cpp b/src/core/gpu.cpp
index 55aa9c8a2..db807a997 100644
--- a/src/core/gpu.cpp
+++ b/src/core/gpu.cpp
@@ -235,7 +235,14 @@ u32 GPU::ReadRegister(u32 offset)
       return ReadGPUREAD();
 
     case 0x04:
+    {
+      // code can be dependent on the odd/even bit, so update the GPU state when reading.
+      // we can mitigate this slightly by only updating when the raster is actually hitting a new line
+      if (IsRasterScanlinePending())
+        m_tick_event->InvokeEarly(true);
+
       return m_GPUSTAT.bits;
+    }
 
     default:
       Log_ErrorPrintf("Unhandled register read: %02X", offset);
@@ -455,6 +462,11 @@ void GPU::UpdateSliceTicks()
   m_tick_event->SetPeriod(GPUTicksToSystemTicks(ticks_until_hblank));
 }
 
+bool GPU::IsRasterScanlinePending() const
+{
+  return (GetPendingGPUTicks() + m_crtc_state.current_tick_in_scanline) >= m_crtc_state.horizontal_total;
+}
+
 void GPU::Execute(TickCount ticks)
 {
   // convert cpu/master clock to GPU ticks, accounting for partial cycles because of the non-integer divider
diff --git a/src/core/gpu.h b/src/core/gpu.h
index d2522c2bf..70bf8e1c7 100644
--- a/src/core/gpu.h
+++ b/src/core/gpu.h
@@ -310,6 +310,9 @@ protected:
   /// Returns the number of pending GPU ticks.
   TickCount GetPendingGPUTicks() const;
 
+  /// Returns true if enough ticks have passed for the raster to be on the next line.
+  bool IsRasterScanlinePending() const;
+
   /// Returns true if scanout should be interlaced.
   bool IsDisplayInterlaced() const { return !m_force_progressive_scan && m_GPUSTAT.In480iMode(); }