VideoInterface: start counting half-lines at 0 instead of 1

- Re-organize VideoInterface::Update() to count half-lines starting at 0 instead of 1 - Use horizontal position when checking if we should assert some display interrupt - Add some more descriptive comments
Ben754 · Aug 27, 2019 · 9e3b867 · 9e3b867
1 parent 230ff47
commit 9e3b867
Showing 1 changed file with 36 additions and 18 deletions.
diff --git a/Source/Core/Core/HW/VideoInterface.cpp b/Source/Core/Core/HW/VideoInterface.cpp
@@ -191,7 +191,7 @@ void Preset(bool _bNTSC)
   m_BorderHBlank.Hex = 0;
 
   s_ticks_last_line_start = 0;
-  s_half_line_count = 1;
+  s_half_line_count = 0;
   s_half_line_of_next_si_poll = num_half_lines_for_si_poll;  // first sampling starts at vsync
   s_current_field = FieldType::Odd;
 
@@ -317,7 +317,7 @@ void RegisterMMIO(MMIO::Mapping* mmio, u32 base)
   // MMIOs with unimplemented writes that trigger warnings.
   mmio->Register(
       base | VI_VERTICAL_BEAM_POSITION,
-      MMIO::ComplexRead<u16>([](u32) { return 1 + (s_half_line_count - 1) / 2; }),
+      MMIO::ComplexRead<u16>([](u32) { return 1 + (s_half_line_count) / 2; }),
       MMIO::ComplexWrite<u16>([](u32, u16 val) {
         WARN_LOG(VIDEOINTERFACE,
                  "Changing vertical beam position to 0x%04x - not documented or implemented yet",
@@ -733,12 +733,30 @@ static void EndField()
 // Run when: When a frame is scanned (progressive/interlace)
 void Update(u64 ticks)
 {
+  // If an SI poll is scheduled to happen on this half-line, do it!
+
   if (s_half_line_of_next_si_poll == s_half_line_count)
   {
     SerialInterface::UpdateDevices();
-
     s_half_line_of_next_si_poll += 2 * SerialInterface::GetPollXLines();
   }
+
+  // If this half-line is at the actual boundary of either field, schedule an SI poll to happen
+  // some number of half-lines in the future
+
+  if (s_half_line_count == 0)
+  {
+    s_half_line_of_next_si_poll = num_half_lines_for_si_poll;  // first results start at vsync
+  }
+  if (s_half_line_count == GetHalfLinesPerEvenField())
+  {
+    s_half_line_of_next_si_poll = GetHalfLinesPerEvenField() + num_half_lines_for_si_poll;
+  }
+
+  // If this half-line is at some boundary of the "active video lines" in either field, we either
+  // need to (a) send a request to the GPU thread to actually render the XFB, or (b) increment
+  // the number of frames we've actually drawn
+
   if (s_half_line_count == s_even_field_first_hl)
   {
     BeginField(FieldType::Even, ticks);
@@ -756,30 +774,30 @@ void Update(u64 ticks)
     EndField();
   }
 
-  for (UVIInterruptRegister& reg : m_InterruptRegister)
-  {
-    if (s_half_line_count + 1 == 2u * reg.VCT)
-    {
-      reg.IR_INT = 1;
-    }
-  }
+  // Move to the next half-line and potentially roll-over the count to zero. If we've reached
+  // the beginning of a new full-line, update the timer
 
   s_half_line_count++;
-
-  if (s_half_line_count > GetHalfLinesPerEvenField() + GetHalfLinesPerOddField())
+  if (s_half_line_count == GetHalfLinesPerEvenField() + GetHalfLinesPerOddField())
   {
-    s_half_line_count = 1;
-    s_half_line_of_next_si_poll = num_half_lines_for_si_poll;  // first results start at vsync
+    s_half_line_count = 0;
   }
 
-  if (s_half_line_count == GetHalfLinesPerEvenField())
+  if (!(s_half_line_count & 1))
   {
-    s_half_line_of_next_si_poll = GetHalfLinesPerEvenField() + num_half_lines_for_si_poll;
+    s_ticks_last_line_start = CoreTiming::GetTicks();
   }
 
-  if (s_half_line_count & 1)
+  // Check if we need to assert IR_INT. Note that the granularity of our current horizontal
+  // position is limited to half-lines.
+
+  for (UVIInterruptRegister& reg : m_InterruptRegister)
   {
-    s_ticks_last_line_start = CoreTiming::GetTicks();
+    u32 target_halfline = (reg.HCT > m_HTiming0.HLW) ? 1 : 0;
+    if ((1 + (s_half_line_count) / 2 == reg.VCT) && ((s_half_line_count & 1) == target_halfline))
+    {
+      reg.IR_INT = 1;
+    }
   }
 
   UpdateInterrupts();