From 2b00f1ecb2b5dcfc41f7fd3ae460fc5b470c1e6f Mon Sep 17 00:00:00 2001
From: Lauchmelder <robert.altner11@gmail.com>
Date: Mon, 28 Feb 2022 17:14:32 +0100
Subject: [PATCH] added latch to ppu

---
 src/PPU.cpp | 28 +++++++++++++++++-----------
 src/PPU.hpp | 38 +++++++++++++++++++++++++++++++++++++-
 2 files changed, 54 insertions(+), 12 deletions(-)

diff --git a/src/PPU.cpp b/src/PPU.cpp
index 531c26f..ebbf88d 100644
--- a/src/PPU.cpp
+++ b/src/PPU.cpp
@@ -35,8 +35,10 @@ void PPU::Reset()
 
 void PPU::Tick()
 {
+	// On this cycle the VBlankStarted bit is set in the ppustatus
 	if (y == 241 && x == 1)
 	{
+		// Set flag and send NMI if necessary
 		ppustatus.Flag.VBlankStarted = 1;
 		if (ppuctrl.Flag.VBlankNMI)
 			bus->NMI();
@@ -44,9 +46,11 @@ void PPU::Tick()
 		isFrameDone = true;
 	}
 
+	// This cycle resets the VBlankStarted flag
 	if (y == 261 && x == 1)
 		ppustatus.Flag.VBlankStarted = 0;
 
+	// Advance pixel counters
 	x++;
 	if (x > 340)
 	{
@@ -59,34 +63,33 @@ void PPU::Tick()
 
 Byte PPU::ReadRegister(Byte id)
 {
-	Byte returnVal = 0x00;
-
+	// Reading from a register fills the latch with the contents of the register
+	// Write-only regs don't fill the latch
+	// But in any case, the latch contents are returned
 	switch (id)
 	{
 	case 0:
-		returnVal = ppuctrl.Raw;
+		latch = ppuctrl.Raw;
 		break;
 
 	case 1:
-		returnVal = ppumask.Raw;
+		latch = ppumask.Raw;
 		break;
 
 	case 2:
-		returnVal =  ppustatus.Raw;
+		latch =  ppustatus.Raw;
 		ppustatus.Flag.VBlankStarted = 0;
 		addressLatch = 0;
 		break;
 
 	case 5:
-		returnVal = 0x00;
 		break;
 
 	case 6:
-		returnVal = 0x00;
 		break;
 
 	case 7:
-		returnVal = bus->ReadPPU(ppuaddr.Raw);
+		latch = bus->ReadPPU(ppuaddr.Raw);
 		ppuaddr.Raw += (ppuctrl.Flag.VRAMAddrIncrement ? 32 : 1);
 		break;
 	
@@ -95,7 +98,7 @@ Byte PPU::ReadRegister(Byte id)
 		break;
 	}
 
-	return returnVal;
+	return latch;
 }
 
 void PPU::WriteRegister(Byte id, Byte val)
@@ -114,13 +117,16 @@ void PPU::WriteRegister(Byte id, Byte val)
 		ppustatus.Raw = val;
 		break;
 
+		// PPUADDR and PPUSCROLL both take 2 accesses to fully set
+		// When writing to them the address latch is switched. The latch
+		// determines whether the hi or lo byte should be written next
 	case 5:
 		if (addressLatch == 0)
 			ppuscroll.x = val;
 		else
 			ppuscroll.y = val;
 
-		addressLatch = 1 - addressLatch;
+		addressLatch = !addressLatch;
 		break;
 
 	case 6:
@@ -129,7 +135,7 @@ void PPU::WriteRegister(Byte id, Byte val)
 		else
 			ppuaddr.Bytes.lo = val;
 
-		addressLatch = 1 - addressLatch;
+		addressLatch = !addressLatch;
 		break;
 
 	case 7:
diff --git a/src/PPU.hpp b/src/PPU.hpp
index 6fc0a85..58cfc9f 100644
--- a/src/PPU.hpp
+++ b/src/PPU.hpp
@@ -4,6 +4,9 @@
 
 class Bus;
 
+/**
+ * @brief The PPU of the NES.
+ */
 class PPU
 {
 	friend class PPUWatcher;
@@ -11,18 +14,49 @@ class PPU
 public:
 	PPU(Bus* bus);
 
+	/**
+	 * @brief Powerup PPU.
+	 * Internal state corresponds to powerup state
+	 */
 	void Powerup();
+
+	/**
+	 * @brief Powerup PPU.
+	 * Internal state corresponds to reset state
+	 */
 	void Reset();
 
+	/**
+	 * @brief Tick PPU forward once.
+	 */
 	void Tick();
 
+	/**
+	 * @brief Read from memory mapped PPU regs.
+	 */
 	Byte ReadRegister(Byte id);
+
+	/**
+	 * @brief Write to memory mapped PPU regs.
+	 */
 	void WriteRegister(Byte id, Byte val);
 
+	/**
+	 * @brief Check whether the PPU finished rendering a frame.
+	 * Returns true if the VBlankStart cycle was hit previously. The function resets
+	 * the boolearn when it is called
+	 */
 	inline bool IsFrameDone() { bool returnVal = isFrameDone; isFrameDone = false; return returnVal; }
 
 private:
+	/**
+	 * @brief Wraps Bus::ReadPPU.
+	 */
 	Byte Read(Word addr);
+	
+	/**
+	 * @brief Wraps Bus::WritePPU.
+	 */
 	void Write(Word addr, Byte val);
 
 private: // Registers
@@ -81,8 +115,10 @@ private: // Registers
 
 	Address ppuaddr;
 
+	// Current x, y position the PPU operates on
 	uint16_t x, y;
-	Byte addressLatch = 0;
+	Byte latch = 0;
+	bool addressLatch = false;
 
 private:
 	bool isFrameDone = false;