From f2437cb257587eb6a30f6efb234e923dd7d0383a Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Tue, 31 Jan 2017 20:30:32 -0500
Subject: [PATCH] Added some additional documentation, started making steps
 towards returning sprites, fixed a counter bug that would exhibit as
 incorrect sync.

---
 Machines/Atari2600/TIA.cpp | 16 +++-------------
 Machines/Atari2600/TIA.hpp | 18 +++++++++++++++---
 2 files changed, 18 insertions(+), 16 deletions(-)

diff --git a/Machines/Atari2600/TIA.cpp b/Machines/Atari2600/TIA.cpp
index 9554ff97c..8398e8ca0 100644
--- a/Machines/Atari2600/TIA.cpp
+++ b/Machines/Atari2600/TIA.cpp
@@ -205,7 +205,7 @@ void TIA::set_player_delay(int player, bool delay)
 
 void TIA::set_player_position(int player)
 {
-	// TODO
+	player_[player].position = ((horizontal_counter_ - 68) + 6)%160;
 }
 
 void TIA::set_player_motion(int player, uint8_t motion)
@@ -267,15 +267,6 @@ void TIA::clear_collision_flags()
 {
 }
 
-//				case 0: case 1: case 2: case 3:					state = OutputState::Blank;									break;
-//				case 4: case 5: case 6: case 7:					state = OutputState::Sync;									break;
-//				case 8: case 9: case 10: case 11:				state = OutputState::ColourBurst;							break;
-//				case 12: case 13: case 14:
-//				case 15: case 16:								state = OutputState::Blank;									break;
-//
-//				case 17: case 18:								state = vbextend ? OutputState::Blank : OutputState::Pixel;	break;
-//				default:										state = OutputState::Pixel;									break;
-
 void TIA::output_for_cycles(int number_of_cycles)
 {
 	/*
@@ -284,7 +275,7 @@ void TIA::output_for_cycles(int number_of_cycles)
 		NTSC colour clock.
 
 		Therefore, each line is composed of:
-		
+
 			16 cycles:	blank					; -> 16
 			16 cycles:	sync					; -> 32
 			16 cycles:	colour burst			; -> 48
@@ -301,7 +292,7 @@ void TIA::output_for_cycles(int number_of_cycles)
 		if(horizontal_counter_ <= target) \
 		{ \
 			crt_->function((unsigned int)((horizontal_counter_ - output_cursor) * 2)); \
-			output_cursor = horizontal_counter_; \
+			horizontal_counter_ %= cycles_per_line; \
 			return; \
 		} \
 		else \
@@ -337,7 +328,6 @@ void TIA::output_for_cycles(int number_of_cycles)
 		}
 		int duration = std::min(228, horizontal_counter_) - output_cursor;
 		crt_->output_blank((unsigned int)(duration * 2));
-		output_cursor += duration;
 	}
 	else
 	{
diff --git a/Machines/Atari2600/TIA.hpp b/Machines/Atari2600/TIA.hpp
index 3883fe0bc..6c76fc9cd 100644
--- a/Machines/Atari2600/TIA.hpp
+++ b/Machines/Atari2600/TIA.hpp
@@ -68,8 +68,12 @@ class TIA {
 		std::shared_ptr<Outputs::CRT::CRT> crt_;
 
 		// drawing methods
-		void output_for_cycles(int number_of_cycles);
-		void output_line();
+		inline void output_for_cycles(int number_of_cycles);
+		inline void output_line();
+
+		inline void draw_background(uint8_t *target, int start, int length);
+		inline void draw_playfield(uint8_t *target, int start, int length);
+		inline void draw_background_and_playfield(uint8_t *target, int start, int length);
 
 		// the master counter; counts from 0 to 228 with all visible pixels being in the final 160
 		int horizontal_counter_;
@@ -84,8 +88,14 @@ class TIA {
 		// playfield state
 		uint8_t playfield_ball_colour_;
 		uint8_t background_colour_;
-		uint32_t background_[2];
 		int background_half_mask_;
+		uint32_t background_[2];	// contains two 20-bit bitfields representing the background state;
+									// at index 0 is the left-hand side of the playfield with bit 0 being
+									// the first bit to display, bit 1 the second, etc. Index 1 contains
+									// a mirror image of index 0. If the playfield is being displayed in
+									// mirroring mode, background_[0] will be output on the left and
+									// background_[1] on the right; otherwise background_[0] will be
+									// output twice.
 
 		// player state
 		struct Player {
@@ -95,6 +105,8 @@ class TIA {
 			uint8_t colour;		// the player colour
 			int reverse_mask;	// 7 for a reflected player, 0 for normal
 			uint8_t motion;		// low four bits used
+			uint8_t position;	// in the range [0, 160) to indicate offset from the left margin, i.e. phase difference
+								// between the player counter and the background pixel counter.
 		} player_[2];
 
 		// missile state