From ab2ad708856a57fd9d1c2e27718aad056c1d4abe Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Thu, 18 Mar 2021 21:29:52 -0400
Subject: [PATCH] Takes a run at interrupts.

---
 Machines/Sinclair/ZXSpectrum/Video.hpp      | 100 +++++++++++++++++++-
 Machines/Sinclair/ZXSpectrum/ZXSpectrum.cpp |   8 +-
 2 files changed, 106 insertions(+), 2 deletions(-)

diff --git a/Machines/Sinclair/ZXSpectrum/Video.hpp b/Machines/Sinclair/ZXSpectrum/Video.hpp
index f72cff6ab..a8e2c7cba 100644
--- a/Machines/Sinclair/ZXSpectrum/Video.hpp
+++ b/Machines/Sinclair/ZXSpectrum/Video.hpp
@@ -9,6 +9,9 @@
 #ifndef Video_hpp
 #define Video_hpp
 
+
+#include "../../../ClockReceiver/ClockReceiver.hpp"
+
 namespace Sinclair {
 namespace ZXSpectrum {
 
@@ -16,8 +19,103 @@ enum class VideoTiming {
 	Plus3
 };
 
-template <VideoTiming timing> class Video {
+/*
+	Timing notes:
 
+	As of the +2a/+3:
+
+		311 lines, 228 cycles/line
+		Delays begin at 14361, follow the pattern 1, 0, 7, 6, 5, 4, 3, 2; run for 129 cycles/line.
+		Possibly delays only affect actual reads and writes; documentation is unclear.
+
+	Unknowns, to me, presently:
+
+		How long the interrupt line held for.
+
+	So...
+
+		Probably two bytes of video and attribute are fetched in each 8-cycle block,
+		with 16 such blocks therefore providing the whole visible display, an island
+		within 28.5 blocks horizontally.
+
+		14364 is 228*63, so I I guess almost 63 lines run from the start of vertical
+		blank through to the top of the display, implying 56 lines on to vertical blank.
+
+*/
+
+template <VideoTiming timing> class Video {
+	private:
+		struct Timings {
+			int cycles_per_line;
+			int lines_per_frame;
+			int first_delay;
+			int first_border;
+			int delays[16];
+		};
+
+		static constexpr Timings get_timings() {
+			constexpr Timings result = {
+				.cycles_per_line = 228 * 2,
+				.lines_per_frame = 311,
+				.first_delay = 14361 * 2,
+				.first_border = 14490 * 2,
+				.delays = {
+					2, 1,
+					0, 0,
+					14, 13,
+					12, 11,
+					10, 9,
+					8, 7,
+					6, 5,
+					4, 3,
+				}
+			};
+			return result;
+		}
+
+	public:
+		void run_for(HalfCycles duration) {
+			constexpr auto timings = get_timings();
+
+			// Advance time. TODO: all the drawing.
+			time_since_interrupt_ = (time_since_interrupt_ + duration.as<int>()) % (timings.cycles_per_line * timings.lines_per_frame);
+		}
+
+	private:
+		// TODO: how long is the interrupt line held for?
+		static constexpr int interrupt_duration = 48;
+
+	public:
+
+		HalfCycles get_next_sequence_point() {
+			if(time_since_interrupt_ < interrupt_duration) {
+				return HalfCycles(interrupt_duration - time_since_interrupt_);
+			}
+
+			constexpr auto timings = get_timings();
+			return timings.cycles_per_line * timings.lines_per_frame - time_since_interrupt_;
+		}
+
+		bool get_interrupt_line() const {
+			return time_since_interrupt_ < interrupt_duration;
+		}
+
+		int access_delay() const {
+			constexpr auto timings = get_timings();
+			if(time_since_interrupt_ < timings.first_delay) return 0;
+
+			const int time_since = time_since_interrupt_ - timings.first_delay;
+			const int lines = time_since / timings.cycles_per_line;
+			if(lines >= 192) return 0;
+
+			const int line_position = time_since % timings.cycles_per_line;
+			if(line_position >= timings.first_border - timings.first_delay) return 0;
+
+			return timings.delays[line_position & 7];
+		}
+
+	private:
+		int time_since_interrupt_ = 0;
 };
 
 }
diff --git a/Machines/Sinclair/ZXSpectrum/ZXSpectrum.cpp b/Machines/Sinclair/ZXSpectrum/ZXSpectrum.cpp
index f404c676d..ab8301fc5 100644
--- a/Machines/Sinclair/ZXSpectrum/ZXSpectrum.cpp
+++ b/Machines/Sinclair/ZXSpectrum/ZXSpectrum.cpp
@@ -82,6 +82,7 @@ template<Model model> class ConcreteMachine:
 		}
 
 		void flush() {
+			video_.flush();
 			update_audio();
 			audio_queue_.perform();
 		}
@@ -102,6 +103,11 @@ template<Model model> class ConcreteMachine:
 		forceinline HalfCycles perform_machine_cycle(const CPU::Z80::PartialMachineCycle &cycle) {
 			time_since_audio_update_ += cycle.length;
 
+			video_ += cycle.length;
+			if(video_.did_flush()) {
+				z80_.set_interrupt_line(video_.last_valid()->get_interrupt_line());
+			}
+
 			// Ignore all but terminal cycles.
 			// TODO: I doubt this is correct for timing.
 			if(!cycle.is_terminal()) return HalfCycles(0);
@@ -287,7 +293,7 @@ template<Model model> class ConcreteMachine:
 
 		// MARK: - Video.
 		static constexpr VideoTiming video_timing = VideoTiming::Plus3;
-		Video<video_timing> video_;
+		JustInTimeActor<Video<video_timing>> video_;
 };