Merge branch 'master' into DesktopScreenshots

Machines/Sinclair/ZXSpectrum/Video.hpp

@@ -167,6 +167,8 @@ template <VideoTiming timing> class Video {
 									last_fetches_[1] = memory_[attribute_address_];
 									last_fetches_[2] = memory_[pixel_address_+1];
 									last_fetches_[3] = memory_[attribute_address_+1];
+									set_last_contended_area_access(last_fetches_[3]);
+
 									pixel_address_ += 2;
 									attribute_address_ += 2;
 
@@ -315,21 +317,37 @@ template <VideoTiming timing> class Video {
 		}
 
 		/*!
-			@returns Whatever the ULA or gate array has fetched this cycle, or 0xff if it has fetched nothing.
+			@returns Whatever the ULA or gate array would expose via the floating bus, this cycle.
 		*/
-		uint8_t get_current_fetch() const {
+		uint8_t get_floating_value() const {
 			constexpr auto timings = get_timings();
 			const int line = time_into_frame_ / timings.cycles_per_line;
 			if(line >= 192) return 0xff;
 
 			const int time_into_line = time_into_frame_ % timings.cycles_per_line;
 			if(time_into_line >= 256 || (time_into_line&8)) {
-				return 0xff;
+				return last_contended_access_;
+			}
+
+			// The +2a and +3 always return the low bit as set.
+			if constexpr (timing == VideoTiming::Plus3) {
+				return last_fetches_[(time_into_line >> 1) & 3] | 1;
 			}
 
 			return last_fetches_[(time_into_line >> 1) & 3];
 		}
 
+		/*!
+			Relevant to the +2a and +3 only, sets the most recent value read from or
+			written to contended memory. This is what will be returned if the floating
+			bus is accessed when the gate array isn't currently reading.
+		*/
+		void set_last_contended_area_access([[maybe_unused]] uint8_t value) {
+			if constexpr (timing == VideoTiming::Plus3) {
+				last_contended_access_ = value | 1;
+			}
+		}
+
 		/*!
 			Sets the current border colour.
 		*/
@@ -367,6 +385,7 @@ template <VideoTiming timing> class Video {
 		bool is_alternate_line_ = false;
 
 		uint8_t last_fetches_[4] = {0xff, 0xff, 0xff, 0xff};
+		uint8_t last_contended_access_ = 0xff;
 
 #define RGB(r, g, b)	(r << 4) | (g << 2) | b
 		static constexpr uint8_t palette[] = {

Machines/Sinclair/ZXSpectrum/ZXSpectrum.cpp

@@ -223,6 +223,10 @@ template<Model model> class ConcreteMachine:
 
 				case PartialMachineCycle::Read:
 					*cycle.value = read_pointers_[address >> 14][address];
+
+					if(is_contended_[address >> 14]) {
+						video_.last_valid()->set_last_contended_area_access(*cycle.value);
+					}
 				break;
 
 				case PartialMachineCycle::Write:
@@ -230,7 +234,13 @@ template<Model model> class ConcreteMachine:
 					if(is_video_[address >> 14] && (address & 0x3fff) < 6912) {
 						video_.flush();
 					}
+
 					write_pointers_[address >> 14][address] = *cycle.value;
+
+					// Fill the floating bus buffer if this write is within the contended area.
+					if(is_contended_[address >> 14]) {
+						video_.last_valid()->set_last_contended_area_access(*cycle.value);
+					}
 				break;
 
 				case PartialMachineCycle::Output:
@@ -331,8 +341,9 @@ template<Model model> class ConcreteMachine:
 
 					// Check for a floating bus read; these are particularly arcane
 					// on the +2a/+3. See footnote to https://spectrumforeveryone.com/technical/memory-contention-floating-bus/
-					if((address & 0xf003) == 0x0001) {
+					// and, much more rigorously, http://sky.relative-path.com/zx/floating_bus.html
-						*cycle.value &= video_->get_current_fetch();
+					if(!disable_paging_ && (address & 0xf003) == 0x0001) {
+						*cycle.value &= video_->get_floating_value();
 					}
 
 					if constexpr (model == Model::Plus3) {